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Fish Abundance and Community Composition in Native and Non-Native Littoral Aquatic Plants at Lake Izabal, Guatemala


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FISH ABUNDANCE AND COMMUNITY COMPOSITION IN NATIVE AND NON-NATIVE LITTORAL AQUATIC PLANTS AT LAKE IZABAL, GUATEMALA By CHRISTIAN ALBERTO BARRIENTOS A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2005

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Copyright 2005 by Christian Alberto Barrientos

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To God, who provides all wisdom and judgment.

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ACKNOWLEDGMENTS I first thank my advisor, Dr. Mike Allen, for his guidance and willingness to do research in Guatemala. I also thank my committee members, Dr. William Haller and Dr. Daniel Canfield. I really appreciate Patrick Cooney and Maritza Aguirres help and support on this project, in the field and by email. Special thanks go to Dr. Mark Brenner who, on one of his research trips to Petn, encouraged me to pursue a higher education level.. Thanks also go to my parents, Jorge and Alicia Barrientos, who provided logistical and moral support. Most importantly, my parents were always there for me. Partial funding for this research was provided by the Wildlife Conservation Society, Mesoamerican Program, especially thanks to Archie Carr III. Funding was also provided by the Fulbright Scholarship and the University of Florida. Last, but not least, I thank my wife Corina and my two daughters Ana Isabel and Ana Cris, who came with me from Guatemala to Gainesville, Florida. iv

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TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................iv LIST OF TABLES .............................................................................................................vi LIST OF FIGURES ..........................................................................................................vii ABSTRACT .....................................................................................................................viii INTRODUCTION ...............................................................................................................1 METHODS ..........................................................................................................................4 Study Site ......................................................................................................................4 Habitat Characterization ...............................................................................................6 Fish Sampling ...............................................................................................................6 Analyses ........................................................................................................................7 RESULTS ..........................................................................................................................12 Habitat Characterization .............................................................................................12 Fish Population Comparisons .....................................................................................12 Low PAC Analysis .....................................................................................................13 Low & High PAC Analysis ........................................................................................14 Hydrilla Analysis ........................................................................................................15 DISCUSSION ....................................................................................................................25 MANAGEMENT IMPLICATIONS .................................................................................32 APPENDIX A BLOCK NETS SUMMARY AT LAKE IZABAL....................................................34 B WATER LEVELS AT LAKE IZABAL.....................................................................36 LIST OF REFERENCES...................................................................................................37 BIOGRAPHICAL SKETCH.............................................................................................41 v

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LIST OF TABLES Table page 1 Block net distribution among treatments.................................................................10 2 Physicochemical variables measured at Lake Izabal...............................................17 3 Taxonomic family and scientific name of fish collected in block nets in Lake Izabal. Mean fish biomass across all treatments......................................................19 4 Taxonomic family and scientific name of fish collected in block nets at Lake Izabal. Plant species and percent area covered (PAC) are located at the top of each column..............................................................................................................20 5 Fish metrics for treatments with low percent area coverage (PAC) including the no plants treatment...................................................................................................21 6 Fish metrics for six treatments included in the high versus low PAC comparison..22 7 Hydrilla data set were treatments are different percent area coverage (PAC). ......23 8 Taxonomic family and scientific name of fish observed in the local market at El Estor, Izabal, Guatemala..........................................................................................24 9 Block nets summary at Lake Izabal (June-July/2004). ..........................................34 vi

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LIST OF FIGURES Figure page 1 Location of Lake Izabal in Guatemala.....................................................................11 2 The relation between plant biomass and percentage area coverage.........................16 3 Fish total length (TL) distribution across all treatments..........................................18 4 Water levels at Lake Izabal 2003-2004....................................................................36 vii

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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science FISH ABUNDANCE AND COMMUNITY COMPOSITION IN NATIVE AND NON-NATIVE LITTORAL AQUATIC PLANTS AT LAKE IZABAL, GUATEMALA By Christian Alberto Barrientos December 2005 Chair: Micheal S. Allen Major Department: Fisheries and Aquatic Sciences I evaluated effects of the coverage of five aquatic plant species on fish community structure and abundance in Lake Izabal, the largest lake in Guatemala, which was recently invaded by a non-native aquatic plant Hydrilla verticillata. The objectives were to assess how hydrilla influenced littoral fish community composition and abundance. Fish were sampled from monotypic beds of five plant species including bulrush Scirpus carrizo, chara Chara phoetida, eelgrass Vallisneria americana, Illinois pondweed Potamogeton illinoensis, hydrilla Hydrilla verticillata, and areas with no plants. Bulrush, hydrilla and eelgrass were found in different densities and were sampled in low (<25%) and high (>75%) percent area coverage (PAC). Fish were sampled using block nets (0.01-ha) treated with rotenone in each plant type and PAC (N = 5 block nets per treatment) during June and July of 2004. Hydrilla had higher aquatic plant biomass relative to native species of the same PAC. Fishes from the family Cichlidae (7 species) viii

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had the highest species richness and highest biomass across all habitats sampled. Total fish species richness was 24, which ranged from 12-15 species and was similar across plant types in the littoral zone. Fish density did not differ significantly among plant types, but fish biomass was significantly different (P=0.01) among plant types with the hydrilla containing the highest fish biomass (186 kg/ha). Fish biomass was higher (P=0.02) in high PAC levels of hydrilla and bulrush than in eelgrass at either low or high PAC. Biomass of mojarra Cichlasoma maculicauda, which support the most important subsistence fisheries in the lake, was significantly different (P=0.01) among treatments and had the greatest biomass in hydrilla. Results of this study show that hydrilla provides suitable habitat for fishes in the littoral areas of Lake Izabal. Areas with hydrilla contained significantly greater biomass of fish compared to native plants or no-plants areas. Although hydrilla may cause problems for lake access and other uses, it is not likely detrimental to Lake Izabal fish community composition or total fish biomass, at levels of growth and coverage found in 2004. ix

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INTRODUCTION Excessive growth of aquatic macrophytes has caused serious problems in many lakes throughout the world (Canfield et al. 1985). Lake Izabal, Guatemala, has recently been colonized by Hydrilla verticillata, a non-native submersed plant. The hydrilla in Lake Izabal was first reported by anglers in 2000, and perhaps entered the system with hurricane Mitch in 1998. The plant is currently too widespread to be eradicated from the lake (Haller 2002). Hydrilla can quickly colonize and dominate systems where it is introduced because it grows rapidly, tolerates low light conditions, and inhabits water depths exceeding four meters (Langeland 1996). Light is often the most limiting factor prohibiting submersed plant growth (Canfield et al. 1985). Arrivillaga (2002) noted that 3 % of the Lake Izabal was covered by hydrilla in 2002, but the plant could potentially cover 10-15% in the dry season when light penetrates into deeper waters, due to increased water clarity. Allen and Tugend (2002) found that areas with dense macrophytes coverage (100 %) had lower fish richness, with centrarchids absent in these areas. Additionally, elevated dead plant material and high plant biomass caused access, fishing and even sampling problems (Allen and Tugend 2002; Allen et al. 2003). Because most attention has focused on the negative aspects of hydrilla, the potential benefits of hydrilla on fish populations have been largely neglected (Tate and Allen 2003a). Like all aquatic plants, hydrilla provides refuge from predators and habitat for fishes that consume macroinvertebrates associated with the plant and associated 1

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2 periphyton. In laboratory studies, hydrilla forms a complex architecture, providing refuge from predators for prey fish and large surface area for macroinvertebrates (Chick and Mclvor 1997; Valley and Bremingan 2002). Aquatic plants influence both fish distribution and abundance by creating structurally complex habitats. Fish are generally not distributed evenly throughout a lake, but are often concentrated within the vegetated littoral zone (Chick and Mclvor 1994). Fish abundance can be substantially higher in areas with aquatic plants than in areas without plants (Barnett and Schneider 1974; Bettoli et al. 1992; Chick and Mclvor 1997; Durant 1980; Killgore et al. 1989). However, foraging success of predators declines as plant density increases (Mittelbach 1981; Savino and Stein 1982). Aquatic macrophytes also influence fish community structure. There is a negative relationship between abundance of submersed plants and planktonic algal biomass (Canfield et al. 1983), and thus, occurrence of aquatic plants favors insectivorous rather than planktivorous fishes. Bettoli et al. (1993) found that of seventeen fish species commonly collected in vegetation, most of them in hydrilla, the biomass of eight species declined after vegetation removal in a Texas reservoir. Density of planktonic fish increased nearly five fold, coincident with a decline in mean size of fish, after hydrilla removal. Maceina et al. (1991) found that dense macrophyte coverage (29-44%) dominated by hydrilla suppressed planktivorous threadfin shad Dorosoma petenense abundance, which seemed to restrict growth of crappie Pomoxis spp., a predator on threadfin shad. In Lake Izabal, fish assemblages found in native vegetation and hydrilla may be different, due to differences in plant architecture complexity. The physical plant structure

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3 influences fish inhabiting the littoral zone (Dionne and Folt 1991). Littoral zones often contain several macrophyte species, occurring either separately or as part of a mosaic, which function as different habitats for fishes (Chick and Mclvor 1994). Management of aquatic vegetation communities usually affects predator-prey interactions, fish community structure, and the quality of component fisheries (Bettoli et al. 1992). Hydrilla control options at Lake Izabal have been assessed (Arrivillaga 2003; Haller 2002), but no studies have evaluated fish communities in hydrilla for lakes in Central America. The relationships between plants, fish abundance, community composition, and fisheries are important management considerations when plant control strategies are developed (Durant 1980; Haller 2002; Killgore et al. 1989; Tate and Allen 2003b). I evaluated the fish communities in various aquatic plant types of littoral areas at Lake Izabal, Guatemala. My objectives were 1) to contribute to the knowledge of the fish community at Lake Izabal; and 2) to determine if composition of fish assemblages and fish abundance differed among types of aquatic habitats and different densities of Hydrilla verticillata and native plants.

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METHODS Study Site Lake Izabal is the largest lake in Guatemala with a length of 43 km, a width of 19 km and an area of 717 km 2 (Brinson and Nordlie 1975). It is an inland lake located in the eastern part of the country, not far (40 km) from the Caribbean coast (Figure 1). The lake surface lies 10 meters above sea level. The mean depth is 12 meters and the maximum depth is 17 meters (Brinson and Nordlie 1975). The lake does not typically stratify and part of the lake volume is below sea level (Brinson and Nordlie 1975). The lake is surrounded by mountains (Sierra de las Minas to the south, Sierra de Santa Cruz to the north) and fed primarily by the Rio Polochic River, one of the largest rivers in Guatemala. The Polochic River discharges 70% of the total freshwater input to Lake Izabal, with smaller streams around the lake contributing the remainder (Brinson and Nordlie 1975). The Dulce River is the only river that flows out of the lake, flowing from the east end of the lake through a smaller lake, El Golfete, and into the Baha de Amatique and the Gulf of Honduras near the town of Livingston (Carr III 1971; Michot et al. 2002; Figure 1). The wetlands of the Polochic River are located in the western part of the lake and are very popular to local subsistence fishers, although large-scale recreational fisheries currently do not occur at the lake. The Izabal system, which is in the Polochic-Izabal division, is one of the four fish subdivisions of the Usumacinta Province (Miller 1966). Most components of the ichthyofauna found at the lake belong to a broadly distributed group of fishes found 4

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5 between the Ro Coatzacoalcos in the north and the Ro Polochic/Ro Sarstun in the south (Miller 1966). This general distribution has been referred to as the Ro Usumacinta Province by Miller (1966; 1976). On a recent revision of fish distribution from the entire Lake Izabal basin, Perez (2004) reported the presence of 81 species and 24 fish families present. Cichlids and livebearers are the most common families with 17 and 13 species, respectively. Lake Izabal also supports important commercial fisheries. However the fisheries are mainly subsistence and help support the local communities around the lake. The main fishing gears are gill nets, but cast nets and hook and line are also used (Carr III 1971; Dickinson III 1974; Arrivillaga 2002; Michot et al. 2002). Prior to the 1960s, the gill nets targeted marine species like snooks (Centropomidae), jacks (Carangidae), and catfishes (Ariidae) in open waters. However, in 1963 anglers noticed a decline in species of marine origin and switched to harvesting freshwater species like mojarra Cichlasoma maculicauda (Dickinson III 1974). Abundance of aquatic macrophytes in Lake Izabal before hydrilla entered the system was described as: Izabal lacks the luxuriant aquatic vegetation that might be expected in a lowland tropical lake. Bottom configuration and wave action provides a possible explanation (Dickinson III 1974). However this description had been modified due to colonization of Lake Izabal by exotic hydrilla. Hydrilla is now part of a plant community that includes many native species. Arrivillaga (2003) reported the presence of native plants including Chara phoetida, Cabomba caroliniana, Eichhornia crassipes, Najas guadalupensis, Pistia stratiotes, Potamogeton illinoensis, P. pussilus, Salvinia molesta, Scirpus carrizo, Typha dominguensis, and Vallisneria americana in the lake.

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6 Arrivillaga (2002) found that about 3% of the lake area contained hydrilla, and the plant was interspersed with native plants around the lake. Habitat Characterization In the littoral zone of Lake Izabal at depth less than 1.5 meters, I set 50 block nets where I sampled vegetation coverage and biomass, water characteristics in each block net (Figure 1; Appendix A). My experimental design included setting block nets in each plant species at low (25%), medium (26% 74%) and high ( 75%) percent of area covered (PAC), but some plants did not occur at the lake in high coverages. I located areas with the aquatic plant and by three different persons visually estimated the PAC in each net in the field, and then averaged, to assign total PAC to each net. Vegetation biomass was estimated using 3 random quadrats (0.25 m 2 ) inside each block net. In each quadrat, above-ground plant material was removed. Sampled plants were placed in a nylon mesh bag and spun to remove excess water. Samples were weighed to the nearest 0.1 kilogram wet weight (Canfield et al. 1990). Temperature (C) and pH were measured in the field using a Merck Multiline P4, after the block net was set and within one meter from the perimeter of each net. Each net was set so that the habitat inside the net was characteristic of the surrounding habitat. Each plant species with different levels of PAC will be referred to as treatments, including no plants, in this document (Table 1). Fish Sampling Fish were collected using block nets (0.01 ha, 10 x 10 m, 6 mm bar mesh) treated with rotenone. The four corners and sides of the nets were anchored with concrete weights to insure contact with the bottom and to maintain a square configuration. After net placement, powder rotenone was mixed with water. The amount of rotenone used was determined by the mean depth inside the net to obtain a desired concentration of 3 mg/L.

