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ASSESSMENT OF FISH AND PLANT COMMUNITIES
IN LAKE APOPKA, FLORIDA
STEPHEN J. MURPHY
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
Stephen J. Murphy
Appreciation is expressed to all those who assisted me with this research. David
Watson and Dan Willis of Florida LAKEWATCH extended their expertise in
electrofishing activities. Alexis Caffrey helped take light meter measurements and
helped survey aquatic plants, along with Heather Hammers and Laura Stockman.
Elizabeth Daneman and Erich Marzolf from the St. Johns River Water Management
District (SJRWMD), William Johnson from the Florida Fish and Wildlife Conservation
Commission (FFWCC), and Mark Hoyer from the University of Florida provided
previous data on the fish and plant communities. Karen Brown from the University of
Florida (UF), Center for Aquatic and Invasive Plants, assisted me in locating journal
articles for my aquatic plant literature review. Dr. Roger Bachmann guided me in
analyzing limnological data. Dr. Daniel E. Canfield, Jr. served as my committee
chairman and advisor, and directed me towards excellence and professionalism in the
scientific field. Finally, Dr. Charles Cichra and Dr. Kenneth Langeland served on my
committee and helped oversee this research.
TABLE OF CONTENTS
A C K N O W L E D G M E N T S ......... ................................................................................... iii
LIST OF TABLES ............................. .......................... v
LIST OF FIGURES ......... ....... .................... .......... ....... ............ vi
ABSTRACT .............. ..................... .......... .............. vii
INTRODUCTION .............. ................................... ..............
M A TERIALS AN D M ETH OD S...................................................................................8
F ish ....................................................... 8
A aquatic M acrophytes............................................................................ 10
Statistical A analyses ................................................................. ....... .. ...... .... 15
RESULTS AND DISCU SSION ............................................................. ................19
F ish .........................................................................1 9
A aquatic M acrophytes............................................................................ 29
LAKE MANAGEMENT RECOMMENDATIONS ................................ ...............46
L IST O F R E F E R E N C E S .......................................................................... ....................53
B IO G R A PH IC A L SK E TCH ...................................................................... ..................62
LIST OF TABLES
1 Electrofishing boat setup used by different studies at Lake Apopka, Florida..........17
2 Electrofishing control box settings used by different studies at Lake Apopka,
F lo rid a ........................................................... ................ 17
3 Electrofishing methods used by different studies at Lake Apopka, Florida. ...........18
4 Common and scientific names of fish collected by individual studies at Lake
A p o p k a, F lo rid a ................................ ......... ... .... ................ ................ 3 6
5 Water chemistry parameters measured for Lake Apopka, Florida, in June -
O ctob er 2004 ...................................................... ................. 37
6 Mean electrofishing catch per unit effort and standard error of fish number
(number/hr) and weight (kg/hr) for Lake Apopka, Florida...................................37
7 Mean yearly and combined electrofishing catch per unit effort and standard error
of fish number (number/hr) for Lake Apopka, Florida, by Johnson and
Crum pton (1998) ................................................................ .. ..........38
8 Annual mean electrofishing catch per unit effort number (number/hr) of two size
groups of largemouth bass, bluegill, and redear sunfish collected in Lake
A p opk a, F lorida .............................................................................. ............... 39
9 Occurrence of plant species in twenty evenly-spaced transects around Lake
A p opk a, F lorida, in 2 004 ............................................................... .....................39
10 Location, area (m2), and maximum depth (m) of eel-grass beds in Lake Apopka,
F lorida, in 2004 .................................................... ................. 40
LIST OF FIGURES
1 Lake Apopka and surrounding area. ............................................... ............... 7
2 Mean electrofishing catch per unit effort estimates of total fish abundance
(number/hr) for Lake Apopka, Florida................................. ....................... 41
3 Mean electrofishing catch per unit effort estimates of total fish abundance
(number/hr) for the combined data of different studies for Lake Apopka, Florida..41
4 Individual fish species percent of total number collected by electrofishing for
Lake Apopka, Florida, by Johnson and Crumpton (1998) for 1989 1993. ...........42
5 Individual fish species percent of total number collected by electrofishing for
Lake Apopka, Florida, in June August 2004.............. .....................................42
6 Length frequency distribution of bluegill collected by electrofishing for Lake
Apopka, Florida, in June August 2004....................................... ............... 43
7 Length frequency distribution of redear sunfish collected by electrofishing for
Lake Apopka, Florida, in June August 2004.............. .....................................43
8 Mean electrofishing catch per unit effort estimates of largemouth bass
abundance (number/hr) for Lake Apopka, Florida........................................44
9 Mean electrofishing catch per unit effort estimates of largemouth bass
abundance (number/hr) for the data sets of combined years from different
studies for Lake Apopka, Florida. ........................................ ......................... 44
10 Length frequency distribution of largemouth bass collected by electrofishing for
Lake Apopka, Florida, in June August 2004.............. .....................................45
11 Electrofishing catch per unit effort of largemouth bass (kg/hr) versus total
chlorophyll a (tg/L) for 60 Florida lakes...................................... ............... 45
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
ASSESSMENT OF FISH AND PLANT COMMUNITIES IN LAKE APOPKA,
Stephen J. Murphy
Chair: Daniel E. Canfield, Jr.
Major Department: Fisheries and Aquatic Sciences
Since the 1960s, an enormous amount of time and resources has been expended
by the State of Florida and other government entities trying to improve the water quality
in Lake Apopka, a hypereutrophic lake in central Florida; however, the lake remains in a
turbid algal state in the early 2000s. The objective of this study was to assess the
effectiveness of the restoration efforts in returning Lake Apopka back to its historic
primary use of recreational fishing for largemouth bass (Micropterus salmoides
floridanus). The assessment was accomplished by surveying the fish populations and
aquatic plant communities in the lake to determine if there has been an increase in the
abundance of largemouth bass or an expansion in the area occupied by aquatic
vegetation. Fish were sampled by electrofishing on three occasions in June August
2004. Submersed macrophyte beds, primarily eel-grass (Vallisneria americana), and
floating-leaved and emergent plants were sampled in October 2004. The mean
largemouth bass catch rate in 2004 was 7 fish/hr. The combined catch rate from 1989 to
1993 was also 7 fish/hr, thus showing that the largemouth bass population did not
increase in relative abundance during the last decade. The abundance of largemouth bass
continues to be lower than expected for Lake Apopka's trophic status in comparison to
other Florida lakes. Submersed, floating-leaved, and emergent plants covered less than
1% of the surface area of the lake in 2004, indicating that the rooted aquatic vegetation
has not expanded in the last several decades. Eel-grass colonized a total lake bottom area
of approximately 900 m2 in 2004 compared with an estimated 11,032 m2 in 1999. Eel-
grass has drastically declined during the past five years since it was replanted around the
shore in 1999. It now occupies only 8% of its former area that it occupied in 1999. The
restoration efforts at Lake Apopka have not yet been successful in restoring the
largemouth bass fishery or in expanding the area occupied by aquatic vegetation.
