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FISH ABUNDANCE AND COMMUNITY COMPOSITION IN NATIVE AND
NON-NATIVE LITTORAL AQUATIC PLANTS AT LAKE IZABAL, GUATEMALA
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
Christian Alberto Barrientos
To God, who provides all wisdom and judgment.
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.
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
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
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
LIST OF TABLES
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
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
Christian Alberto Barrientos
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.
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
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
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.
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,
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.
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 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.
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:
H'- (pi/p) (log2 pi/p) (eq. 1)
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
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
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
Low (<25%) 5 5 5 5 5
Med (26-74%) 5
High (>75%) 5 5 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).
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
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
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
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).
y = 0.0097x + 0.1742
R2 = 0.4723
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.
Plant species PAC
[11. pondweed 25
No plants 0
8.07 + 0.28
7.91 + 0.12
7.67 + 0.39
8.05 + 0.11
7.62 + 0.09
8.02 + 0.37
8.08 + 0.30
7.90 + 0.53
7.90 + 0.35
29.94 + 0.78
29.60 + 0.79
29.90 + 0.76
29.82 + 0.77
29.34 + 0.72
1.12 + 0.08
1.16 + 0. 17
1.22 + 0.24
1.24 + 0.27
1.10 + 0.20
1.24 + 0.36
1.34 + 0.22
1.28 + 0.17
1.18 + 0.22
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
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).
Ct/it/ 1,i i p spp
,S/l i i\ aI "/lllt notata
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).
C, ahiiti p spp
Siq linl\h\t Ota notata
low low medium high
low high low high
x x x
x x x
x x x
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
84 + 48
64 + 71
155 + 68
42 + 44
51,120 + 41,632
40,480 + 37,356
1.47 + 0.61
1 + 0.81
0.72 + 0.46
62 + 50
46 + 55
121 + 67
22 + 46
67 + 57
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.
84 + 48
64 + 71
52 + 30
155 + 68
186 + 199
25,500 + 12,644
21,400 + 7,437
26,940 + 29,843
34,320 + 26,781
51,120 + 41,632
23,980 + 8,748
1.47 + 0.61
1.95 + 0.57
1.49 + 0.79
62 + 50
46 + 55
29 + 20
121 + 67
132 + 116
64 + 95
22 + 46
67 + 56
81 + 91
18 + 12
16 + 16
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
186 + 199
51,120 + 41,623
35,760 + 30,471
23,980 + 8,748
1.91 + 0.81
121 + 67
67 + 56
81 + 91
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
biomass rank in
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
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
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
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
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
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
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.
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
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
Net Plant species
Total Total fish Fish
fish density richness
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
25 2.5 747.0
25 8.5 539.4
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
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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.