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7 The rotenone slurry was sprayed onto the water surface within each net using a small pump. Fish were collected as they surfaced using dip nets for one hour after the application, and usually three persons were inside the net retrieving fish (Bettoli and Maceina 1996; Timmons et al. 1978). Sampling began June 25 and continued through July 18 2005. Sampling each day started at 07:00 and ended at 15:00 (6 nets/day). Fish collected from each net were placed on ice and moved to the field laboratory at the end of the day. At the laboratory, fish were sorted by species, counted and weighed. When high numbers of individuals were collected, a random sub-sample of 20 individuals by species were measured to the nearest millimeter total length (TL) and individually weighed (0.1 g), and then all the rest were weighed as a batch. Samples of all species were fixed in 10% formalin to assure field identification. Samples of all species were returned to the Florida Natural History Museum in Gainesville and identified to the species level. Analyses Fish metrics including total fish biomass (kg/ha), total density (fish/ha), fish species richness, and fish diversity were estimated for each block net. Diversity was calculated using the Shannon-Wiener index (H) for numbers of individual fish of each species collected in each block net (Krebs 1999). Diversity attempts to account for evenness and richness by looking at both the number of species and how evenly distributed the number of individuals is among the species in each habitat. The Shannon-Weiner index of species diversity was estimated as: 2 H= (pi/p) (log 2 pi/p) (eq. 1) i=1

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8 where pi is the number of individuals or total weight of the ith species, P is the total number of individuals or total weight of all species, and s is the total number of species in each habitat. For biological communities, H ranges from zero to five (Krebs 1999), and is expressed in bits per individual (bits/individual). The mojarra is the local name for several Cichlids in Guatemala, however in Lake Izabal the name is mainly used for Cichlasoma maculicauda. Biomass of mojarra was also included as a dependent variable and tested independently because it is the most common species captured in the fishery at the lake. Mojarra harvest is estimated in tons on a weekly basis (personal comment Maritza Aguirre 2004). The Cichlids family was the most abundant with seven species; hence cichlids biomass was tested independently in all analysis. Silversides Atherinella spp accounted over half of fish density across all habitats; hence silversides biomass was also tested independently and total fish density where tested without silversides. The data were analyzed in three different tests. First, I tested whether total fish density (fish/ha), total fish biomass (kg/ha) and fish diversity (H) differed among five plant type treatments at low PAC (25%) using a one-way ANOVA. The plant treatments were bulrush Scirpus carrizo, chara Chara phoetida, eelgrass Vallisneria americana, illinois pondweed Potamogeton illinoensis and hydrilla Hydrilla verticillata, and no plants. This first test is referred to as the low PAC assessment in this document. (Procedure GLM SAS 1996). Prior to the ANOVA, a Shapiro-Wilks test was used to test for normality for all fish variables except diversity, which is already on a log scale (Procedure UNIVARIATE NORMAL SAS 1996). Density and biomass were log 10 (x+1)

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9 transformed to improve normality. When the one-way ANOVA was significant, Tukeys test was performed to separate the means. The second analysis included only three of the plant species (i.e., bulrush, eelgrass, and hydrilla) and two coverage levels (low vs. high PAC) using a two-way ANOVA (Procedure GLM SAS 1996). The two-way ANOVA was used to assess if fish metrics differed among plant species, PAC levels, and the interaction of the two fixed effects. Dependent variables were total fish density, biomass and diversity (H). The density and biomass were transformed as described above. When the two-way ANOVA was significant, Least Squared Means test was performed at fixed levels of the interest variable. The third analysis tested whether the fish metrics differed among three levels of PAC (low, medium, high) for hydrilla alone. Hydrilla was evaluated alone because it is exotic at the lake and because it was the only aquatic plant found at all levels of PAC. When the one-way ANOVA was significant, Tukeys test was performed to separate the means. Differences were declared significant at P 0.10 for all analyses. Physicochemical variables were tested in each of the analysis

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10 Table 1. Block net distribution among treatments sorted by plants species and PAC at Lake Izabal 2004. Notice that hydrilla was the only plant species with all levels of PAC present. PAC Bulrush Chara Eelgrass Hydrilla Illinois pondweed No plants Low ( 25%) 5 5 5 5 5 Med (26-74%) 5 High (75%) 5 5 5 None 5

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11 Figure 1. Location of Lake Izabal in Guatemala (inset right bottom corner). The littoral zones sampled are show in red. From: Arrivillaga, A. (2003).

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12 RESULTS Habitat Characterization Plants species and percentage area c overage (PAC) were the factors that experimental design included (Table 1). Pl ant biomass was positively related to PAC, but correlation was weak (r2 = 0.47), which was likely due to differences in biomassPAC relationships among species and the relatively low num ber of quadrats (N=3) used to sample plant biomass (Figure 2). For ex ample, bulrush was the only emergent plant with cylindrical stems in this study. Eelgra ss was short (< 30 cm) present in the bottom and Illinois pondweed with medium sized lance shaped leaves close to the surface. Chara had little filaments always close to the botto m not taller than 10 cm. Hydrilla formed extremely dense canopy (high plant biomass) close to the surface. Physicochemical variables were measured along with plant species and PAC. Water temperature, depth sampled, and pH were similar across all nets. Mean depth was 1.2 meters across all treatments (Table 2). Temperature averaged 30 oC and was nearly constant across treatments and days (Table 2) and pH averaged 7.9 across all treatments. No significant differences were found in temper ature (P = 0.72), depth inside the nets (P = 0.92) or pH (P = 0.12) using analysis of va riance (ANOVA) with plant type as the fixed effect. Fish Population Comparisons Twenty four fish species were collected from 50 block nets placed in the littoral areas at Lake Izabal. A total of 16,610 fish were collected, and total length (TL) was

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13 measured from 4,565 fish. Size (TL) across all species and treatments ranged from 1 cm to 24 cm and averaged 5.8 cm; 95% of the fish were under 13 cm (Figure 3). Eight of twenty four species were collected in all treatments, and 8 uncommon species were found in two or less treatments (Table 3). Six species had less than 10 individuals per hectare (Table 3). Individual biomass ranged from 0.1 g to 266 g with an average of 5 g. Three species (Atherinella spp, Cichlasoma aureum and C. maculicauda) accounted for 70% of the total biomass across all nets. Some species were uncommon and found only in certain habitats. For example, Oligoplites saurus and Eugerres plumieri were found only in nets with no plants, and Poecilia mexicana and Brycon guatemalensis were found only in bulrush (Table 4). Jaguar guapote Cichlasoma managuense introduced to Lake Izabal in 1950 (personal comment Herman King), was present in high densities in hydrilla, and just a few individuals were captured in other plant types. Low PAC Analysis The low PAC analysis demonstrated that fish richness was similar (12 to 14 species) among treatments (Table 5). Mean total fish density ranged from 16,620 (fish/ha) in chara to 51,120 (fish/ha) in hydrilla, and 40,480 (fish/ha) in no-plant treatment, but fish density was not significantly different (P > 0.1) among habitats. However, when I removed silversides Atherinella spp from the analysis the total fish density was significantly different (P = 0.04).The no-plant treatment had significantly less fish compare to hydrilla, which had cichlids accounting for most of the density in this treatment. Mean total fish biomass ranged from 17 (kg/ha) to 155 (kg/ha) and was significantly different (P = 0.02) among treatments (Table 5). Chara and no plants had lower total fish biomass than hydrilla, but fish biomass did not differ between hydrilla,

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14 eelgrass, or bulrush (Table 5). The Shannon-Wiener diversity index (H) by number ranged from 0.72 in no-plants to 2.06 in Illinois pondweed and differed between these treatments but not the others (P = 0.07, Table 5). Regarding specific fish groups, there were differences in biomass among treatments. Cichlid biomass differed among treatments (P = 0.003), where chara (5 kg/ha) and no-plants (10 kg/ha) had lower cichlids biomass than hydrilla (162 kg/ha) (Table 5). Mojarra biomass was significantly (P = 0.02) higher in hydrilla (62 kg/ha) than in the no plant treatment (5 kg/ha), but the other treatments did not differ significantly (Table 5). The silversides was the most common fish in our samples (caught in 44/50 nets), but I did not detect significant differences in biomass across treatments due to highly variable catches, despite high silverside density in the no plant treatment. Low & High PAC Analysis This data set comprised fish metrics for three plant species (hydrilla, eelgrass and bulrush) and two PAC levels (low and high) as fixed effects. Species richness ranged from 13 to 15 species across treatments. The plant type and PAC interaction was not significant (P > 0.1) for total fish density, total fish biomass or diversity (H), indicating that differences in these metrics did not differ among the levels of each treatment. These variables did not differ significantly due to PAC (P > 0.1); indicating that the coverage level of plants did not influence fish metrics. However, total fish biomass varied (P = 0.02) due to plant type (Table 6), with eelgrass having significantly lower biomass than hydrilla and bulrush (Table 6). Specific groups and species were tested with the two-way ANOVA. The plant species and PAC interaction was not significant (P > 0.1) for Cichlids, mojarra or

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15 silversides biomass. However, Cichlids differed in biomass, due to plant type (P = 0.02), where eelgrass had lower cichlid biomass than the other two plant species. Similarly, mojarra had the lower biomass in eelgrass than in hydrilla and bulrush (P < 0.01). Silverside biomass was different due to plant type (P = 0.09), and hydrilla had the lower biomass. Silversides biomass was the only variable different due to PAC (P = 0.06), with high biomass in low PAC areas compared to high PAC areas across all plant types (Table 6). Hydrilla Analysis Hydrilla was the only non-native plant in this study and was present around all 50 nets at Lake Izabal. Hydrilla had the highest plant biomass of all treatments (Figure 2). Total fish density, total fish biomass or diversity (H) did not differ with different levels of hydrilla PAC. Cichlids, mojarra and silversides biomass did not differ significantly (P > 0.1) among PAC levels of hydrilla (Table 7).

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16 00.20.40.60.811.21.41.61.8020406080100Percentage Area CoverageLog10 plant biomass kg/m2 hydrilla bulrush eelgrass illinoispondweed chara y = 0.0097x + 0.1742R2 = 0.4723 Figure 2. The relation between plant biomass and percentage area coverage (PAC) (r2= 0.47 N=50) at Lake Izabal June-July 2004.

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17 Table 2. Physicochemical variables measured at Lake Izabal from June to July 2004. Treatments were plant species with different percentage area coverage (PAC) and no plants. Variables were measured at each block net and then averaged by treatments (N=5/treatment). Mean standard deviation of temperature (oC), depth and pH are shown. Treatments Plant species PoC (meters) 8.07 0.2829.94 0.78 1.06 0.31 AC pH Temperature Depth Chara 15 Ill. pond we ed 7.91 0.1229.60 0.79 1.12 0.08 nts 7.67 0.3929.96 1.18 1.16 0. 17 8.05 0.1129.90 0.76 1.22 0.24 7.62 0.0929.82 0.77 1.24 0.27 8.25 0.2330.46 1.94 1.14 0.16 ss 8.02 0.3730.64 1.19 1.10 0.20 25 7.65 0.2629.34 0.72 1.24 0.36 54 8.08 0.3030.28 1.27 1.34 0.22 Hydri83 7.90 0.5331.16 1.66 1.28 0.17 All treatments 7.90 0.3530.10 1.15 1.18 0.22 25 No pla 0 19 Bulrush 80 23 Eelgra 77 lla

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18 Figure 3. Fish total length (TL) distribution across all treatments. The x axis shows fish total length in centimeters, and the y axis is the number of fish (N= total measured individuals). 010012345678910111213141Total Length (cm) 2003004005006007008005161718192021222324Number of Fish N=4565

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Table 3. Taxonomic family and scientific name of fish collected in block nets in Lake Izabal. Mean fish biomass across all treatments is in kg/ha, and mean fish density is fish/ha, mean total length (TL, mm), and SD is the standard deviation for respective mean. Values represent means across all block nets (N=50). 19y me Famil Scientific na kg/ha SD fish/ha SD TL (mm) SD Cichlid ae 31.53 29.50 2018 1439 Cichlasoma maculicauda 76 32 Cichlid ae 20.60 12.44 2204 1241 ae 11.52 7.39 10897 e i 9.24 8.94 1484 1393 60 17 e 5.90 5.57 1436 1224 58 15 e 4.18 5.61 1148 1633 47 28 3.27 8.58 114 ae 2.91 4.08 2224 3691 ae 1.27 1.27 ae 0.66 0.75 134 189 ae 0.30 0.48 ae i 0.19 0.50 108 Cichlasoma bocourti 0.07 0.14 ae p0.07 0.11 88 ae 0.07 0.09 dae 0.04 0.12 10 19 ae 0.03 0.10 0.03 0.07 14 hidae 0.02 0.04 80 112 ae 0.02 0.05 0.02 0.03 222 284 120 45 0.01 0.02 12 10 118 7 0.01 0.01 8 19 64 0 Hemiramphidae Hyporhamphus roberti hildebrandi 0.01 0.01 2 6 64 16 Cichlasoma aureum 77 16 Atherinid Atherinella spp 17784 43 12 Cichlida Cichlasoma salvin Cichlida Cichlasoma spilurum Cichlida Cichlasoma managuense Ariidae Cathorops spp 230 428 44 Poeciliid Carlhubbsia stuarti 40 14 Eleotrid Gobiomorus dormitor 462 326 49 31 Characid Astianax aeneus 60 25 Characid Brycon guatemalensis 20 50 96 25 Cichlid Cichlasoma robertson 14 19 14 Cichlida eEngraulid 12 19 67 18 Anch s oap 89 44 11 Poeciliid Poecilia mexicana 4 13 90 24 Carangi Oligoplites saurus 62 13 Gerreid Eugerres plumieri 2 6 80 18 Achiridae Trinectes paulistanus 4 8 69 Sygnat Pseudophallus mindii 13 7 Gobiid Gobiodes broussoneti 6 14 81 49 Gobiidae Gobiosoma spp Belonidae Strongylura notata Pimelodidae Rhamdia guatemalensis