Reducing the abundance of planktonic algae through nutrient management is the primary
restoration strategy for agencies such as the St. Johns River Water Management District
(SJRWMD). Other factors besides light attenuation by planktonic algae, however, are
involved in limiting the abundance of largemouth bass and expansion of aquatic
macrophytes. The depth and fluidity of sediments, and wind resuspension of sediments,
are also major factors. It could be several more decades before the largemouth bass
fishery is improved while relying on the current restoration strategy unless alternative
management strategies are also utilized. Three alternative management strategies or
ideas that have been discussed are drawdown, artificial reefs or barriers, and stocking.
Lake Apopka is the fourth largest lake in Florida with a surface area of 12,465 ha
(Hoge et al. 2003). It is located northwest of Orlando, Florida, in both Orange and Lake
counties (Figure 1). It is shallow (1.6 m mean depth) and polymictic. It is
hypereutrophic with high nutrient and chlorophyll levels (100 [g/L mean total
phosphorus (TP) and 59 [g/L mean chlorophyll a (chla)), and low transparency (0.35 m
mean Secchi depth (SD)) (Hoge et al. 2003). These limnological parameter values (TP,
chla, and SD) are mean 2004 values obtained from the St. Johns River Water
Management District (SJRWMD). The lake is nearly round except that it is shaped like a
funnel or gourd neck at the southern end of the lake (Figure 1). Vegetable farms (muck
farms) were established on the north end of the lake after 1940 for World War II wartime
Prior to 1950, Lake Apopka contained clear water, and dense growths of Illinois
pondweed (Potamogeton illinoensis) and eel-grass (Vallisneria americana) down to
depths of 2.4 m (Clugston 1963, Chestnut and Barman 1974). About 80% of the lake
was inhabited by aquatic plants. Clugston (1963) reported that Illinois pondweed began
at about 180 m from the shoreline and extended across the entire length and breath of the
lake except in deep-water areas (> 2.4 m). Water hyacinth (Eichhornia crassipes) grew
profusely around the entire shoreline of the lake (Clugston 1963). Boating around the
lake was restricted to trails through submersed vegetation and to openings not covered by
extensive mats of water hyacinth (Clugston 1963).
Lake Apopka was nationally renowned as a premier largemouth bass
(Micropterus salmoidesfloridanus) fishing lake (Dequine 1950, Dequine and Hall 1951,
Shofner 1982). Anglers came from throughout the United States to fish for trophy-sized
largemouth bass (Davis 1946, Dequine 1950, Clugston 1963, Chestnut and Barman
1974). By the early 1950s, there were an estimated 13 fishing camps with a value to the
local economy of over one million dollars annually (Dequine and Hall 1951, Shofner
Lake Apopka was changed in the early twentieth century by flood plain alteration,
water level stabilization, and urban and agricultural runoff (Huffstutler et al. 1965). Also,
in the 1940s, farms at the north end of the lake expanded by draining marsh areas.
Following the expansion of the farms, farmers began to pump an increased amount of
nutrient-rich water back into Lake Apopka (Clugston 1963). A hurricane in 1947
uprooted many of the macrophytes in the lake according to Mr. John Dequine (retired
biologist, Florida Game and Fresh Water Fish Commission, personal communication),
Clugston (1963), Schneider and Little (1969), Lake Apopka Restoration Council (1986),
and Bachmann et al. (1999). Within several years, the remaining macrophytes
disappeared in other parts of the lake as well, coinciding with a switch to a turbid algal
state (Clugston 1963). There is controversy about the role of the hurricane in the loss of
the macrophytes and about whether the switch from rooted macrophytes to algae was the
result of natural causes or the result of the numerous human activities (Schelske and
Brezonik 1992, Bachmann et al. 1999, Lowe et al. 1999, 2001, Canfield et al. 2000,
Schelske et al. 2000, Schelske and Kenney 2001). Regardless of the mechanisms, sport
fishing for largemouth bass declined in the late 1950s and early 1960s (Clugston 1963,
Huffstutler et al. 1965, Lake Apopka Restoration Council 1986).
According to Johnson and Crumpton (1998), the largemouth bass fishery and
aquatic vegetation community have been functionally non-existent at Lake Apopka since
the 1960s. Dequine and Hall (1951) estimated that over 9,513 largemouth bass were
harvested in January 1951 alone. Johnson and Crumpton (1998) stated that the combined
harvest over the last two decades (from 1978 to 1998) probably would not equal that one
Aquatic vegetation in the 1980s and 1990s has only occupied a narrow belt
around the shoreline of the lake, comprising less than 1% of the surface area of lake
(Canfield and Hoyer 1992, Johnson and Crumpton 1998). Dominant emergent plant
species were cattail (Typha spp.) and woody wetland species (red maple (Acer rubrum)
and Carolina willow (Salix caroliniana)), which occupied 38% and 44% of the shoreline,
respectively, in 1997 (Johnson and Crumpton 1998). There were isolated stands of
aquatic grasses (maidencane (Panicum hemitomon), torpedograss (Panicum repens),
Egyptian paspalidium (Paspalidium geminatum)), and soft-stem bulrush (Scirpus
validus), which together occupied 8% of shoreline (Johnson and Crumpton 1998). The
dominant floating-leaved plant species was water hyacinth in 1986, occurring in 27% of
transects (Canfield and Hoyer 1992). The occurrence of floating-leaved or submersed
plant species was not reported by Johnson and Crumpton (1998). The relative species
richness of aquatic macrophytes (10 species) and fish (16 species) in 1986 was low in
comparison with other Florida lakes (Canfield and Hoyer 1992).