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20Table 4. Taxonomic family and scientific name of fish collected in block nets (N=5/treatment) at Lake Izabal. Plant species an d percent area covered (PAC) are located at the top of each column (x= present). no plants chara hydrilla Illinois pondweed bulrush eelgrass Family Scientific name none low low medium high low low high low high Achiridae Achirus spp x x Ariidae Cathorops spp x x x x x x Atherinidae Atherinella spp x x x x x x x x x x Belonidae Strongylura notata x x x x x x Carangidae Oligoplites saurus x x x Astianax aeneus x x x x x x x x x Characidae Brycon guatemalensis x x x Cichlasoma aureum x x x x x x x x x x Cichlasoma bocourti x x x x Cichlasoma managuense x x x x x x x x Cichlasoma robertsoni x x x x Cichlasoma salvini x x x x x x x x x Cichlasoma spilurum x x x x x x x x x Cichlidae Cichlasoma maculicauda x x x x x x x x x x Eleotridae Gobiomorus dormitor x x x x x x x x x x Engraulidae Anchoa spp x x x x x x x x Gerreidae Eugerres plumieri x Gobiodes broussoneti x x Gobiidae Gobiidae x x x x x x x x x x Hemiramphidae Hyporhamphus spp x x Pimelodidae Rhamdia guatemalensis x Carlhubbsia stuarti x x x x x x Poeciliidae Poecilia mexicana x Sygnathidae Pseudophallus mindii x x x x x x x

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21Table 5. F ificantly different (Tukeys test = 0.1). No letters indicate no significant difference between me ish metrics for treatments with low percent area coverage (PAC) including the no plants treatment. Treatments are plantspecies and no-plants. Fish richness is the cumulative richness across all nets (N=5) for each treatment. Mean and standard deviation of total fish biomass (kilograms/hectare), total fish density (fish/hectare), Shannon-Wiener diversity index (H) are shown. Cichlid, mojarra, and silverside biomass (kilograms/hectare) are shown. Values along column without a letter in common are sign an values. Treatment Plant species PAC Richness (cumulative) biomass Total fish density Shannon-Wiener Cichlids biomass mojarra biomass biomass Total fish (kg/ha) (fish/ha) (H) (kg/ha) (kg/ha) silversides (kg/ha) Bulrush Low 14 84 48 ab 25,500 12,644 1.47 0.61 ab 62 50 ab 30 29 ab 18 12 Chara Low 12 16,620 15,1248 9 ass Hydrilla Low 14 b 51,120 41,632ab a a 16 21 Illinois pondweed Low 13 57 30 ab 19,960 14,6032.06 0.80 a 41 27 ab 8 7 ab 7 10 No plants 0 13 42 44 a 40,480 37,3560.72 0.46 b 10 17 c 5 10 b 26 26 17 8 a 1 0.81 ab 5 4 bc 3 3 ab Eelgr Low 13 64 71 ab 26,940 29,843 1.19 0.25 ab 46 55 abc 22 46 ab 16 16 155 68 1.79 1.13 121 67 67 57

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Table 6. Fish metrics for six treatments included in the high versus low PAC comparison. Treatments are plant species with low (<25%) or high (>75%) PAC. Fish richness is the cumulative richness across all nets (N=5) for each treatment. Mean and standard deviation of total fish biomass (kilograms/hectare), total fish density (fish/hectare), Shannon-Wiener diversity index (H) are shown. Cichlid, mojarra, and silverside biomass (kilograms/hectare) are shown. Values along column without a letter in common are significantly different (Least Squares Means test P < 0.1). No letters indicate no significant difference between mean values. 22nt Treatme Plant species PAC Richness (cumulative) Total fish biomass (kg/ha) (fish/ha) (kg/ha) (kg/ha) Total fish density Shannon-Wiener (H) Cichlids biomass mojarra biomass silversides biomass (kg/ha) Low 14 84 48 b 25,500 12,644 1.47 0.61 62 50 b 30 29 b 18 12 b Bulrush High 15 21,400 7,437 1.95 0.57 Eelgrass High 15 34,320 26,7811.49 0.79 8 8 Low 14 b 51,120 41,6321.79 1.13 b b b Hydrilla High 14 186 199 b 23,980 8,748 2.26 0.43 132 116 b 81 91 b 1 1 a 152 124 b 130 121 b 64 95 b 7 3 a Low 13 64 71 a 26,940 29,843 1.19 0.25 46 55 a 22 46 a 16 16 b 52 30 a 29 20 a 2 2 a a 155 68 121 67 67 56 16 21

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Table 7. Hydrilla data set were treatments are different percent area coverage (PAC). Fish richness is the cumulative richness across all nets (N=5) for each treatment. Mean and standard deviation of total fish biomass (kilograms/hectare), total fish density (fish/hectare), Shannon-Wiener diversity index (H) are shown. Cichlid, mojarra, and silverside biomass (kilograms/hectare) are shown. Values along column without a letter in common are significantly different (Tukeys test = 0.1). 23ts Treatmen PAC Richness (cumulative) Total fish biomass (kg/ha) Total fish density (fish/ha) (H) (kg/ha) (kg/ha) Shannon-Wiener Cichlids biomass mojarra biomass silversides biomass (kg/ha) low 14 155 68 51,120 41,623 1.79 1.13 121 67 67 56 16 21 mediu m 18 112 57 35,760 30,471 1.91 0.81 82 38 33 23 7 11 high 14 186 199 23,980 8,748 2.26 0.43 132 116 81 91 1 1

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24 y me Table 8. Taxonomic family and scientific name of fish observed in the local market at El Estor, Izabal, Guatemala (July/1-15/2004). Common Spanish name obtained from the anglers. Abundance was the presence in the market as indicated by categories of highly common: > 10 days, common: 5-10 days, and uncommon: < 5 days. Biomass rank obtained in block nets across all treatments. Famil Scientific na common name (spanish) abundance biomass rank in block nets Centropom idae imalis hion Centropomus undec robalo ghly comm Not present Cnropomus para et llelus ichlidae naguense rti maculicauda ma highmon obertsoni mis spp c t atus tae higon unn ent Not present Carangidae Oligopterus saurus zapatera uncommon 16th Characidae Brycon guatemalensis machaca common 11th Eleotridae Gobiomorus dormitor guavina common 9th Gerreidae Eugerres plumieri mojarra blanca uncommon 17th robalo blanco common Not present C Cichlasoma ma guapote common 6th Cichlasoma bou cu cagona uncommon 12th Cichlasoma ojarr ly com 1st Cihlaoma cs r rar shca un ncommo 13th Oreocro tilapia ommon Not presen Ictaluridae Ictalru urc usf cazon t hly comm Not present Ariidae Bagre marinus bagre commo Not pres Ariidae spp punta estrella common Not present Megalopidae Tarpon atlanticus sabalo uncommon

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DISCUSSION It is not certain when hydrilla entered Lake Izabal, but the plant has probably been in the lake since at least 1997 (Haller 2002; Arrivillaga 2003). Historical data regarding the distribution and composition of native plant and fish communities prior to hydrilla introduction do not exist for Lake Izabal. Bettoli et al. (1993) found numerous changes in fish community composition and biomass related to vegetation removal by grass carp in a Texas reservoir over a seven-year span, and the changes occurred as early as the first year after the vegetation removal. I compared fish community metrics among hydrilla and five others types of habitats, including no-plants and four native species of aquatic plants. My assumption was that fish community characteristics in sites without hydrilla would approximate conditions found in littoral areas of the lake prior to hydrilla introduction. Lake Izabal contains native aquatic plant communities with hydrilla mixed in almost all of them, consistent with reports by Arrivillaga (2003). I found hydrilla areas ranged from low percent area coverage (PAC) to high PAC (Appendix A); no co-occurring plants were present when hydrilla had high PAC. Chick and McIvor (1994) found similar results at Lake Okeechobee, where thick stands of hydrilla usually contained no other plant species. Since previous reports at Lake Izabal before hydrilla described low aquatic macrophyte because of the bottom configuration and wave action (Dickinson III 1974), hydrilla beds may actually protect the shoreline from wave action and enhance the ability of native macrophytes to grow. Overall habitat complexity was 25

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26 frequently governed by hydrilla in this study, but there were exceptions. Bulrush was the only littoral plant in this study that had no evident hydrilla colonization. No vast monocultures of native plants existed at the time of this study, and thus, chara and Illinois pondweed were not found in high PAC. Hydrilla was in the proximity or mixed with all the other native submerged plants sampled. Fish species richness did not vary greatly among treatments with low PAC in my study. Preliminary findings at Lake Izabal reported less fish species richness in habitats that contained hydrilla versus native plants (Arrivillaga 2003). However, Arrivillaga (2003) used seines to sample fish, whereas we used block nets and rotenone. Seining in hydrilla could be extremely inefficient due to its dense architecture, making comparisons between these studies difficult. My total fish species collected (24) was lower than previous studies at the lake. Perez (2004) reported 81 at the lake and rivers around it, but her study included all the historical records from 1935 throughout 2003, different fishing gears, sampling across most months, and samples from the entire basin including the associated rivers. Perez (2004) reported 21 species restricted to rivers and 16 with unknown distribution and 44 present in the lake. She also found that about 35 species in Lake Izabal basin were from estuarine origin, and abundance of estuarine fishes in the lake likely varies strongly with season. The most abundant families in Lake Izabal are Poecilids and Cichlids. I only collected two Poecilid species, whereas Perez (2004) reported twelve species from this family, however seven were collected only in rivers. The Usumacinta province exhibited high richness in the Poecilidae family (Miller 1976), and other studies using multiple

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27 gears in areas inside the province found more species, but included rivers (Greenfield 1997; Willink et al. 1999). Perez (2004) reported 17 species of Cichlids, three collected only in rivers and two with unknown distribution in the lake, whereas I collected about nine of the twelve in the lake. Thus, my sampling using block nets in shallow areas of the lake found relatively low species richness (24 species) compared to the historical records in the lake (44 species). Use of 10x10 m block nets in shallow areas resulted in small fish sizes collected in this study, probably resulting in incomplete sampling of the whole-lake fish community. However, my sample size of fish species was probably reflective of the fish community inhabiting inshore littoral areas at Lake Izabal during summer. Total fish density was not significantly different among low PAC treatments. The lack of a statistical difference is surprising, because other studies had found differences due to macrophytes type (Chick and McIvor 1994) or between open water and vegetated areas (Bettoli et al 1993; Gelwick and Mathews 1990; Killgore et al. 1989; Shireman et al. 1981). Although total fish density did not differ among habitats, certain species or groups seemed to prefer different habitats. For example, silversides Atherinella spp were commonly collected in no-plant areas similar to Zaret and Paine (1973) and Bettoli and Morris (1991). Removing silversides from the total fish density resulted in differences among treatments with hydrilla having more fish than no plants treatment. Conversely, cichlids density was lower in no-plant or chara areas compared to other plant areas. Silversides contributed greatly to high within-habitat variability in fish density, preventing the detection of significant differences in total fish density (i.e., low statistical power).

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28 I was able to detect differences in total fish biomass among low PAC treatment types, with bulrush, eelgrass, and hydrilla having higher fish biomass than the other low PAC treatments. Six species comprised 85% of the biomass across all treatments; five of the six species were cichlids. Cichlids (including mojarra) at Lake Izabal exhibited increased biomass in hydrilla, bulrush, and eelgrass, with lower biomass in chara and no-plants. Cichlids occupy much the same ecological position in the tropics as do the temperate North American sunfishes (Centrarchidae, Miller 1976). Therefore, the higher total fish biomass with vegetation present was consistent with other studies in temperate zones (Barnett and Schneider 1974; Bettoli et al. 1993; Durant 1980; Gelwick and Matthews 1990; Killgore et al. 1989; Shireman et al. 1981) and the life history of cichlids (Greenfield 1997; Miller 1976). Subsistence anglers recognized bulrush as a high abundance area for cichlids, consistent with my results. Bulrush was the only native plant with high cichlid biomass similar to non-native hydrilla. The fish community indices also varied among treatments. Diversity by number was lower in no-plants, which was predicted given studies showing that complex habitats increase fish diversity (Barnett and Schneider 1974; Bettoli et al. 1993; Durant 1980; Killgore et al. 1989; Shireman et al. 1981). Diversity showed the lowest value in no-plants and chara, which had almost no plant biomass. Moreover, the littoral fish community changed from cichlids dominated in vegetated to pelagic dominated (Atherinadae, Engraulidae, Gerreidae and Carangidae) in areas without plants and chara. The two-way ANOVA was useful to indicate that the fish population parameters did not vary between plant species and PAC levels, because the interaction between plant species and PAC level was not significant. Total fish biomass was again the only

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29 parameter that showed significant differences due to plant type. The silversides are pelagic oriented (Greenfield 1997; Zaret and Paine 1973) and showed lower biomass in high PAC consistent with Killgore et al. (1989) who found that fish that feed directly on zooplankton could be negatively affected by dense hydrilla coverage. The mojarra and total cichlid biomass was lower in eelgrass, but bulrush and hydrilla had higher fish biomass, consistent with the one-way analysis. Hydrilla was the most common plant around Lake Izabal, thus low, medium and high PAC was present, and hydrilla had the highest plant biomass among all plant species (Appendix A). However, none of the fish community parameters estimated in this study was significantly different among levels of PAC in hydrilla. Conversely, Killgore et al. (1989) found that among seasons intermediate and high densities of hydrilla beds had higher fish abundance and biomass compared with hydrilla beds in lower densities. High variability within different hydrilla PAC levels (i.e., standard deviation that exceeded the mean for the high PAC level) may have prevented the detection of significant differences. Alternately, the fish metrics in this study may not have varied with hydrilla PAC, because the mean fish biomass was 155 kg/ha at low hydrilla PAC and 186 kg/ha at high PAC. Another interesting note was that jaguar guapote Cichlasoma managuense, an introduced species to Lake Izabal was commonly collected in hydrilla but not in other plant species in this study. The tilapia (Oreocromis spp), another introduced species, was found commonly in the market (Table 8) and reported to be caught on the edge of hydrilla beds, but was not present in my samples. Estimates of fish population parameters are inherently influenced by sampling techniques, plant densities and patchy fish distribution (Killgore et al. 1989). However,

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30 small block nets (0.01 ha) used in this study allowed more treatments and repetition than other methods and allowed for adequate collection of smaller fish sizes (Bettoli and Maceina 1996; Haller et al. 1980; Shireman et al.1981; Timmons et al. 1979) but not fish over about 180 mm TL. Hence, fish collected in this study were small. The small juvenile cichlids probably chose habitats with vegetation to avoid predation or increase food availability, similar to juvenile bluegill in temperate zones (Gotceitas and Colgan 1987; Savino and Stein 1982; Mittelbach 1981; Valley and Bremingan 2002). Results of this study should be considered representative of the juvenile littoral fish assemblage at Lake Izabal but do not reflect densities of larger fish or open-water species. Seasonal distribution of fish could be a factor in my study. Data for this study were collected in June-July of 2004. The family Poecilidae was represented by two species (one species by one individual) in the lake at the sampling time when the water levels were the highest in the year (Appendix B). Six species of Poecilidae were collected throughout the year at the lake using dip nets (personal communication Maritza Aguirre), but I collected only two. Because my samples were concentrated in June/July, temporal variation in fish community structure and richness was not evaluated in this study. Similarly Chick and McIvor (1994) found differences in Poecilidae densities among months and depths in hydrilla. Another example of seasonality would be silversides, which account for 60% of all fish collected in this study. Gelwick and Matthews (1990) found differences in silverside Menidia beryllina abundance over seasons which peaked in April-June. Silverside Menidia beryllina was also present in high density in open waters, similar to this study. Thus, silverside density, like other species, may vary with season.