A thick layer of flocculent sediments (1.5 m mean thickness) covers 90% of the
lake bottom in almost all areas of the lake (Schneider and Little 1969). There is a firm
bottom in some parts of the west shore and in some parts of the shoreline in the gourd
neck area. These unconsolidated sediments are often resuspended by the wind,
contributing to the high turbidity, and do not allow plants to anchor their roots (Carter et
al. 1985, Doyle 2001, Doyle and Smart 2001) or largemouth bass to successfully nest
(Porak et al. 1999).
Point sources of nutrient loading from sewage and citrus processing plants were
eliminated by the 1980s, and discharges from farming operations were reduced in 1992
(Johnson and Crumpton 1998, Bachmann et al. 1999, Canfield et al. 2000, Hoge et al.
2003). The majority of these farmlands were purchased by the SJRWMD in the late
1990s. The SJRWMD continues to conduct management efforts to restore Lake Apopka
under the Lake Apopka Surface Water Improvement and Management (SWIM) program
(Conrow et al. 1993, Hoge et al. 2003).
The current restoration program strategy, headed by the SJRWMD, is based
primarily on reducing external nutrient loading, focusing primarily on phosphorus (P)
reductions (Hoge et al. 2003). The P criterion proposed by the SJRWMD for the lake is
55 [g/L (Hoge et al. 2003). Other projects of the current restoration program have
included removing gizzard shad (Dorosoma cepedianum) from the lake and planting
macrophytes around the shore in the 1990s. Despite the enormous amount of time and
resources that have been expended trying to improve the water quality in Lake Apopka
and to return the lake to its former clear-water, macrophyte-dominated state, nationally
renowned for its largemouth bass fishery, the lake remains in a turbid algal state
(Bachmann et al. 1999).
There is a lack of unanimity regarding the efficacy of restoration efforts
(Bachmann et al. 1999, 2001a, b, Lowe et al. 1999, 2001, Canfield et al. 2000, Schelske
et al. 2000, Schelske and Kenney 2001). Some authors do not believe that the restoration
program, based primarily on an external P reduction program, will be successful in
improving the water clarity or in expanding the aquatic vegetation needed as habitat for
largemouth bass in the near-future (Bachmann et al. 1999, 2001a, b, Canfield et al. 2000).
Alternative hypotheses regarding the limiting factor responsible for causing the turbidity
of the water, for example wind resuspension of sediments (Bachmann et al. 1999, 2000a,
b), have been put forth but have not yet been acted upon.
Recent management strategies have not included alternative methods and ideas
such as drawdown or artificial barriers that could reduce the turbidity in the water from
resuspended particles and potentially improve the largemouth bass fishery. For example,
a major drawdown could consolidate and compact the sediments (Wegener and Williams
1974, Moyer et al. 1995) if the technical problems, such as the amount of water to be
moved and the length of time that the lake would stay drained, could be resolved (United
States Environmental Protection Agency (USEPA) 1979). Also, placing artificial barriers
around the perimeter of the lake could provide calm, protected waters and improve the
habitat for largemouth bass in those areas (Canfield et al. 2000).
No studies have been conducted to find out if the aquatic plants, which were
planted by the SJRWMD in or before 1999, have expanded in area to determine if the
restoration program has been successful in one of its primary objectives of restoring lake
habitat (Hoge et al. 2003), and to determine if the money that has been expended has
been well spent. For example, more than $100,000,000 in Federal and State funds were
used to purchase farms, with drainage to the lake, to take them out of production. A
marsh flow-way was also constructed to remove phosphorus-rich particles from the lake
water (Conrow et al. 1993, Hoge et al. 2003). A study was needed to assess the response
of the fish and plant communities in Lake Apopka to these restoration activities because
of the controversy over the successfulness of the restoration program.
Previous studies to assess the fish and plant communities in Lake Apopka were
conducted by Canfield and Hoyer (1992) and Johnson and Crumpton (1998). The
objective of this study was to evaluate the effectiveness of the restoration program in
restoring the largemouth bass fishery and the aquatic plant community. This objective
was accomplished by sampling the littoral fish populations and the aquatic macrophyte
community. The criteria, for determining if the restoration program was effective, was
whether or not the largemouth bass population has increased in abundance (catch rate as
number/hr or kg/hr) in the last two decades, or the aquatic macrophyte community, used
as habitat by largemouth bass, has expanded in area (m2 or ha) since 1999. To make this
determination, the abundance and structure of the littoral fish populations and aquatic
macrophyte community, surveyed by this study in 2004, were compared, and tested as
appropriate, to those of previous studies.
Lake County Orange County
Ferndale .": i]
b b.. . . ......b..b..b ..b.. .. .. .. .. .
.. .. .b..b..b..b..b..b..b..b..b..b...b ..b....b..
0 1 2 Kilometers W- -E
I I I s
Figure 1. Lake Apopka and surrounding area (modeled using Hoge et al. 2003, page 14).
MATERIALS AND METHODS
Fish were electrofished in near-shore areas of the main part of Lake Apopka,
excluding the gourd neck area and the adjoining canals. Ten transects were sampled once
monthly in June, July, and August 2004 using a Coffelt Mfg., Inc. control box. Ten
additional transects were sampled in June using a Smith-Root, Inc. control box as part of
a control box comparison study, thus totaling 40 transects. Transects were evenly spaced
around the lake in all habitats. Each transect was 10 minutes in duration, with continuous
current input. Methods were similar to those used by Canfield and Hoyer (1992), Hoyer
(University of Florida (UF), unpublished data), and Johnson and Crumpton (1998).