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31 Responses from fishes to aquatic plants in Lake Izabal were predictable with other plant-fish interaction literature (Dibble et al 1996), but my data suggest that plants, and moreover hydrilla is highly utilized by small (< 13 cm) cichlids in Lake Izabal regardless the level of PAC. The only description of aquatic macrophytes before hydrilla infestation mentions the lack of vegetation in Lake Izabal (Dickinson III 1974). Hayes et al. (1996) showed that if a habitat is limited and if the habitat increases from the baseline occur, an increase in fish population parameters will be expected. However, detecting fish responses to habitat changes remains problematic for fisheries managers due to variability in fish responses (e.g., Minns et al. 1996; Allen et al. 2003). Finally, fisheries at Lake Izabal included marine origin species and adult cichlids caught from areas outside the littoral zones (more than 90 % of Lake Izabal area), and I did not evaluated fish response to hydrilla outsides littoral areas. Nonetheless, littoral areas in the lake are directly important for at least eight species present in the fisheries (Table 8), and indirectly for others important predators. Thus, the littoral fish assemblage I sampled included juveniles of species that were important in the fisheries

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MANAGEMENT IMPLICATIONS Hydrilla has been a nuisance plant around the world. Many negative effects have been documented including the occupation of the entire water column of shallow lakes by hydrilla. However, Lake Izabals mean depth prevents hydrilla from occupying the entire water body, as Arrivillaga (2003) estimated that only about 9% of the lake could be colonized by hydrilla. Furthermore Hoyer and Canfield (1996) state the need of aquatic macrophytes in large lakes as refuge for juvenile fish of certain types, like bass. Hydrilla was not detrimental for fish in this study and generally had higher fish biomass and similar richness to areas with no plants and those with other native plants. Hydrilla creates littoral fish habitat at Lake Izabal. Juvenile cichlid biomass was higher in hydrilla than in other plant types, and cichlids support important fisheries at the lake. Further field work should target adult fish and assess the entire cichlid population and not just juveniles or forage fish. Other long-term fish population responses to hydrilla are unknown for Lake Izabal. Parameters such as fish growth, recruitment or mortality rates should be assessed in the future. Results of this study suggest that hydrilla is not detrimental to the littoral fish community at Lake Izabal, and because the plant is unlikely to occupy over 10% of the lake surface area, it could actually benefit the lakes fisheries. The anglers in El Estor community at Lake Izabal recognize hydrilla as a high abundance fish area, and they commonly seek hydrilla edges to set gillnets for adult fish. However, Lake Izabal is not used only for fishing, but also access, transportation, tourist 32

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33 and aesthetic values should be addressed in a holistic approach before making decisions about hydrilla management at the lake.

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APPENDIX A BLOCK NETS SUMMARY AT LAKE IZABAL Table 9. Block nets summary at Lake Izabal (June-July/2004). Treatments were plant species with different levels based on the percentage area coverage (PAC) and no plants. Total plant biomass is mean kilogram per square meter (N = 3). Total fish biomass in kilograms. Total fish density present and richness present in each net. Net Plant species level PAC Total plant biomass kg/m 2 Total fish biomass kg/ha Total fish density number/ha Fish richness 1 bulrush low 25 3.6 286.6 84 7 2 bulrush low 20 2.1 769.6 302 9 3 bulrush low 20 2.2 1180.2 414 7 4 bulrush low 15 0.1 1452.1 180 9 5 bulrush low 15 1.3 511.2 295 6 6 bulrush high 75 5.4 241.91 117 4 7 bulrush high 80 3.7 1039.2 169 8 8 bulrush high 80 4.2 3578.2 259 11 9 bulrush high 80 1.5 1221.6 218 8 10 bulrush high 85 8.2 1517.5 307 7 11 chara low 25 0.9 258.4 433 2 12 chara low 20 0.3 162.3 93 7 13 chara low 10 0.2 148.8 135 5 14 chara low 15 0.2 45.1 65 5 15 chara low 15 0.2 228.9 105 7 16 eelgrass low 25 1.8 259.4 114 7 17 eelgrass low 25 1.6 103.5 102 5 18 eelgrass low 25 1.9 465.6 52 6 19 eelgrass low 20 0.3 491.2 304 6 20 eelgrass low 20 1.5 1866.1 775 9 21 eelgrass high 75 2.9 716.9 363 10 22 eelgrass high 75 1.8 491.7 338 7 23 eelgrass high 80 2.8 352.7 223 7 24 eelgrass high 80 2.9 142.2 31 8 25 eelgrass high 75 7.0 910.2 761 7 26 hydrilla low 25 5.2 1499.8 203 10 27 hydrilla low 25 6.0 2284.8 259 9 28 hydrilla low 25 0 2038.5 187 8 29 hydrilla low 25 4.4 1409.3 815 10 30 hydrilla low 25 4.0 520.0 1092 8 31 hydrilla medium 50 6.7 601.4 887 12 34

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35 Net Plant species level PAC Total plant biomass kg/m2 Total fish biomass kg/ha Total fish density number/ha Fish richness 32 hydrilla medium50 13.0 2082.0 296 8 33 hydrilla medium50 5.6 941.5 298 10 34 hydrilla medium60 8.0 860.0 169 8 35 hydrilla medium60 13.9 1116.9 138 7 36 hydrilla high 85 33.4 365.0 121 10 37 hydrilla high 90 15.2 1466.7 201 8 38 hydrilla high 75 6.9 580.3 239 8 39 hydrilla high 80 28.7 1603.5 354 10 40 hydrilla high 85 28.4 5287.4 284 10 41 Illinois pondweed low 25 7.4 673.4 110 8 42 Illinois pondweed low 20 2.0 65.7 48 5 43 Illinois pondweed low 25 1.7 832.9 193 10 44 Illinois pondweed low 25 2.5 747.0 432 11 45 Illinois pondweed low 25 8.5 539.4 215 9 46 no-plants none 0 0 322.6 348 5 47 no-plants none 0 0 515.6 587 4 48 no-plants none 0 0 23.1 43 3 49 no-plants none 0 0 111.4 97 5 50 no-plants none 0 0 1130.6 949 7

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APPENDIX B WATER LEVELS AT LAKE IZABAL 00.20.40.60.811.21.41.61.8Oct-03Nov-03Dic-03Ene-04Feb-04Mar-04Abr-04May-04Jun-04Jul-04MesMetros Figure 4. Water levels at Lake Izabal 2003-2004. From: AMASURLI Boletn Hidrolgico 2005. Metros = meters and Mes = month. 36

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LIST OF REFERENCES Allen, M. S. and K. I Tugend. 2002. Effects of large-scale habitat enhancement project in habitat quality for age-0 largemouth bass at Lake Kissimmee, Florida. Pages 265-276 in D.P. Phillip and M.S. Ridgway, editors. Black Bass: ecology, conservation and management. American Fisheries Society, Symposium 31, Bethesda, Maryland. Allen, M. S., K. I. Tugend, and Mann M. J. 2003. Largemouth bass abundance and anglers catch rates following a habitat enhancement project at Lake Kissimmee, Florida. North American Journal of Fisheries Management 23: 845-855. Arrivillaga, A. 2002. Evaluacin de la presencia de Hydrilla verticillata en la region de Rio Dulce y Lago de Izabal: Diagnstico general e identificacin de medidas de control. Oficina tcnica de biodiversidad. Consejo Nacional de Areas Protegidas (CONAP). Fondo Nacional para la Conservacin de la Naturaleza (FONACON). Arrivillaga, A. 2003. Estudio de Impacto Ambiental para la Aplicacin de Medidas de Control y Mitigacin de la Especie Invasora Hydrilla Verticillata en Izabal. Ministerio de Ambiente y Recursos Naturales, Guatemala. 120pp Barnett, B. S. and R.W. Schneider. 1974. Fish populations in dense submersed plant communities. Hyacinth Control Journal 12:12-14. Bettoli, P. W. and Morris J. E. 1991. Changes in the abundance of two atherinid species after aquatic vegetation removal. Transactions of the American Fisheries Society 120: 90-97. Bettoli, P. W., M. J. Maceina, R. L. Noble, and R. K. Betsill. 1992. Piscivory in largemouth bass as a function of aquatic vegetation abundance. North American Journal of Fisheries Management 12: 509-516 Bettoli, P. W., M. J. Maceina, R. L. Noble, and R. K. Betsill. 1993. Response of a reservoir fish community to aquatic vegetation removal. North American Journal of Fisheries Management 13: 110-124. Bettoli, P. W. and M. J. Maceina. 1996. Sampling with toxicants. Pages 303-333 in B.R. Murphy and D.W. Willis, editors. Fisheries techniques, 2 nd edition. American Fisheries Society, Bethesda, Maryland. Brinson, M. M. and F. G. Nordlie. 1975. Lake Izabal, Guatemala. Verh. Internat. Verein. Limnol. 19: 1468-1479. 37

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38 Canfield, D. E., Jr., K. A. Langeland, M. J. Maceina, W. T. Haller, J. V. Shireman, and J. R. Jones. 1983. Trophic state classification of lakes with aquatic macrophytes. Canadian Journal of Fisheries and Aquatic Sciences 40: 1713-1718 Canfield, D. E., Jr., K. A. Langeland, S. B. Linda, and W. T. Haller. 1985. Relations between water transparency and maximum depth of macrophyte colonization in Lakes. Journal of Aquatic Plant Management. 23: 25-28. Canfield, D. E., M. V. Hoyer, and C. M. Duarte. 1990. An empirical method for characterizing standing crops of aquatic vegetation. Journal of Aquatic Plant Management 28: 64-69. Carr III, A. F. 1971. The commercial snook (Centropomus undecimalis) fishery of Lake Izabal, Guatemala. Master of Sciences thesis. University of Florida. Chick, J. H. and C.C. Mclvor. 1994. Patterns in the abundance and composition of fishes among beds of different macrophytes: viewing a littoral zone as a landscape. Canadian Journal of Fisheries and Aquatic Sciences 51:2873-2883. Chick, J. H. and C.C. Mclvor. 1997. Habitat selection by three littoral zone fishes: effects of predation pressure, plant density and macrophyte type. Ecology of Freshwater Fish 6:27-35. Dibble, E. D., K. J. Killgore, and S. L. Harrel. 1996. Assessment of Fish-Plant interaction. American Fisheries Society Symposium 16: 357-372. Dickinson III, J. C. 1974. Fisheries of Lake Izabal, Guatemala. Geographical review. Vol 64, No.3 385-409. Dionne, M. and C. L. Folt. 1991. An experimental analysis of macrophyte growth forms as fish foraging habitat. Canadian Journal of Fisheries and Aquatic Sciences 48: 123-131. Durant, D. F. 1980. Fish Distribution among habitats in hydrilla infested Orange Lake, Florida. Master of Science thesis. University of Florida. Gelwick, F. P. and W. J. Matthews. 1990. Temporal and spatial patterns in littoral-zone fish assemblages of a reservoir (Lake Texoma, Oklahoma-Texas, U.S.A.). Environmental Biology of Fishes 27:107-120. Gotceitas, V. and P.Colgan. 1987. Selection between densities of artificial vegetation by young bluegills avoiding predation. Transactions of the American Fisheries Society 116: 40-49. Greenfield, D. W. and J. E. Thomerson. 1997. Fishes of the continental waters of Belize. University Press of Florida, Gainesville, Florida

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39 Haller, W .T. 2002. Hydrilla in Lake Izabal, Guatemala current status and future prospects. Final Report to USAID. University of Florida. Haller, W. T., J. V. Shireman, and D. F. Durant. 1980. Fish harvest resulting from mechanical control of hydrilla. Transactions of the American Fisheries Society 109:517-520. Hayes, D. B., C. P. Ferreri, and W. W. Taylor. 1996. Linking fish habitat to their population dynamics. Canadian Journal of Fisheries and Aquatic Sciences 53: 383-390. Hoyer, M. V. and D.E. Canfield. 1996. Largemouth Bass Abundance and Aquatic Vegetation in Florida Lakes: An Empirical Analysis. Journal of Aquatic Plant Management. 34: 23-32. Killgore, K. J., R. P. Morgan II, and N. B. Rybicki. 1989. Distribution and abundance of fishes associated with submersed aquatic plants in the Potomac River. North American Journal of Fisheries Management 9:101-111. Krebs, C. J. 1999. Ecological methodology. 2 nd edition. University of British Columbia. Addison-Wesley Educational Publishers, Inc, Menlo Park, California Langeland, K. A. 1996. Hydrilla verticillata (L.F.) Royle (Hydrocharitaceae), The perfect weed. Castanea 61: 293-304. Maceina, M. J., P. W. Bettoli, W. G. Klussmann, R. K. Betsill, and R. L. Noble. 1991. Effect of aquatic macrophyte removal on recruitment and growth of black crappies and white crappies in Lake Conroe, Texas. North American Journal of Fisheries Management 11:556-563. Michot, T. C., R. G. Bounstany, A. Arrivillaga, and B. Perez. 2002. Impacts of Hurracane Mitch on water quality and sediments of Lake Izabal, Guatemala. USGS Open File Report 03-180, 20p. Miller, R. R. 1966. Geographical distribution of Central American fishes. Copeia 773-802. Miller, R. R. 1976. Geographical distribution of Central American fishes, with addendum. pp. 125-156 in Investigations of the Ichthyofauna of Nicaraguan Lakes (T. B. Thorson, ed.). University of Nebraska Press, Lincoln, NE. Minns, C. K., J. R. M. Kelso, and R. G. Randall. 1996. Detecting the response of fish to habitat alterations in freshwater ecosystems. Canadian Journal of Fisheries and Aquatic Sciences 53: 403-414. Mittelbach, G. G. 1981. Foraging efficiency and body size: a study of optimal diet and habitat use by bluegills. Ecology 62(5): 1370-1386