Fish were collected, identified to species, measured to the nearest mm total length
(TL), and released back into the lake. Individual fish weights were calculated from
measured lengths using total weight to length regression formulas developed for
individual Florida freshwater fish species by Hoyer and Canfield (1994). The total
number and weight of fish were calculated by transect and then averaged across the 40
transects to yield catch per unit electrofishing effort (number or kg/hr of electrofishing)
At each electrofishing transect, the depth and dominant vegetation, if present,
were recorded. Dissolved oxygen (DO) concentration (mg/L), Secchi disc depth (SD)
(m), specific conductance ([iS/cm at 25 C), and water temperature (C) were measured at
four near-shore stations, when the electrofishing was conducted (one station in each of
the north, east, south, and west regions of the lake). Dissolved oxygen concentration,
specific conductance, and water temperature were measured by using a Yellow Springs
Instrument Company Model 85 Meter.
Coffelt Mfg., Inc. control boxes were used in collecting fish in previous studies at
Lake Apopka by Canfield and Hoyer (1992) and by Hoyer (UF, unpublished data).
Coffelt Mfg., Inc. recently went out of business and the only company that now
manufactures electrofishing control boxes is now Smith-Root, Inc. The Florida Fish and
Wildlife Conservation Commission (FFWCC) replaced all of its Coffelt Mfg., Inc.
control boxes with control boxes manufactured by Smith-Root, Inc. in the 1990s, but the
UF, Department of Fisheries and Aquatic Sciences (FAS) uses both brands of control
boxes. Because the catch rate of this study (which used both brands of control boxes)
was compared to previous studies by Canfield and Hoyer (1992), Hoyer (UF,
unpublished data), and Johnson and Crumpton (1998) that did not all use the same brand
of control box, it was important to conduct a comparison study to account for any
possible differences that might be due to using different control boxes.
A comparison study was conducted in June 2004 by using control boxes
manufactured by Smith-Root, Inc. and Coffelt Mfg., Inc. to simultaneously collect fish in
10 paired transects in similar habitat. The data from paired transects were used to test for
differences in catch rate between the two control boxes using a paired t test. Each boat
started from the middle of the habitat, 10 m apart, and worked away from each other to
the ends of the habitat. The two boats used in the comparison study were set up
identically except for the control boxes. Both boats were 5 m in length, made of
aluminum, powered by 50-hp Mercury motors, and equipped with 5000-watt Honda
EG5000X generators (Table 1). Both boats had one boom with an attached 62-cm
diameter ring, with six 1-cm diameter, 1.2-m long, stainless steel cables in place, for
transmitting electricity into the water. Both control boxes were set at 7-9 amps, 180-190
volts, 60-80 pulses per second, and (pulsed) alternating current output (Table 2). All
persons participating in the study had prior experience in electrofishing. One person, in
each boat, dipped fish.
Although the equipment and sampling design used in this study were similar to
previous studies on Lake Apopka, there were some differences. Johnson and Crumpton
(1998) was the only study that used two electric booms. A study was not conducted to
determine if having two booms would result in collecting more fish than using only one
boom. Additionally, no studies were found in the literature that compared the efficiency
of using two booms versus one boom.
Johnson and Crumpton (1998) had the most samples (870 transects in 1989 -
1993, and 150 transects in 1996 1997) (Table 3). They collected fish in 5-min transects
using random sampling in 1989 1993, and both random and selected sampling in 1996
- 1997. All the other studies collected fish in 10-min transects using evenly distributed
sampling. Hoyer (UF, unpublished data) sampled only the northern half of the lake,
whereas, all of the other studies sampled the entire lake.
Dominant plant species were recorded at each of the evenly spaced, electrofishing
transects during each sampling occasion. Four fathometer transects (north to south, west
to east, northwest to southeast, and southwest to northeast) were run across the entire
diameter (-17 km) of the lake in August 2004 to record the location and amount, if any,
of submersed vegetation in Lake Apopka. The procedures for using a recording
fathometer and determining quantitative vegetation parameters have been described
previously (Maceina and Shireman 1980). The widths of floating-leaved and emergent
plant zones (m) were estimated by eye at 30 transects evenly spaced around the lake in
October 2004. Estimating fixed distances was practiced just prior to sampling and
consistent methods were used to make and record estimations of distance. Dissolved
oxygen concentration and SD were measured at four near-shore stations, on the date that
vegetation was sampled (one station in each of the north, east, south, and west regions of
The 16 largest beds of eel-grass, reported by the SJRWMD (Conrow and Peterson
2000), were sampled in October 2004. Eel-grass was the only submersed macrophyte
sampled in this study because it was the only submersed macrophyte reported to be
abundant (- 1 ha) in the lake besides hydrilla (Hydrilla verticillata, < 0.5 ha) (Conrow
and Peterson 2000). Hydrilla was not sampled during this study because contact
herbicide has been applied regularly by the SJRWMD to keep it from expanding (Erich
Marzolf, environmental scientist, SJRWMD, Palatka, FL, personal communication). The
location and abundance of hydrilla also changes frequently.
All sampled eel-grass beds were located on the west side of the lake, with a few
exceptions. Only a few small beds were reported on the north, east, or south sides of the
lake (Conrow and Peterson 2000). Seven additional sites were sampled in the north (3
sites), east (1 site), and south (3 sites) regions of the lake, even though they were small
(Conrow and Peterson 2000), so that the entire lake would be represented by the locations
that were chosen. Sites were found using GPS coordinates obtained from the SJRWMD.
At each site, the area (m2) of the plant bed was estimated by eye from the boat for
all but the largest beds. Estimating fixed distances was practiced just prior to sampling
and consistent methods were used to make and record estimations of distance. Plant
beds, over 100 m2 in area, were walked around while simultaneously measuring their area
(m2) with a handheld GPS receiver. The percent density (from 0 to 100% for sparse to
dense aggregations) was estimated by eye. The type of soil (sand or some degree of
silt/mud) was determined by pushing on the substrate with a rod. The maximum water
depth (m) that the plant bed was growing in was determined by lowering a Secchi disk
until it rested on the substrate and measuring the vertical distance (m) from the substrate
to the water surface with the attached cord.