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40 Prez, L. 2004. La ictiofauna del lago de Izabal y sus afluentes: composicin, distribucin y ecologa. Te sis para optar a grado de Licenciatura en Biologa. Universidad del Valle de Guatemala. 259 pp Savino, J. F. and Stein, R. A. 1982. Pred ator-prey interaction between largemouth bass and bluegills as influenced by simulated submersed vegetation. Transactions of the American Fisheries Society 111: 255-266. Shireman, J. V., D. E. Colle, and D. F. DuRa nt. 1981. Efficiency of rotenone with large and small blocknets in vegetated and open water habitats. Transactions of the American Fisherie s Society 110:77-80. Tate, W. B., and M. S. Allen. 2003a. Relation of age-0 largemouth bass abundance to Hydrilla coverage and water level at Lochl oosa and Orange lakes, Florida. North American Journal of Fish eries Management 23:251-257 Tate, W. B., and M. S. Allen. 2003b. Co mparison of electrofishing and rotenone for sampling largemouth bass in vegetated areas of two Florida lakes. North American Journal of Fisheries Management 23: 181-188. Timmons, T. J., W. L. Shelton, and W. D. Davies. 1979. Sampling of reservoir fish populations with rotenone in littoral areas Proceedings of the Annual Conference Southeastern Association of Fish and Wildlife Agencies 32 (1978): 474-484 Valley, R. D., and M. T. Bremigan. 2002. Effects of macrophyte bed architecture on largemouth bass foraging: implications of non-native macrophyte invasions. Transactions of the American Fisheries Society 131:234-244. Willink, P., C. Barrientos, H.King, and B.Cher nnoff. 2000. An Ichthyological survey of the Laguna del Tigre National Park, Pe tn, Guatemala. Chapter 4. In: Bestelmeyer, B.T. [Ed.], Alonso, L.E. [Ed.]. A Biological Assessment of Laguna del Tigre National Park, Petn, Guatemala. RAP Bulletin of Biological Assessment 16. Zaret T. M., and Paine R. T. 1973. Species introduction in a tropi cal lake. Science, New Series. Volume 182 No. 4111 449-455

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BIOGRAPHICAL SKETCH Christian Barrientos is originally from Guatemala, Central America. He grew up in Guatemala City, where he obtained a biologist degree from University of San Carlos in Guatemala, married another biologist and moved to Peten, home of the Mayan Biosphere Reserve. He worked in the Scarlet Macaw Biological Station located at Laguna del Tigre National Park, spending most of his time in the beautiful San Pedro River. During this time he worked in conservation of terrestrial systems. But his heart was always loyal to the aquatic systems and his research started to move to the water. During that time he always enjoyed fishing trips on the San Pedro River. During the time as a coordinator of a Global Environment Facility/World Bank Project for Laguna Del Tigre, he improved his writing and communication abilities in English. Finally, and following advice from several friends and parents he decided to pursue a new level of education at the University of Florida. During this new student phase, and now as a father he realized how valuable the aquatic systems are for this and the next generation, and he will continue to work on the conservation and management of the aquatic systems. 41


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Permanent Link: http://ufdc.ufl.edu/UFE0012100/00001

Material Information

Title: Fish Abundance and Community Composition in Native and Non-Native Littoral Aquatic Plants at Lake Izabal, Guatemala
Physical Description: Mixed Material
Copyright Date: 2008

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Source Institution: University of Florida
Holding Location: University of Florida
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System ID: UFE0012100:00001

Permanent Link: http://ufdc.ufl.edu/UFE0012100/00001

Material Information

Title: Fish Abundance and Community Composition in Native and Non-Native Littoral Aquatic Plants at Lake Izabal, Guatemala
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0012100:00001


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FISH ABUNDANCE AND COMMUNITY COMPOSITION IN NATIVE AND
NON-NATIVE LITTORAL AQUATIC PLANTS AT LAKE IZABAL, GUATEMALA















By

CHRISTIAN ALBERTO BARRIENTOS


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2005

































Copyright 2005

by

Christian Alberto Barrientos

































To God, who provides all wisdom and judgment.















ACKNOWLEDGMENTS

I first thank my advisor, Dr. Mike Allen, for his guidance and willingness to do

research in Guatemala. I also thank my committee members, Dr. William Haller and Dr.

Daniel Canfield.

I really appreciate Patrick Cooney and Maritza Aguirre's help and support on this

project, in the field and by email. Special thanks go to Dr. Mark Brenner who, on one

of his research trips to Peten, encouraged me to pursue a higher education level..

Thanks also go to my parents, Jorge and Alicia Barrientos, who provided logistical

and moral support. Most importantly, my parents were always there for me.

Partial funding for this research was provided by the Wildlife Conservation

Society, Mesoamerican Program, especially thanks to Archie Carr III. Funding was also

provided by the Fulbright Scholarship and the University of Florida.

Last, but not least, I thank my wife Corina and my two daughters Ana Isabel and

Ana Cris, who came with me from Guatemala to Gainesville, Florida.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ......... ................................................................................... iv

L IST O F T A B L E S ................................................................... .............. vi

LIST OF FIGURES ......... ....... .................... ............ .... ........... vii

A B S T R A C T ......... .................................. ................................................... v iii

INTRODUCTION .............. ................................... ..............

M E T H O D S .......................................................................... . 4

S tu d y S ite .................................................................................. 4
H habitat Characterization ............................................................6
F ish Sam pling ................................................................... 6
A n aly se s ............................................... 7

R E S U L T S ................................................................................12

H habitat C characterization .................................................................... .. ..................12
Fish Population Com prisons .......................................................... ............... 12
L ow PA C A analysis ................................................................ ........................ 13
Low & High PAC Analysis .................. .................. ................... .. ................ 14
H y drilla A n aly sis .............................................................................................. 15

D IS C U S S IO N ............................................................................................................... 2 5

M ANAGEM ENT IM PLICATION S ............................................. .......................... 32

APPENDIX

A BLOCK NETS SUMMARY AT LAKE IZABAL ........................................... 34

B WATER LEVELS AT LAKE IZABAL.................................. ....................... 36

LIST OF REFEREN CE S ............................................. ........................ ............... 37

B IO G R A PH IC A L SK E T C H ...................................................................... ..................41



v
















LIST OF TABLES


Table pge

1 Block net distribution among treatments ...................................... ............... 10

2 Physicochemical variables measured at Lake Izabal ............................................17

3 Taxonomic family and scientific name offish collected in block nets in Lake
Izabal. Mean fish biomass across all treatments.. ....................................................19

4 Taxonomic family and scientific name of fish collected in block nets at Lake
Izabal. Plant species and percent area covered (PAC) are located at the top of
each co lu m n .................................................... ................ 2 0

5 Fish metrics for treatments with low percent area coverage (PAC) including the
no plants treatm ent. .......................... .. ......................... .............. ..... .......... 2 1

6 Fish metrics for six treatments included in the high versus low PAC comparison..22

7 Hydrilla data set were treatments are different percent area coverage (PAC). ......23

8 Taxonomic family and scientific name offish observed in the local market at El
Estor, Izabal, Guatem ala. ............................................... .............................. 24

9 Block nets summary at Lake Izabal (June-July/2004). ......................................34
















LIST OF FIGURES

Figure page

1 Location of Lake Izabal in Guatemala. .................................... .... ........... 11

2 The relation between plant biomass and percentage area coverage......................16

3 Fish total length (TL) distribution across all treatments. .......................................18

4 W after levels at Lake Izabal 2003-2004 ........................................ ............... 36















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

FISH ABUNDANCE AND COMMUNITY COMPOSITION IN NATIVE AND
NON-NATIVE LITTORAL AQUATIC PLANTS AT LAKE IZABAL, GUATEMALA

By

Christian Alberto Barrientos


December 2005

Chair: Micheal S. Allen
Major Department: Fisheries and Aquatic Sciences

I evaluated effects of the coverage of five aquatic plant species on fish community

structure and abundance in Lake Izabal, the largest lake in Guatemala, which was

recently invaded by a non-native aquatic plant Hydrilla verticillata. The objectives were

to assess how hydrilla influenced littoral fish community composition and abundance.

Fish were sampled from monotypic beds of five plant species including bulrush Scirpus

carrizo, chara Charaphoetida, eelgrass Vallisneria americana, Illinois pondweed

Potamogeton illinoensis, hydrilla Hydrilla verticillata, and areas with no plants. Bulrush,

hydrilla and eelgrass were found in different densities and were sampled in low (<25%)

and high (>75%) percent area coverage (PAC). Fish were sampled using block nets

(0.01-ha) treated with rotenone in each plant type and PAC (N = 5 block nets per

treatment) during June and July of 2004. Hydrilla had higher aquatic plant biomass

relative to native species of the same PAC. Fishes from the family Cichlidae (7 species)









had the highest species richness and highest biomass across all habitats sampled. Total

fish species richness was 24, which ranged from 12-15 species and was similar across

plant types in the littoral zone. Fish density did not differ significantly among plant

types, but fish biomass was significantly different (P=0.01) among plant types with the

hydrilla containing the highest fish biomass (186 kg/ha). Fish biomass was higher

(P=0.02) in high PAC levels of hydrilla and bulrush than in eelgrass at either low or high

PAC. Biomass of mojarra Cichlasoma maculicauda, which support the most important

subsistence fisheries in the lake, was significantly different (P=0.01) among treatments

and had the greatest biomass in hydrilla. Results of this study show that hydrilla provides

suitable habitat for fishes in the littoral areas of Lake Izabal. Areas with hydrilla

contained significantly greater biomass of fish compared to native plants or no-plants

areas. Although hydrilla may cause problems for lake access and other uses, it is not

likely detrimental to Lake Izabal fish community composition or total fish biomass, at

levels of growth and coverage found in 2004.















INTRODUCTION

Excessive growth of aquatic macrophytes has caused serious problems in many

lakes throughout the world (Canfield et al. 1985). Lake Izabal, Guatemala, has recently

been colonized by Hydrilla verticillata, a non-native submersed plant. The hydrilla in

Lake Izabal was first reported by anglers in 2000, and perhaps entered the system with

hurricane Mitch in 1998. The plant is currently too widespread to be eradicated from the

lake (Haller 2002).

Hydrilla can quickly colonize and dominate systems where it is introduced because

it grows rapidly, tolerates low light conditions, and inhabits water depths exceeding four

meters (Langeland 1996). Light is often the most limiting factor prohibiting submersed

plant growth (Canfield et al. 1985). Arrivillaga (2002) noted that 3 % of the Lake Izabal

was covered by hydrilla in 2002, but the plant could potentially cover 10-15% in the dry

season when light penetrates into deeper waters, due to increased water clarity. Allen and

Tugend (2002) found that areas with dense macrophytes coverage (100 %) had lower fish

richness, with centrarchids absent in these areas. Additionally, elevated dead plant

material and high plant biomass caused access, fishing and even sampling problems

(Allen and Tugend 2002; Allen et al. 2003).

Because most attention has focused on the negative aspects of hydrilla, the

potential benefits of hydrilla on fish populations have been largely neglected (Tate and

Allen 2003a). Like all aquatic plants, hydrilla provides refuge from predators and habitat

for fishes that consume macroinvertebrates associated with the plant and associated









periphyton. In laboratory studies, hydrilla forms a complex architecture, providing

refuge from predators for prey fish and large surface area for macroinvertebrates (Chick

and Mclvor 1997; Valley and Bremingan 2002).

Aquatic plants influence both fish distribution and abundance by creating

structurally complex habitats. Fish are generally not distributed evenly throughout a lake,

but are often concentrated within the vegetated littoral zone (Chick and Mclvor 1994).

Fish abundance can be substantially higher in areas with aquatic plants than in areas

without plants (Barnett and Schneider 1974; Bettoli et al. 1992; Chick and Mclvor 1997;

Durant 1980; Killgore et al. 1989). However, foraging success of predators declines as

plant density increases (Mittelbach 1981; Savino and Stein 1982).

Aquatic macrophytes also influence fish community structure. There is a negative

relationship between abundance of submersed plants and planktonic algal biomass

(Canfield et al. 1983), and thus, occurrence of aquatic plants favors insectivorous rather

than planktivorous fishes. Bettoli et al. (1993) found that of seventeen fish species

commonly collected in vegetation, most of them in hydrilla, the biomass of eight species

declined after vegetation removal in a Texas reservoir. Density of planktonic fish

increased nearly five fold, coincident with a decline in mean size of fish, after hydrilla

removal. Maceina et al. (1991) found that dense macrophyte coverage (29-44%)

dominated by hydrilla suppressed planktivorous threadfin shad Dorosomapetenense

abundance, which seemed to restrict growth of crappie Pomoxis spp., a predator on

threadfin shad.

In Lake Izabal, fish assemblages found in native vegetation and hydrilla may be

different, due to differences in plant architecture complexity. The physical plant structure









influences fish inhabiting the littoral zone (Dionne and Folt 1991). Littoral zones often

contain several macrophyte species, occurring either separately or as part of a mosaic,

which function as different habitats for fishes (Chick and Mclvor 1994).

Management of aquatic vegetation communities usually affects predator-prey

interactions, fish community structure, and the quality of component fisheries (Bettoli et

al. 1992). Hydrilla control options at Lake Izabal have been assessed (Arrivillaga 2003;

Haller 2002), but no studies have evaluated fish communities in hydrilla for lakes in

Central America. The relationships between plants, fish abundance, community

composition, and fisheries are important management considerations when plant control

strategies are developed (Durant 1980; Haller 2002; Killgore et al. 1989; Tate and Allen

2003b).

I evaluated the fish communities in various aquatic plant types of littoral areas at

Lake Izabal, Guatemala. My objectives were 1) to contribute to the knowledge of the fish

community at Lake Izabal; and 2) to determine if composition of fish assemblages and

fish abundance differed among types of aquatic habitats and different densities of

Hydrilla verticillata and native plants.















METHODS

Study Site

Lake Izabal is the largest lake in Guatemala with a length of 43 km, a width of 19

km and an area of 717 km2 (Brinson and Nordlie 1975). It is an inland lake located in the

eastern part of the country, not far (40 km) from the Caribbean coast (Figure 1). The lake

surface lies 10 meters above sea level. The mean depth is 12 meters and the maximum

depth is 17 meters (Brinson and Nordlie 1975). The lake does not typically stratify and

part of the lake volume is below sea level (Brinson and Nordlie 1975).

The lake is surrounded by mountains (Sierra de las Minas to the south, Sierra de

Santa Cruz to the north) and fed primarily by the Rio Polochic River, one of the largest

rivers in Guatemala. The Polochic River discharges 70% of the total freshwater input to

Lake Izabal, with smaller streams around the lake contributing the remainder (Brinson

and Nordlie 1975). The Dulce River is the only river that flows out of the lake, flowing

from the east end of the lake through a smaller lake, El Golfete, and into the Bahia de

Amatique and the Gulf of Honduras near the town of Livingston (Carr III 1971; Michot

et al. 2002; Figure 1). The wetlands of the Polochic River are located in the western part

of the lake and are very popular to local subsistence fishers, although large-scale

recreational fisheries currently do not occur at the lake.

The Izabal system, which is in the Polochic-Izabal division, is one of the four fish

subdivisions of the Usumacinta Province (Miller 1966). Most components of the

ichthyofauna found at the lake belong to a broadly distributed group of fishes found









between the Rio Coatzacoalcos in the north and the Rio Polochic/Rio Sarstun in the south

(Miller 1966). This general distribution has been referred to as the Rio Usumacinta

Province by Miller (1966; 1976). On a recent revision of fish distribution from the entire

Lake Izabal basin, Perez (2004) reported the presence of 81 species and 24 fish families

present. Cichlids and livebearers are the most common families with 17 and 13 species,

respectively.

Lake Izabal also supports important commercial fisheries. However the fisheries

are mainly subsistence and help support the local communities around the lake. The

main fishing gears are gill nets, but cast nets and hook and line are also used (Carr III

1971; Dickinson III 1974; Arrivillaga 2002; Michot et al. 2002). Prior to the 1960s, the

gill nets targeted marine species like snooks (Centropomidae), jacks (Carangidae), and

catfishes (Ariidae) in open waters. However, in 1963 anglers noticed a decline in species

of marine origin and switched to harvesting freshwater species like mojarra Cichlasoma

maculicauda (Dickinson III 1974).

Abundance of aquatic macrophytes in Lake Izabal before hydrilla entered the

system was described as: "Izabal lacks the luxuriant aquatic vegetation that might be

expected in a lowland tropical lake. Bottom configuration and wave action provides a

possible explanation" (Dickinson III 1974). However this description had been modified

due to colonization of Lake Izabal by exotic hydrilla. Hydrilla is now part of a plant

community that includes many native species. Arrivillaga (2003) reported the presence

of native plants including Charaphoetida, Cabomba caroliniana, Eichhornia crassipes,

Najas guadalupensis, Pistia stratiotes, Potamogeton illinoensis, P. pussilus, Salvinia

molesta, Scirpus carrizo, Typha dominguensis, and Vallisneria americana in the lake.









Arrivillaga (2002) found that about 3% of the lake area contained hydrilla, and the plant

was interspersed with native plants around the lake.

Habitat Characterization

In the littoral zone of Lake Izabal at depth less than 1.5 meters, I set 50 block nets

where I sampled vegetation coverage and biomass, water characteristics in each block net

(Figure 1; Appendix A). My experimental design included setting block nets in each

plant species at low (<25%), medium (26% 74%) and high (> 75%) percent of area

covered (PAC), but some plants did not occur at the lake in high coverages. I located

areas with the aquatic plant and by three different persons visually estimated the PAC in

each net in the field, and then averaged, to assign total PAC to each net. Vegetation

biomass was estimated using 3 random quadrats (0.25 m2) inside each block net. In each

quadrat, above-ground plant material was removed. Sampled plants were placed in a

nylon mesh bag and spun to remove excess water. Samples were weighed to the nearest

0.1 kilogram wet weight (Canfield et al. 1990). Temperature (oC) and pH were measured

in the field using a Merck Multiline P4, after the block net was set and within one meter

from the perimeter of each net. Each net was set so that the habitat inside the net was

characteristic of the surrounding habitat. Each plant species with different levels of PAC

will be referred to as treatments, including no plants, in this document (Table 1).

Fish Sampling

Fish were collected using block nets (0.01 ha, 10 x 10 m, 6 mm bar mesh) treated

with rotenone. The four covers and sides of the nets were anchored with concrete

weights to insure contact with the bottom and to maintain a square configuration. After

net placement, powder rotenone was mixed with water. The amount of rotenone used was

determined by the mean depth inside the net to obtain a desired concentration of 3 mg/L.









The rotenone slurry was sprayed onto the water surface within each net using a small

pump. Fish were collected as they surfaced using dip nets for one hour after the

application, and usually three persons were inside the net retrieving fish (Bettoli and

Maceina 1996; Timmons et al. 1978).

Sampling began June 25 and continued through July 18 2005. Sampling each day

started at 07:00 and ended at 15:00 (6 nets/day). Fish collected from each net were

placed on ice and moved to the field laboratory at the end of the day. At the laboratory,

fish were sorted by species, counted and weighed. When high numbers of individuals

were collected, a random sub-sample of 20 individuals by species were measured to the

nearest millimeter total length (TL) and individually weighed (0.1 g), and then all the rest

were weighed as a batch. Samples of all species were fixed in 10% formalin to assure

field identification. Samples of all species were returned to the Florida Natural History

Museum in Gainesville and identified to the species level.

Analyses

Fish metrics including total fish biomass (kg/ha), total density (fish/ha), fish species

richness, and fish diversity were estimated for each block net. Diversity was calculated

using the Shannon-Wiener index (H') for numbers of individual fish of each species

collected in each block net (Krebs 1999). Diversity attempts to account for evenness and

richness by looking at both the number of species and how evenly distributed the number

of individuals is among the species in each habitat. The Shannon-Weiner index of

species diversity was estimated as:

2
H'- (pi/p) (log2 pi/p) (eq. 1)
i=l









where p, is the number of individuals or total weight of the ith species, P is the total

number of individuals or total weight of all species, and s is the total number of species in

each habitat. For biological communities, H' ranges from zero to five (Krebs 1999), and

is expressed in bits per individual (bits/individual).

The mojarraa" is the local name for several Cichlids in Guatemala, however in

Lake Izabal the name is mainly used for Cichlasoma maculicauda. Biomass of mojarra

was also included as a dependent variable and tested independently because it is the most

common species captured in the fishery at the lake. Mojarra harvest is estimated in tons

on a weekly basis (personal comment Maritza Aguirre 2004). The Cichlids family was

the most abundant with seven species; hence cichlids biomass was tested independently

in all analysis. Silversides Atherinella spp accounted over half of fish density across all

habitats; hence silversides biomass was also tested independently and total fish density

where tested without silversides.

The data were analyzed in three different tests. First, I tested whether total fish

density (fish/ha), total fish biomass (kg/ha) and fish diversity (H') differed among five

plant type treatments at low PAC (<25%) using a one-way ANOVA. The plant treatments

were bulrush Scirpus carrizo, chara Charaphoetida, eelgrass Vallisneria americana,

illinois pondweed Potamogeton illinoensis and hydrilla Hydrilla verticillata, and no

plants. This first test is referred to as the low PAC assessment in this document.

(Procedure GLM SAS 1996). Prior to the ANOVA, a Shapiro-Wilks test was used to test

for normality for all fish variables except diversity, which is already on a log scale

(Procedure UNIVARIATE NORMAL SAS 1996). Density and biomass were logio(x+l)









transformed to improve normality. When the one-way ANOVA was significant, Tukey's

test was performed to separate the means.

The second analysis included only three of the plant species (i.e., bulrush, eelgrass,

and hydrilla) and two coverage levels (low vs. high PAC) using a two-way ANOVA

(Procedure GLM SAS 1996). The two-way ANOVA was used to assess if fish metrics

differed among plant species, PAC levels, and the interaction of the two fixed effects.

Dependent variables were total fish density, biomass and diversity (H'). The density and

biomass were transformed as described above. When the two-way ANOVA was

significant, Least Squared Means test was performed at fixed levels of the interest

variable.

The third analysis tested whether the fish metrics differed among three levels of

PAC (low, medium, high) for hydrilla alone. Hydrilla was evaluated alone because it is

exotic at the lake and because it was the only aquatic plant found at all levels of PAC.

When the one-way ANOVA was significant, Tukey's test was performed to separate the

means. Differences were declared significant at P < 0.10 for all analyses.

Physicochemical variables were tested in each of the analysis






10


Table 1. Block net distribution among treatments sorted by plants species and PAC at
Lake Izabal 2004. Notice that hydrilla was the only plant species with all
levels of PAC present.

PAC Bulrush Chara Eelgrass Hydrilla Illinois No plants
pondweed
Low (<25%) 5 5 5 5 5

Med (26-74%) 5

High (>75%) 5 5 5

None 5




































Figure 1. Location of Lake Izabal in Guatemala (inset right bottom corner). The littoral zones sampled are show in red. From:
Arrivillaga, A. (2003).














RESULTS

Habitat Characterization

Plants species and percentage area coverage (PAC) were the factors that

experimental design included (Table 1). Plant biomass was positively related to PAC,

but correlation was weak (r2 = 0.47), which was likely due to differences in biomass-

PAC relationships among species, and the relatively low number of quadrats (N=3) used

to sample plant biomass (Figure 2). For example, bulrush was the only emergent plant

with cylindrical stems in this study. Eelgrass was short (< 30 cm) present in the bottom

and Illinois pondweed with medium sized lance shaped leaves close to the surface. Chara

had little filaments always close to the bottom not taller than 10 cm. Hydrilla formed

extremely dense canopy (high plant biomass) close to the surface.

Physicochemical variables were measured along with plant species and PAC.

Water temperature, depth sampled, and pH were similar across all nets. Mean depth was

1.2 meters across all treatments (Table 2). Temperature averaged 30 oC and was nearly

constant across treatments and days (Table 2) and pH averaged 7.9 across all treatments.

No significant differences were found in temperature (P = 0.72), depth inside the nets (P

= 0.92) or pH (P = 0.12) using analysis of variance (ANOVA) with plant type as the fixed

effect.

Fish Population Comparisons

Twenty four fish species were collected from 50 block nets placed in the littoral

areas at Lake Izabal. A total of 16,610 fish were collected, and total length (TL) was









measured from 4,565 fish. Size (TL) across all species and treatments ranged from 1 cm

to 24 cm and averaged 5.8 cm; 95% of the fish were under 13 cm (Figure 3). Eight of

twenty four species were collected in all treatments, and 8 uncommon species were found

in two or less treatments (Table 3). Six species had less than 10 individuals per hectare

(Table 3). Individual biomass ranged from 0.1 g to 266 g with an average of 5 g. Three

species (Atherinella spp, Cichlasoma aureum and C. maculicauda) accounted for 70% of

the total biomass across all nets.

Some species were uncommon and found only in certain habitats. For example,

Oligoplites saurus and Eugerres plumieri were found only in nets with no plants, and

Poecilia mexicana and Brycon guatemalensis were found only in bulrush (Table 4).

Jaguar guapote Cichlasoma managuense introduced to Lake Izabal in 1950 (personal

comment Herman King), was present in high densities in hydrilla, and just a few

individuals were captured in other plant types.

Low PAC Analysis

The low PAC analysis demonstrated that fish richness was similar (12 to 14

species) among treatments (Table 5). Mean total fish density ranged from 16,620

(fish/ha) in chara to 51,120 (fish/ha) in hydrilla, and 40,480 (fish/ha) in no-plant

treatment, but fish density was not significantly different (P > 0.1) among habitats.

However, when I removed silversides Atherinella spp from the analysis the total fish

density was significantly different (P = 0.04).The no-plant treatment had significantly

less fish compare to hydrilla, which had cichlids accounting for most of the density in this

treatment. Mean total fish biomass ranged from 17 (kg/ha) to 155 (kg/ha) and was

significantly different (P = 0.02) among treatments (Table 5). Chara and no plants had

lower total fish biomass than hydrilla, but fish biomass did not differ between hydrilla,









eelgrass, or bulrush (Table 5). The Shannon-Wiener diversity index (H') by number

ranged from 0.72 in no-plants to 2.06 in Illinois pondweed and differed between these

treatments but not the others (P = 0.07, Table 5).

Regarding specific fish groups, there were differences in biomass among

treatments. Cichlid biomass differed among treatments (P = 0.003), where chara (5

kg/ha) and no-plants (10 kg/ha) had lower cichlids biomass than hydrilla (162 kg/ha)

(Table 5). Mojarra biomass was significantly (P = 0.02) higher in hydrilla (62 kg/ha)

than in the no plant treatment (5 kg/ha), but the other treatments did not differ

significantly (Table 5). The silversides was the most common fish in our samples

(caught in 44/50 nets), but I did not detect significant differences in biomass across

treatments due to highly variable catches, despite high silverside density in the no plant

treatment.

Low & High PAC Analysis

This data set comprised fish metrics for three plant species (hydrilla, eelgrass and

bulrush) and two PAC levels (low and high) as fixed effects. Species richness ranged

from 13 to 15 species across treatments. The plant type and PAC interaction was not

significant (P > 0.1) for total fish density, total fish biomass or diversity (H'), indicating

that differences in these metrics did not differ among the levels of each treatment. These

variables did not differ significantly due to PAC (P > 0.1); indicating that the coverage

level of plants did not influence fish metrics. However, total fish biomass varied (P =

0.02) due to plant type (Table 6), with eelgrass having significantly lower biomass than

hydrilla and bulrush (Table 6).

Specific groups and species were tested with the two-way ANOVA. The plant

species and PAC interaction was not significant (P > 0.1) for Cichlids, mojarra or









silversides biomass. However, Cichlids differed in biomass, due to plant type (P = 0.02),

where eelgrass had lower cichlid biomass than the other two plant species. Similarly,

mojarra had the lower biomass in eelgrass than in hydrilla and bulrush (P < 0.01).

Silverside biomass was different due to plant type (P = 0.09), and hydrilla had the lower

biomass. Silversides biomass was the only variable different due to PAC (P = 0.06), with

high biomass in low PAC areas compared to high PAC areas across all plant types (Table

6).