Light attenuation in Lake Apopka was measured (in quanta units) using a
photometer (LI-COR model LI-1400 data logger) attached to an underwater quantum
sensor (LI-192SA) and a spherical quantum sensor (LI-193SA). Three samples, spaced
15 m apart, were taken at the center of the lake on three sampling occasions (once
monthly in June, August, and October 2004) for a total of nine samples. Irradiance
measurements were taken at the center of the lake because of depth limitations near
shore. A sample consisted of taking a series of downwelling irradiation measurements
every 0.2 m into the water column starting from the surface until either depth or light
became limited (as seen by negative irradiance measurements), whichever was reached
first. Measurements were taken with the spherical quantum sensor laying facing up on
the deck to measure ambient atmospheric irradiance and with the underwater sensor
attached to a graduated rod that was held straight out and away from the sampler's body
on the sunny side of the boat with the rod aligned vertically.
The maximum depth of colonization (MDC) is the maximum depth at which an
aquatic macrophyte species is capable of colonizing. Mittelboe and Markager (1997)
stated that the MDC was defined by the deepest growing plant observed. Since MDC is
thought to be primarily limited by irradiance (Chambers and Kalff 1985, Carter et al.
1985), measurements of downwelling irradiance in the water column, and calculations of
the percent of downwelling surface irradiance at depth, can be used to determine the
theoretical MDC (Korschgen et al. 1997). One percent of downwelling surface irradiance
(Io) is generally taken as the theoretical MDC of aquatic macrophytes (Dennison 1987,
Valiela 1995) because it is the compensation depth at which photosynthesis and plant
respiration are equivalent (Korschgen et al. 1997).
Aquatic macrophytes are not always able to colonize down to their theoretical
MDC except under optimal conditions (Spence 1972, Barko and Smart 1981, Chambers
and Kalff 1985, Scheffer et al. 1992, Hudon et al. 2000). The lower depth limit, with
some exceptions, more commonly occurs at light levels between 5 and 10% Io (Sheldon
and Boylen 1977, Barko et al. 1982, Chambers and Kalff 1985, Kimber et al. 1995a, b).
Dennison (1987) stated that the maximum depth limit for freshwater macrophytes was
roughly equivalent to the Secchi disc depth, which is often taken as the 10% light level
The maximum depth of growth (MDG) is defined in this paper as the maximum
depth corresponding to the minimum percent of Io at which an aquatic macrophyte
species predominantly grows down to, as limited by light attenuation (Sheldon and
Boylen 1977, Kenworthy and Fonseca 1996, Nichols 1997, Moore et al. 2003). Other
studies have more strictly defined the MDG as the maximum depth corresponding to the
minimum percent of Io needed for there to be a specified production (number or g) or
elongation (cm) of leaf, meristem, or shoot, or production (g) of biomass, within a
specified period of time (days) (Barko and Smart 1981, Barko et al. 1982, Loreny and
Herderndorf 1982, Dennison 1987, Duarte and Kalff 1987, Sand-Jensen and Madsen
1991, Korschgen et al. 1997, Blanch et al. 1998, Bintz and Nixon 2001, Grimshaw et al.
2002, Nielsen et al. 2002). The theoretical MDG for eel-grass was established in this
study by referencing previous studies on the minimum percent of Io needed for eel-grass
to grow. For example, the maximum depth corresponding to a minimum referenced
percent of lo, needed for eel-grass to grow, was taken as the MDG.
The data from the light meter measurements were used to determine the
theoretical MDC and MDG of eel-grass in Lake Apopka. The theoretical MDC and
MDG were chosen as 1% Io and 8% Io, respectively, because authors in the literature
have reported that eel-grass and other aquatic vascular plants are capable of colonizing
down to depths of 1% Io and that they grow down to about 8% Io (Sheldon and Boylen
1977, Barko et al. 1982, Carter and Rybicki 1985, 1990, Duarte and Kalff 1987,
Goldsborough and Kemp 1988, Sand-Jensen and Madsen 1991, Zimmerman et al. 1994).
For each sample, the ratio of downwelling irradiance at depth (Id) to Io, taken as a percent,
was calculated using Microsoft Office Excel 2003 to determine theoretical MDC and
MDG. For example, the ratio of Id to Io, taken as a percent, yields the percent
downwelling irradiance at a given depth in the water column. Then, the water depth
corresponding to 1% and 8% Io equals the theoretical MDC and MDG, respectively.
The total area (m2) of eel-grass that was found in Lake Apopka during this study
was subtracted from the total area reported by the SJRWMD in 1999 (Conrow and
Peterson 2000) to determine if eel-grass was expanding, remaining constant, or declining.
The observed and theoretical MDC and MDG were compared to determine if the
maximum depth at which eel-grass was found to be growing agrees with the theoretical
depth to which it should be able to grow. The observed MDG used in my analysis was
derived from the maximum depth (m) at which eel-grass was measured to be
predominantly growing. Similarly, the observed MDC used in my analysis was derived
from the maximum depth at which eel-grass was found to be growing at the 23 sites
The area of the lake, that was available for eel-grass to colonize and grow based
on the observed MDC and MDG, was calculated using a bathymetric map (Danek and
Tomlinson 1989). This area was compared to the total area of eel-grass found to
determine the percentage of available area that eel-grass inhabited under the current light
regime. The percentage of the total lake area that eel-grass was theoretically able to
colonize and grow, under the current light regime, was also calculated.
The catch rates of total fish and largemouth bass of this study were compared to
the catch rates of previous studies conducted in 1986 by Canfield and Hoyer (1992); in
1987 1996, excluding 1990, by Hoyer (UF, unpublished data); and in 1989 1993 and
1996 1997 (largemouth bass only) by Johnson and Crumpton (1998). Ttests were
used, where appropriate, to determine if there had been a statistical difference in the
abundance of total fish or individual fish species in the near-shore region of Lake
Apopka. Raw data were available only for Johnson and Crumpton (1998) for 1989 -
1993, and not for 1996 1997. Their report and tables were used to determine the catch
rate of largemouth bass, bluegill (Lepomis macrochirus), and redear sunfish (Lepomis
microlophus) for 1996 1997. Microsoft Office Excel 2003 was used to analyze all data
(the fish and plant data collected in this study, as well as data collected by other
The population of largemouth bass at Lake Apopka, from 1986 to 2004, was
graphically compared to 60 other Florida lakes sampled by Canfield and Hoyer (1992),
by plotting the mean catch per unit effort (CPUE) (kg/hr) of largemouth bass for all of the
lakes against their corresponding mean chla ([tg/L) value. Four separately identified data
points were given for the individual studies that sampled Lake Apopka from 1986 to
2004. Chlorophyll data points, for the individual studies conducted at Lake Apopka,
were the mean yearly chlorophyll values, for the years in which Lake Apopka was
sampled, obtained from the SJRWMD. Chlorophyll data points, for the 60 lakes sampled
by Canfield and Hoyer (1992) were the mean chlorophyll values reported in Canfield and
Hoyer (1992). Data were transformed to their logarithms (base 10) to reduce
heterogeneity of variance. The resulting plot allows comparison of the mean CPUE of
largemouth bass of all the lakes based on their mean chla values.