Hydrilla Analysis

Hydrilla was the only non-native plant in this study and was present around all 50

nets at Lake Izabal. Hydrilla had the highest plant biomass of all treatments (Figure 2).

Total fish density, total fish biomass or diversity (H') did not differ with different levels

of hydrilla PAC. Cichlids, mojarra and silversides biomass did not differ significantly (P

> 0.1) among PAC levels of hydrilla (Table 7).












1.8

CE 1.6

1.4

S1.2

E 1
o
.C
0.8

0.6

0.4
o
J 0.2

0
0


y = 0.0097x + 0.1742
R2 = 0.4723


* *


U
I
k


: I


* hydrilla



A bulrush



* eelgrass



* illinois
pondweed


x chara


*X


Percentage Area Coverage


Figure 2. The relation between plant biomass and percentage area coverage (PAC) (r2
0.47 N=50) at Lake Izabal June-July 2004.









Table 2. Physicochemical variables measured at Lake Izabal from June to July 2004.
Treatments were plant species with different percentage area coverage (PAC)
and no plants. Variables were measured at each block net and then averaged
by treatments (N=5/treatment). Mean standard deviation of temperature
(oC), depth and pH are shown.


I
]


Treatments
Plant species PAC
Chara 15
[11. pondweed 25
No plants 0
Bulrush 19
80
Eelgrass 23
77


Hydrilla


All treatments


8.07 + 0.28
7.91 + 0.12
7.67 + 0.39
8.05 + 0.11
7.62 + 0.09
8.25 0.23
8.02 + 0.37
7.65 0.26
8.08 + 0.30
7.90 + 0.53
7.90 + 0.35


Temperature
oC
29.94 + 0.78
29.60 + 0.79
29.96 1.18
29.90 + 0.76
29.82 + 0.77
30.46 1.94
30.64 1.19
29.34 + 0.72
30.28 1.27
31.16 1.66
30.10+ 1.15


Depth
(meters)
1.06 0.31
1.12 + 0.08
1.16 + 0. 17
1.22 + 0.24
1.24 + 0.27
1.14 0.16
1.10 + 0.20
1.24 + 0.36
1.34 + 0.22
1.28 + 0.17
1.18 + 0.22












800

700 N=4565

600
,-C
.M 500
U-
400

-300
E


200

100

0
1 2 3 4 5 6 7 8 9 101112131415161718192021222324
Total Length (cm)

Figure 3. Fish total length (TL) distribution across all treatments. The x axis shows fish
total length in centimeters, and the y axis is the number of fish (N= total
measured individuals).













Table 3. Taxonomic family and scientific name of fish collected in block nets in Lake Izabal. Mean fish biomass across all treatments
is in kg/ha, and mean fish density is fish/ha, mean total length (TL, mm), and SD is the standard deviation for respective
mean. Values represent means across all block nets (N=50).


Family
Cichlidae
Cichlidae
Atherinidae
Cichlidae
Cichlidae
Cichlidae
Ariidae
Poeciliidae
Eleotridae
Characidae
Characidae
Cichlidae
Cichlidae
Engraulidae
Poeciliidae
Carangidae
Gerreidae
Achiridae
Sygnathidae
Gobiidae
Gobiidae
Belonidae
Pimelodidae
Hemiramphidae


Scientific name
Cichlasoma maculicauda
Cichlasoma aureum
Atherinella spp
Cichlasoma salvini
Cichlasoma spilurum
Cichlasoma managuense
Ct/it/ 1,i i p spp
Carlhubbsia stuarti
Gobiomorus dormitor
Astianax aeneus
Brycon guatemalensis
Cichlasoma robertsoni
Cichlasoma bocourti
Anchoa spp
Poecilia mexicana
Oligoplites saurus
Eugerres plumieri
Trinectes paulistanus
Pseudophallus mindii
Gobiodes broussoneti
Gobiosoma spp
,S/l i i\ aI "/lllt notata
Rhamdia guatemalensis
Hyporhamphus roberti
hildebrandi


kg/ha
31.53
20.60
11.52
9.24
5.90
4.18
3.27
2.91
1.27
0.66
0.30
0.19
0.07
0.07
0.07
0.04
0.03
0.03
0.02
0.02
0.02
0.01
0.01
0.01


SD
29.50
12.44
7.39
8.94
5.57
5.61
8.58
4.08
1.27
0.75
0.48
0.50
0.14
0.11
0.09
0.12
0.10
0.07
0.04
0.05
0.03
0.02
0.01
0.01


fish/ha
2018
2204
17784
1484
1436
1148
230
2224
462
134
20
14
12
88
4
10
2
4
80
6
222
12
8
2


SD
1439
1241
10897
1393
1224
1633
428
3691
326
189
50
19
19
89
13
19
6
8
112
14
284
10
19
6


TL (mm)
76
77
43
60
58
47
114
40
49
60
96
108
67
44
90
62
80
69
13
81
120
118
64
64


SSD
32
16
12
17
15
28
44
14
31
25
25
14
18
11
24
13
18
14
7
49
45
7
0
16














Table 4. Taxonomic family and scientific name of fish collected in block nets (N=5/treatment) at Lake Izabal. Plant species and
percent area covered (PAC) are located at the top of each column (x= present).


Family



Achiridae
Ariidae
Atherinidae
Belonidae
Carangidae
Characidae

Cichlidae








Eleotridae
Engraulidae
Gerreidae
Gobiidae

Hemiramphidae
Pimelodidae
Poeciliidae


Scientific name


Achirus spp
C, ahiiti p spp
Atherinella spp
Siq linl\h\t Ota notata
Oligoplites saurus
Astianax aeneus
Brycon guatemalensis
Cichlasoma aureum
Cichlasoma bocourti
Cichlasoma managuense
Cichlasoma robertsoni
Cichlasoma salvini
Cichlasoma spilurum
Cichlasoma maculicauda
Gobiomorus dormitor
Anchoa spp
Eugerres plumieri
Gobiodes broussoneti
Gobiidae
Hyporhamphus spp
Rhamdia guatemalensis
Carlhubbsia stuarti
Poecilia mexicana
Pseudophallus mindii


no
plants
none


chara


hydrilla


low low medium high


Illinois
pondweed
low


bulrush


low high low high


x x
x


x x
x
x x


x x
x
x x
x
x x
x x
x x


x x

x x
x
x x
x
x x
x x
x x
x x
x


x
x x x


x x x


x x x


eelgrass


Sygnathidae


x x x x













Table 5. Fish metrics for treatments with low percent area coverage (PAC) including the no plants treatment. Treatments are plant
species and no-plants. Fish richness is the cumulative richness across all nets (N=5) for each treatment. Mean and
standard deviation of total fish biomass (kilograms/hectare), total fish density (fish/hectare), Shannon-Wiener diversity
index (H') are shown. Cichlid, mojarra, and silverside biomass (kilograms/hectare) are shown. Values along column


without a letter in common are significantly different (Tukey's test a
between mean values.


0.1). No letters indicate no significant difference


Treatment
Plant
species
Bulrush

Chara

Eelgrass

Hydrilla

Illinois
pondweed
No plants


Richness
PAC (cumulative)


Low

Low

Low

Low

Low

0


Total fish
biomass
(kg/ha)

84 + 48
ab
17+8
a
64 + 71
ab
155 + 68
b
57 30
ab
42 + 44


Total fish
density
(fish/ha)

25,500 12,644

16,620 15,124

26,940 29,843

51,120 + 41,632

19,960 14,603

40,480 + 37,356


Shannon-
Wiener
(H')
1.47 + 0.61
ab
1 + 0.81
ab
1.19 0.25
ab
1.79 1.13
ab
2.06 0.80
a
0.72 + 0.46
b


Cichlids
biomass
(kg/ha)

62 + 50
ab
5+4
bc
46 + 55
abc
121 + 67
a
41 27
ab
10 17
c


mojarra
biomass
(kg/ha)

30 29
ab
3+3
ab
22 + 46
ab
67 + 57
a
87
ab
5 10
b


silversides
biomass
(kg/ha)

18 12

8+9

16 16

16 21

7 10

26 + 26













Table 6. Fish metrics for six treatments included in the high versus low PAC comparison. Treatments are plant species with low
(<25%) or high (>75%) PAC. Fish richness is the cumulative richness across all nets (N=5) for each treatment. Mean and
standard deviation of total fish biomass (kilograms/hectare), total fish density (fish/hectare), Shannon-Wiener diversity
index (H') are shown. Cichlid, mojarra, and silverside biomass (kilograms/hectare) are shown. Values along column
without a letter in common are significantly different (Least Squares Means test P < 0.1). No letters indicate no significant
difference between mean values.


Treatment
Plant
species


Richness
PAC (cumulative)


Bulrush Low

High

Eelgrass Low


High


Hydrilla Low

High


Total fish
biomass
(kg/ha)

84 + 48
b
152 124
b
64 + 71
a
52 + 30
a
155 + 68
b
186 + 199


Total fish
density
(fish/ha)


25,500 + 12,644

21,400 + 7,437

26,940 + 29,843

34,320 + 26,781

51,120 + 41,632

23,980 + 8,748


Shannon-
Wiener
(H')

1.47 + 0.61

1.95 + 0.57

1.19 0.25

1.49 + 0.79

1.79 1.13

2.26 0.43


Cichlids
biomass
(kg/ha)

62 + 50
b
130 121
b
46 + 55
a
29 + 20
a
121 + 67
b
132 + 116
b


mojarra
biomass
(kg/ha)

30 29
b
64 + 95
b
22 + 46
a
2+2
a
67 + 56
b
81 + 91
b


silversides
biomass
(kg/ha)


18 + 12
b
7+3
a
16 + 16
b
8+8
a
16 21
b
1+1
a













Table 7. Hydrilla data set were treatments are different percent area coverage (PAC). Fish richness is the cumulative richness across
all nets (N=5) for each treatment. Mean and standard deviation of total fish biomass (kilograms/hectare), total fish density
(fish/hectare), Shannon-Wiener diversity index (H') are shown. Cichlid, mojarra, and silverside biomass
(kilograms/hectare) are shown. Values along column without a letter in common are significantly different (Tukey's test a
=0.1).


Treatments

PAC
low
medium
high


Richness
(cumulative)


Total fish
biomass
(kg/ha)
155+ 68
112 57
186 + 199


Total fish
density
(fish/ha)
51,120 + 41,623
35,760 + 30,471
23,980 + 8,748


Shannon-
Wiener
(H')
1.79 1.13
1.91 + 0.81
2.26 0.43


Cichlids
biomass
(kg/ha)
121 + 67
82+ 38
132 116


mojarra
biomass
(kg/ha)
67 + 56
33 23
81 + 91


silversides
biomass
(kg/ha)
16 21
7 11
S11














Table 8. Taxonomic family and scientific name of fish observed in the local market at El Estor, Izabal, Guatemala (July/1-15/2004).
Common Spanish name obtained from the anglers. Abundance was the presence in the market as indicated by categories
of highly common: > 10 days, common: 5-10 days, and uncommon: < 5 days. Biomass rank obtained in block nets across
all treatments.


Family


Centropomidae

Cichlidae





Ictaluridae
Ariidae

Megalopidae
Carangidae
Characidae
Eleotridae
Gerreidae


Scientific name


Centropomus undecimalis
Centropomus parallels
Cichlasoma managuense
Cichlasoma boucurti
Cichlasoma maculicauda
Cichlasoma robertsoni
Oreocromis spp
Ictalurusfurcatus
Bagre marinus
Ariidae spp
Tarpon atlanticus
Oligopterus saurus
Brycon guatemalensis
Gobiomorus dormitor
Eugerres plumieri


common name
(spanish)
robalo
robalo blanco
guapote
cagona
mojarra
rashcar
tilapia
tacazonte
bagre
punta estrella
sabalo
zapatera
machaca
guavina
mojarra blanca


abundance


highly common
common
common
uncommon
highly common
uncommon
common
highly common
uncommon
common
uncommon
uncommon
common
common
uncommon


biomass rank in
block nets
Not present
Not present
6th
12th
1st
13th
Not present
Not present
Not present
Not present
Not present
16th
Slth
9th
17th















DISCUSSION

It is not certain when hydrilla entered Lake Izabal, but the plant has probably been

in the lake since at least 1997 (Haller 2002; Arrivillaga 2003). Historical data regarding

the distribution and composition of native plant and fish communities prior to hydrilla

introduction do not exist for Lake Izabal. Bettoli et al. (1993) found numerous changes

in fish community composition and biomass related to vegetation removal by grass carp

in a Texas reservoir over a seven-year span, and the changes occurred as early as the first

year after the vegetation removal. I compared fish community metrics among hydrilla

and five others types of habitats, including no-plants and four native species of aquatic

plants. My assumption was that fish community characteristics in sites without hydrilla

would approximate conditions found in littoral areas of the lake prior to hydrilla

introduction.

Lake Izabal contains native aquatic plant communities with hydrilla mixed in

almost all of them, consistent with reports by Arrivillaga (2003). I found hydrilla areas

ranged from low percent area coverage (PAC) to high PAC (Appendix A); no co-

occurring plants were present when hydrilla had high PAC. Chick and McIvor (1994)

found similar results at Lake Okeechobee, where thick stands of hydrilla usually

contained no other plant species. Since previous reports at Lake Izabal before hydrilla

described low aquatic macrophyte because of the bottom configuration and wave action

(Dickinson III 1974), hydrilla beds may actually protect the shoreline from wave action

and enhance the ability of native macrophytes to grow. Overall habitat complexity was









frequently governed by hydrilla in this study, but there were exceptions. Bulrush was the

only littoral plant in this study that had no evident hydrilla colonization. No vast

monocultures of native plants existed at the time of this study, and thus, chara and Illinois

pondweed were not found in high PAC. Hydrilla was in the proximity or mixed with all

the other native submerged plants sampled.

Fish species richness did not vary greatly among treatments with low PAC in my

study. Preliminary findings at Lake Izabal reported less fish species richness in habitats

that contained hydrilla versus native plants (Arrivillaga 2003). However, Arrivillaga

(2003) used seines to sample fish, whereas we used block nets and rotenone. Seining in

hydrilla could be extremely inefficient due to its dense architecture, making comparisons

between these studies difficult.

My total fish species collected (24) was lower than previous studies at the lake.

Perez (2004) reported 81 at the lake and rivers around it, but her study included all the

historical records from 1935 throughout 2003, different fishing gears, sampling across

most months, and samples from the entire basin including the associated rivers. Perez

(2004) reported 21 species restricted to rivers and 16 with unknown distribution and 44

present in the lake. She also found that about 35 species in Lake Izabal basin were from

estuarine origin, and abundance of estuarine fishes in the lake likely varies strongly with

season.

The most abundant families in Lake Izabal are Poecilids and Cichlids. I only

collected two Poecilid species, whereas Perez (2004) reported twelve species from this

family, however seven were collected only in rivers. The Usumacinta province exhibited

high richness in the Poecilidae family (Miller 1976), and other studies using multiple









gears in areas inside the province found more species, but included rivers (Greenfield

1997; Willink et al. 1999). Perez (2004) reported 17 species of Cichlids, three collected

only in rivers and two with unknown distribution in the lake, whereas I collected about

nine of the twelve in the lake. Thus, my sampling using block nets in shallow areas of

the lake found relatively low species richness (24 species) compared to the historical

records in the lake (44 species). Use of 10x10 m block nets in shallow areas resulted in

small fish sizes collected in this study, probably resulting in incomplete sampling of the

whole-lake fish community. However, my sample size of fish species was probably

reflective of the fish community inhabiting inshore littoral areas at Lake Izabal during

summer.

Total fish density was not significantly different among low PAC treatments. The

lack of a statistical difference is surprising, because other studies had found differences

due to macrophytes type (Chick and McIvor 1994) or between open water and vegetated

areas (Bettoli et al 1993; Gelwick and Mathews 1990; Killgore et al. 1989; Shireman et

al. 1981). Although total fish density did not differ among habitats, certain species or

groups seemed to prefer different habitats. For example, silversides Atherinella spp were

commonly collected in no-plant areas similar to Zaret and Paine (1973) and Bettoli and

Morris (1991). Removing silversides from the total fish density resulted in differences

among treatments with hydrilla having more fish than no plants treatment. Conversely,

cichlids density was lower in no-plant or chara areas compared to other plant areas.

Silversides contributed greatly to high within-habitat variability in fish density,

preventing the detection of significant differences in total fish density (i.e., low statistical

power).









I was able to detect differences in total fish biomass among low PAC treatment

types, with bulrush, eelgrass, and hydrilla having higher fish biomass than the other low

PAC treatments. Six species comprised 85% of the biomass across all treatments; five of

the six species were cichlids. Cichlids (including mojarra) at Lake Izabal exhibited

increased biomass in hydrilla, bulrush, and eelgrass, with lower biomass in chara and no-

plants. Cichlids occupy much the same ecological position in the tropics as do the

temperate North American sunfishes (Centrarchidae, Miller 1976). Therefore, the higher

total fish biomass with vegetation present was consistent with other studies in temperate

zones (Barnett and Schneider 1974; Bettoli et al. 1993; Durant 1980; Gelwick and

Matthews 1990; Killgore et al. 1989; Shireman et al. 1981) and the life history of cichlids

(Greenfield 1997; Miller 1976). Subsistence anglers recognized bulrush as a "high

abundance" area for cichlids, consistent with my results. Bulrush was the only native

plant with high cichlid biomass similar to non-native hydrilla.

The fish community indices also varied among treatments. Diversity by number

was lower in no-plants, which was predicted given studies showing that complex habitats

increase fish diversity (Barnett and Schneider 1974; Bettoli et al. 1993; Durant 1980;

Killgore et al. 1989; Shireman et al. 1981). Diversity showed the lowest value in no-

plants and chara, which had almost no plant biomass. Moreover, the littoral fish

community changed from cichlids dominated in vegetated to pelagic dominated

(Atherinadae, Engraulidae, Gerreidae and Carangidae) in areas without plants and chara.

The two-way ANOVA was useful to indicate that the fish population parameters

did not vary between plant species and PAC levels, because the interaction between plant

species and PAC level was not significant. Total fish biomass was again the only









parameter that showed significant differences due to plant type. The silversides are

pelagic oriented (Greenfield 1997; Zaret and Paine 1973) and showed lower biomass in

high PAC consistent with Killgore et al. (1989) who found that fish that feed directly on

zooplankton could be negatively affected by dense hydrilla coverage. The mojarra and

total cichlid biomass was lower in eelgrass, but bulrush and hydrilla had higher fish

biomass, consistent with the one-way analysis.

Hydrilla was the most common plant around Lake Izabal, thus low, medium and

high PAC was present, and hydrilla had the highest plant biomass among all plant species

(Appendix A). However, none of the fish community parameters estimated in this study

was significantly different among levels of PAC in hydrilla. Conversely, Killgore et al.

(1989) found that among seasons intermediate and high densities of hydrilla beds had

higher fish abundance and biomass compared with hydrilla beds in lower densities. High

variability within different hydrilla PAC levels (i.e., standard deviation that exceeded the

mean for the high PAC level) may have prevented the detection of significant differences.

Alternately, the fish metrics in this study may not have varied with hydrilla PAC, because

the mean fish biomass was 155 kg/ha at low hydrilla PAC and 186 kg/ha at high PAC.

Another interesting note was that jaguar guapote Cichlasoma managuense, an introduced

species to Lake Izabal was commonly collected in hydrilla but not in other plant species

in this study. The tilapia (Oreocromis spp), another introduced species, was found

commonly in the market (Table 8) and reported to be caught on the edge of hydrilla beds,

but was not present in my samples.

Estimates of fish population parameters are inherently influenced by sampling

techniques, plant densities and patchy fish distribution (Killgore et al. 1989). However,









small block nets (0.01 ha) used in this study allowed more treatments and repetition than

other methods and allowed for adequate collection of smaller fish sizes (Bettoli and

Maceina 1996; Haller et al. 1980; Shireman et al.1981; Timmons et al. 1979) but not fish

over about 180 mm TL. Hence, fish collected in this study were small. The small

juvenile cichlids probably chose habitats with vegetation to avoid predation or increase

food availability, similar to juvenile bluegill in temperate zones (Gotceitas and Colgan

1987; Savino and Stein 1982; Mittelbach 1981; Valley and Bremingan 2002). Results of

this study should be considered representative of the juvenile littoral fish assemblage at

Lake Izabal but do not reflect densities of larger fish or open-water species.

Seasonal distribution of fish could be a factor in my study. Data for this study were

collected in June-July of 2004. The family Poecilidae was represented by two species

(one species by one individual) in the lake at the sampling time when the water levels

were the highest in the year (Appendix B). Six species of Poecilidae were collected

throughout the year at the lake using dip nets (personal communication Maritza Aguirre),

but I collected only two. Because my samples were concentrated in June/July, temporal

variation in fish community structure and richness was not evaluated in this study.

Similarly Chick and Mclvor (1994) found differences in Poecilidae densities among

months and depths in hydrilla. Another example of seasonality would be silversides,

which account for 60% of all fish collected in this study. Gelwick and Matthews (1990)

found differences in silverside Menidia beryllina abundance over seasons which peaked

in April-June. Silverside Menidia beryllina was also present in high density in open

waters, similar to this study. Thus, silverside density, like other species, may vary with

season.









Responses from fishes to aquatic plants in Lake Izabal were predictable with other

plant-fish interaction literature (Dibble et al 1996), but my data suggest that plants, and

moreover hydrilla is highly utilized by small (< 13 cm) cichlids in Lake Izabal regardless

the level of PAC. The only description of aquatic macrophytes before hydrilla infestation

mentions the "lack" of vegetation in Lake Izabal (Dickinson III 1974). Hayes et al.

(1996) showed that if a habitat is limited and if the habitat increases from the baseline

occur, an increase in fish population parameters will be expected. However, detecting

fish responses to habitat changes remains problematic for fisheries managers due to

variability in fish responses (e.g., Minns et al. 1996; Allen et al. 2003).

Finally, fisheries at Lake Izabal included marine origin species and adult cichlids

caught from areas outside the littoral zones (more than 90 % of Lake Izabal area), and I

did not evaluated fish response to hydrilla outsides littoral areas. Nonetheless, littoral

areas in the lake are directly important for at least eight species present in the fisheries

(Table 8), and indirectly for others important predators. Thus, the littoral fish assemblage

I sampled included juveniles of species that were important in the fisheries















MANAGEMENT IMPLICATIONS

Hydrilla has been a nuisance plant around the world. Many negative effects have

been documented including the occupation of the entire water column of shallow lakes by

hydrilla. However, Lake Izabal's mean depth prevents hydrilla from occupying the entire

water body, as Arrivillaga (2003) estimated that only about 9% of the lake could be

colonized by hydrilla. Furthermore Hoyer and Canfield (1996) state the need of aquatic

macrophytes in large lakes as refuge for juvenile fish of certain types, like bass.

Hydrilla was not detrimental for fish in this study and generally had higher fish

biomass and similar richness to areas with no plants and those with other native plants.

Hydrilla creates littoral fish habitat at Lake Izabal. Juvenile cichlid biomass was higher

in hydrilla than in other plant types, and cichlids support important fisheries at the lake.

Further field work should target adult fish and assess the entire cichlid population

and not just juveniles or forage fish. Other long-term fish population responses to

hydrilla are unknown for Lake Izabal. Parameters such as fish growth, recruitment or

mortality rates should be assessed in the future. Results of this study suggest that hydrilla

is not detrimental to the littoral fish community at Lake Izabal, and because the plant is

unlikely to occupy over 10% of the lake surface area, it could actually benefit the lakes'

fisheries. The anglers in El Estor community at Lake Izabal recognize hydrilla as a high

abundance fish area, and they commonly seek hydrilla edges to set gillnets for adult fish.

However, Lake Izabal is not used only for fishing, but also access, transportation, tourist









and aesthetic values should be addressed in a holistic approach before making decisions

about hydrilla management at the lake.
















APPENDIX A
BLOCK NETS SUMMARY AT LAKE IZABAL

Table 9. Block nets summary at Lake Izabal (June-July/2004). Treatments were plant
species with different levels based on the percentage area coverage (PAC) and
no plants. Total plant biomass is mean kilogram per square meter (N = 3).
Total fish biomass in kilograms. Total fish density present and richness
present in each net.


Net Plant species


bulrush
bulrush
bulrush
bulrush
bulrush
bulrush
bulrush
bulrush
bulrush
bulrush
chara
chara
chara
chara
chara
eelgrass
eelgrass
eelgrass
eelgrass
eelgrass
eelgrass
eelgrass
eelgrass
eelgrass
eelgrass
hydrilla
hydrilla
hydrilla
hydrilla
hydrilla
hydrilla


level PAC Total
plant
biomass
kg/m2
low 25 3.6
low 20 2.1
low 20 2.2
low 15 0.1
low 15 1.3
high 75 5.4
high 80 3.7
high 80 4.2
high 80 1.5
high 85 8.2
low 25 0.9
low 20 0.3
low 10 0.2
low 15 0.2
low 15 0.2
low 25 1.8
low 25 1.6
low 25 1.9
low 20 0.3
low 20 1.5
high 75 2.9
high 75 1.8
high 80 2.8
high 80 2.9
high 75 7.0
low 25 5.2
low 25 6.0
low 25 0
low 25 4.4
low 25 4.0
medium 50 6.7


Total
fish
biomass
kg/ha
286.6
769.6
1180.2
1452.1
511.2
241.91
1039.2
3578.2
1221.6
1517.5
258.4
162.3
148.8
45.1
228.9
259.4
103.5
465.6
491.2
1866.1
716.9
491.7
352.7
142.2
910.2
1499.8
2284.8
2038.5
1409.3
520.0
601.4


Total fish
density
number/ha

84
302
414
180
295
117
169
259
218
307
433
93
135
65
105
114
102
52
304
775
363
338
223
31
761
203
259
187
815
1092
887


Fish
richness


7
9
7
9
6
4
8
11
8
7
2
7
5
5
7
7
5
6
6
9
10
7
7
8
7
10
9
8
10
8
12











Net Plant species


32 hydrilla
33 hydrilla
34 hydrilla
35 hydrilla
36 hydrilla
37 hydrilla
38 hydrilla
39 hydrilla
40 hydrilla
41 Illinois
pondweed
42 Illinois
pondweed
43 Illinois
pondweed
44 Illinois
pondweed
45 Illinois
pondweed
46 no-plants
47 no-plants
48 no-plants
49 no-plants
50 no-plants


level PAC


medium
medium
medium
medium
high
high
high
high
high
low

low

low

low

low

none
none
none
none
none


Total
plant
biomass
kg/m2
13.0
5.6
8.0
13.9
33.4
15.2
6.9
28.7
28.4
7.4


20 2.0


Total Total fish Fish
fish density richness
biomass number/ha
kg/ha
2082.0 296 8
941.5 298 10
860.0 169 8
1116.9 138 7
365.0 121 10
1466.7 201 8
580.3 239 8
1603.5 354 10
5287.4 284 10
673.4 110 8


65.7


1.7 832.9


25 2.5 747.0

25 8.5 539.4


322.6
515.6
23.1
111.4
1130.6
















APPENDIX B
WATER LEVELS AT LAKE IZABAL


Oct- Nov- Dic- Ene- Feb- Mar- Abr- May- Jun- Jul-04
03 03 03 04 04 04 04 04 04


Figure 4. Water levels at Lake Izabal 2003-2004. From:
Hidrol6gico 2005. Metros = meters and Mes


AMASURLI Boletin
= month.















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BIOGRAPHICAL SKETCH

Christian Barrientos is originally from Guatemala, Central America. He grew up in

Guatemala City, where he obtained a biologist degree from University of San Carlos in

Guatemala, married another biologist and moved to Peten, home of the Mayan Biosphere

Reserve. He worked in the Scarlet Macaw Biological Station located at Laguna del Tigre

National Park, spending most of his time in the beautiful San Pedro River. During this

time he worked in conservation of terrestrial systems. But his heart was always loyal to

the aquatic systems and his research started to move to the water. During that time he

always enjoyed fishing trips on the San Pedro River. During the time as a coordinator of

a Global Environment Facility/World Bank Project for Laguna Del Tigre, he improved

his writing and communication abilities in English. Finally, and following advice from

several friends and parents he decided to pursue a new level of education at the

University of Florida. During this new student phase, and now as a father he realized

how valuable the aquatic systems are for this and the next generation, and he will

continue to work on the conservation and management of the aquatic systems.