The efficiency of collecting total fish using control boxes manufactured by
different companies, Smith-Root, Inc. versus Coffelt Mfg, Inc., was tested for significant
differences using a paired t test. The efficiency of collecting two categories of fish was
also tested using paired t tests. Centrarchids (warmouth (Lepomis gulosus), bluegill,
redear sunfish, and largemouth bass) were lumped into one category. Other non-
centrarchids (Florida gar (Lepisosteus platyrhincus), bowfin (Amia calva), gizzard shad,
blue tilapia (Tilapia aurea), yellow bullhead (Ameiurus natalis), brown bullhead
(Ameiurus nebulosus), and channel catfish (Ictaluruspunctatus)) were lumped into a
Table 1. Electrofishing boat setup used by different studies at Lake Apopka, Florida.
Number of Persons
Study Generator Model Number of Booms Dipping Fish
This Study (2004) Honda EG5000X 1 1
Johnson & Crumpton (1998) Honda EG5000X 2 1
Hoyer (Unpublished data) Honda EG5000X 1 1
Canfield & Hoyer (1992) Honda EG5000X 1 1
Table 2. Electrofishing control box settings used by different studies at Lake Apopka,
Control Box Output Output Output Pulse
Study Model Mode Amps Volts Width Frequency
This Study (2004) Coffelt VVP-15 Pulsed AC 7-9 180 50% 80- 100
Smith-Root VI-A Pulsed AC 6-8 177 2 ms -
Johnson & Crumpton (1998) Smith-Root VI-A Pulsed AC or 5-7 177- 2 5 ms -
Pulsed DC 500
Hoyer (Unpublished data) Coffelt VVP-15 Pulsed AC 7-9 180 50% 80 100
Canfield & Hoyer (1992) Coffelt VVP-15 Pulsed AC 7-9 180 50% 80 100
Table 3. Electrofishing methods used by different studies at Lake Apopka, Florida.
Elapsed Time of Each
Month and Years Number of Samples Transect With
Study Sampled (N) Continuous Current Sampling Method Region of Lake
This Study (2004) Jun 2004 20 10 Proportioned by habitat Entire lake
Jul 2004 Aug 2004 20 10 Evenly distributed Entire lake
Johnson & Crumpton (1998) Mar 1989 -Nov 1993b 870 5 Random Entire lake
Nov 1996 Sept 1997c 180 5 75 random, 75 selected Entire lake
Hoyer (Unpublished data) Mar 1987 Jan 1996d 62 10 Evenly distributed Northern half only
Canfield and Hover (1992) Aug 1986 9 10 Evenly distributed Entire lake
aNot including the gourd neck area or canals.
bTwenty-nine months were sampled bimonthly in the odd months of the year.
CSix months were sampled bimonthly in the odd months of the year.
dSampling was conducted only in January, February, or March.
RESULTS AND DISCUSSION
Twenty species of fish were collected in 2004 (Table 4). Canfield and Hoyer
(1992), Hoyer (UF, unpublished data), and Johnson and Crumpton (1998) collected 16,
22, and 26 species, respectively. These differences in species richness are within a range
that would be expected from a stable fish community when temporal and personnel
differences in sampling are accounted for (Reynolds 1983, Hardin and Conner 1992,
Andrus 2000, Ott and Longnecker 2001, Bayley and Austen 2002).
Since Johnson and Crumpton (1998) took the most number of samples (870),
bimonthly, over a five-year period, using natural laws of probabilities (Ott and
Longnecker 2001), it would be expected for them to encounter more rare species than the
other studies that took less samples or took all of their samples in only one or a few
months, or only in a certain season of the year. Also, since Canfield and Hoyer (1992)
only sampled on one date, it is reasonable that they collected the smallest number of
Differences in the manner in which personnel sample can result in changes in the
number and type of fish collected (Reynolds 1983, Hardin and Conner 1992, Andrus
2000, Bayley and Austen 2002). Personnel, that are meticulous in collecting every fish
regardless of size, are likely to collect a greater quantity of smaller, rare species. Also,
misidentification can result in an incorrect number of fish species being recorded.
There was no significant difference in the number of total fish, centrarchids, or
non-centrarchids collected between Smith-Root, Inc. and Coffelt Mfg., Inc. control boxes
(p > 0.05 for all three tests), although there were some interesting findings. When the
Smith-Root, Inc. control box was used, slightly more fish (23) were collected overall for
all 10 combined transects than when the Coffelt Mfg., Inc. control box was used. There
was higher variation in the total number of fish collected with the Coffelt Mfg., Inc.
control box (standard error = 7.1/hr) than with the Smith-Root, Inc. control box (standard
error = 6.1/hr). Also, when the Coffelt Mfg., Inc. control box was used, proportionally
more centrarchid fish were caught than non-centrarchids compared to the Smith-Root,
Inc. control box.
These results may apply only to conditions present at Lake Apopka during the
time of sampling (Bayley and Austen 2002). For example, the high mean conductivity
(453 pS/cm at 25 C) in Lake Apopka at the time of sampling, decreased the efficiency of
collecting fish, while using either brand of control box (Kolz 1989). Reynolds (2000)
stated that conductivity was the single most important environmental factor in
electrofishing. Temperature is also important. It affects the ability of the fish to float,
respond, and escape (Reynolds 2000). Mean water chemistry parameters during
sampling were 5.9 mg/L DO, 0.37 m SD, and 31.6 C water temperature (Table 5).
The total catch rate in 2004 was 156 fish/hr. Mean catch for Canfield and Hoyer
(1992), Hoyer (UF, unpublished data), and Johnson and Crumpton (1998) (1989 1993
data) were 203, 268, and 365 fish/hr, respectively (Figures 2 and 3). Comparison of the
yearly mean total catch rate over time suggests that the mean total abundance of fish has
declined since 1993 in the near-shore region in Lake Apopka (p < 0.05).
The total catch rate of this study was only tested for differences with Johnson and
Crumpton (1998), for the data from 1989 1993, because the data of that time period
were the most representative, of the different studies, of the fish populations in Lake
Apopka, based on the fact that they took the most samples (870 transects), bimonthly,
over an extended period of time (5 years) (Materials and Methods, Table 3). Canfield
and Hoyer (1992) only sampled on one occasion (9 transects). Hoyer (UF, unpublished
data) only sampled the northern half of the lake (62 transects).
Johnson and Crumpton (1998) was the only study that used two electric booms.
Using two booms could help explain why their mean total catch rate was higher than
Canfield and Hoyer (1992) and Hoyer (UF, unpublished data) during the same, or close
to the same, time period. If using two booms enabled Johnson and Crumpton (1998) to
collect more total fish than they would have collected using just one, it would result in a
larger reported difference in the catch rate of total fish over time.
Additionally, the mean total catch rate remained stable or increased from 1986 -
1993 (Figure 3). In 1993, the total catch rate seems to have declined dramatically.
Johnson and Crumpton (1998) stated that the abundance of open water fish species
(gizzard shad, threadfin shad (Dorosomapetenense), blue tilapia, and black crappie
(Pomoxis nigromaculatus)) as well as centrarchids (bluegill, redear sunfish, and
largemouth bass) all declined from 1989 1993 to 1996 1997. It was appropriate,
therefore, to test for a decrease in the total catch rate from 1993 to 2004.
Part of the reason for the decline in the total abundance of fish may be attributed
to the extensive harvesting of gizzard shad. For example, Crumpton and Godwin (1997)
reported that, from January 1993 through June 1997, commercial fishermen harvested
over 5 million gizzard shad weighing about 2.3 million kg from Lake Apopka.
Secondly, adverse environmental conditions present at the time of sampling could
also have reduced the total catch rate in 2004. For example, Hurricane Charley passed
over central Florida on 13 August (National Oceanic and Atmospheric Administration
(NOAA) 2005), just five days prior to the date that fish were sampled on 18 August. A
widespread mud plume was observed in the center of the lake while sampling fish on that
date. A widespread fish kill was also observed across the entire north half of the lake.
Additionally, the mean DO concentration (3.48 mg/L) for the near-shore stations in
August was low.
Lastly, it is possible that the decrease in mean yearly chla from about 97[g/L in
1990 to 59 [g/L in 2004 could have resulted in decreased productivity in the lake as well
as a reduction in the abundance of open water fish species (Bachmann et al. 1996). These
chla values were based on unpublished data obtained from the SJRWMD.
Because it was suspected that the adverse environmental conditions in August
2004 affected the total catch rate, the 2004 total catch rate was divided by month. The
mean total catch rates for June, July, and August were 149, 285, and 44 fish/hr,
respectively. The mean total catch rate in August was much lower than the two previous
months and contributed to the low total catch rate in 2004. Comparison of total catch rate
among all the studies, by using the same months that were sampled in 2004, would have
provided a direct comparison. Unfortunately, none of the previous studies sampled in
the same months (June, July, and August) as this study in 2004 (see Materials and
Methods, Table 3).
Comparison of the 2004 individual fish species percent of total, with Johnson and
Crumpton (1998), for 1989 1993, suggests that there has been a decrease in percent of
total fish for open water fish species (gizzard shad, threadfin shad, black crappie, blue
tilapia, and brown bullhead) over time (Figures 4 and 5). These open water species
composed 11% of the total fish number in 2004, a decrease from 56% of the total number
in 1989 1993. These values represent a species composition change from open water
species being most abundant in the near-shore region in previous years (1989 1993) to
centrarchids being dominant in 2004. A decrease in gizzard shad and threadfin shad
(from 34 to 5% of total fish numbers) contributed the largest proportion of the change.
No dramatic changes in the abundance of rare species were observed.
Centrarchids (warmouth, bluegill, redear sunfish, and largemouth bass) comprised
78% of the fish collected in 2004, an increase from 29% of the fish in 1989 1993
(Figures 4 and 5). The most abundant species in 2004 were bluegill and redear sunfish
with catch rates of 86 and 16 fish/hr, respectively (Table 6). The catch rates for these
species in 1989 1993 were 73 and 18 fish/hr, respectively (Table 7). Comparison of the
mean catch rate over time suggests that bluegill abundance is similar from previous years,
even though there is a statistical difference (p < 0.05).
This difference in the catch rate of bluegill and redear sunfish over time is within
a range expected from a stable fish population, given the natural fluctuations in fish
populations, and the temporal differences in sampling (Swingle 1950, Latta 1975, Ott and
Longnecker 2001). Latta (1975) stated that the abundance of fish populations changes
dynamically in response to changing environmental (e.g., temperature, wind, dissolved
oxygen, competition, predation) and biological (e.g., reproduction, growth, and mortality)
factors. Swingle (1950) stated that fish populations are regulated by the number of
offspring in the present year, and the number remaining in the population from the
previous year. He also stated that fish populations dynamically change depending on the
number and ratio of predator and forage fish present. Ott and Longnecker (2001) stated,
that as time increases between sampling occasions, greater differences in abundance
between samples can be expected. These changes in the abundance of bluegill and redear
sunfish over time are not very big considering all of the possible stochastic environmental
and biological changes over time, the amount of time that has passed between sampling
occasions, and the natural variation in sampling.
The catch rate of small (< 14 cm total length (TL)) bluegill and redear sunfish was
66 and 2.6 fish/hr, respectively, in 2004 (Table 8). The catch rate of large (> 14 cm TL)
bluegill and redear sunfish was 20 and 14 fish/hr, respectively, in 2004. Bluegill and
redear sunfish were dominated by small length classes in 2004, as in the past, based on
summary data taken from Table 5 in Johnson and Crumpton (1998), and presented in this
study (Table 8). Redear sunfish reached slightly larger mean length than bluegill
(Figures 6 and 7). Johnson and Crumpton (1998) reported that the annual mean catch
rate of large (> 14 cm) bluegill and redear sunfish were lower in 1996 1997 than values
from 1989 1993. Note that Johnson and Crumpton (1998) summary data was from
1989 1992 instead of for 1989 1993. The catch rate of bluegill and redear sunfish >
14 cm seems to have rebounded in 2004 to be similar to values reported for 1989 1993
by Johnson and Crumpton (1998).
The lake-wide largemouth bass catch rate in 2004 was 6.9 fish/hr. Canfield and
Hoyer (1992), Hoyer (UF, unpublished data), and Johnson and Crumpton (1998) (for
1989 1993, and 1996 1997) collected 1.3, 0.8, 6.5, and 3.6 fish/hr, respectively
(Figures 8 and 9). Comparison of the yearly mean largemouth bass catch rate over time
shows that the mean catch rate of 6.9 fish/hr in 2004 was not statistically different from
the combined mean catch rate of 6.5 fish/hr in 1989 1993 (p > 0.05), slightly more than
one decade ago. This indicates that the largemouth bass population has not significantly
increased in abundance over the last decade.
The mean largemouth bass catch rate of this study (6.9 fish/hr) was only tested for
an increase in largemouth bass abundance with that of Johnson and Crumpton (1998) for
1989 1993 because mean catch rate of that study was the highest (6.5 fish/hr) of the
previous studies (as well as the fact that their data were most representative of the fish
populations in Lake Apopka, as explained earlier). Testing for an increase in largemouth
bass abundance with Johnson and Crumpton (1998) for 1989 1993, was more
conclusive than testing for an increase with the other studies, which had lower mean
catch rates. It should be noted, however, that if using two booms enabled Johnson and
Crumpton (1998) to collect more largemouth bass than they would have collected using
just one boom, it would result in a smaller reported difference in the catch rate of
largemouth bass over time.
Largemouth bass collected in 2004 ranged in size from 7.1 to 51.6 cm TL (Figure
10), and in calculated weight from 3.4 to 2096 g. There was a low catch rate of small
(<24 cm) largemouth bass, as in previous years (Table 8). The fact that small fish (< 24
cm) are represented shows that reproduction is occurring in Lake Apopka.
Largemouth bass were absent from the 16 23.9 cm TL size group. This
suggests that juveniles may be particularly vulnerable to predation and other sources of
mortality at this size (Beverton and Holt 1957). On a statewide basis, largemouth bass
that average 24 cm TL are about 1 to 2 years old (Porak et al. 1999). The dominant size
group in 2004 was 24 31.9 cm TL. If one-year-old fish are undergoing extremely high
mortality, this size group would logically be a mixture of two-year-old and older fish.
Largemouth bass that are recruited into the larger size groups (> 24 cm) seem to be less
susceptible to mortality as shown by the greater frequency of fish in those size groups.
An alternative explanation for the absence of 16 23.9 cm fish is that fish in Lake
Apopka have rapid growth (Dr. Charles Cichra, professor, UF, personal communication).
Sampling was conducted from June through August while largemouth bass in Apopka
likely spawn from February through April. The small fish could thus be young-of-the
year fish. Rapid growth could result in fish between one and two years old exceeding 24
cm TL by summer. Largemouth bass from Lake Apopka should thus be aged to
determine whether the low abundance of 16 23.9 cm TL largemouth bass is due to high
mortality or rapid growth.
Largemouth bass are reproducing, growing, and recruiting to large size in Lake
Apopka, but on a population basis, their abundance is low (< 7 fish/hr), especially when
compared with other Florida lakes. Many Florida lakes have mean electrofishing catch
rates of more than 20 largemouth bass/hr. For example, Lakes Okahumpka, Miona,
Wales, Clear, Baldwin, and Susannah were reported to have 26, 60, 92, 232, 42, and 129
fish/hr, respectively (Canfield and Hoyer 1992).
Comparison of largemouth bass populations from Lake Apopka and 60 other
Florida lakes, sampled by Canfield and Hoyer (1992), shows that the abundance of
largemouth bass at Lake Apopka in 2004 continues to be lower than expected for its
trophic status as estimated by its chlorophyll content (Figure 11). The relationship
between largemouth bass and trophic status is positive suggesting that eutrophic and
hypereutrophic Florida lakes will have a high abundance of largemouth bass (Bachmann
et al. 1996). Lake Apopka is a hypereutrophic lake and in the absence of other limiting
factors, it would be expected to have a high abundance of largemouth bass. But the
results of this study as well as two out of three other previous studies on Lake Apopka
indicate that the abundance of largemouth bass is low (Figure 11), suggesting that
something is limiting their abundance. The amount of quality habitat is most likely the
limiting factor (Porak et al. 1999).
The first year of life is critical to the survival of largemouth bass. The structure
provided by aquatic vegetation, terrestrial brush, and rockpiles offers age-zero
largemouth bass protection from predators (Aggus and Elliot 1975, Crowder and Cooper
1979, Savino and Stein 1982, Durocher et al. 1984, Dibble and Kilgore 1994). Suitable
prey such as zooplankton and small forage fish are more readily available in aquatic
vegetation and other structure than in open areas (Durocher et al. 1984, Gutreuter and
Anderson 1985, Dibble and Kilgore 1994).
Availability of food of the proper type and size contributes to differential growth
in young largemouth bass (Aggus and Elliot 1975, Shelton et al. 1979, Timmons et al.
1980, Gutreuter and Anderson 1985, Olson 1996). Faster growing juveniles make the
switch to piscivory earlier than slower growing fish (Gutreuter and Anderson 1985,
Olson 1996). Rate of growth is one of the primary determinants of the recruitment of
largemouth bass to stock size (Kramer and Smith 1962, Aggus and Elliot 1975, Olson
1996). A higher percentage of fast growing largemouth bass that reach a large size in
their first winter are recruited than slow growing fish (Kramer and Smith 1962, Davies et
al. 1982, Gutreuter and Anderson 1985).
Expanding cover in Lake Apopka would result in a decrease in mortality of age-
zero largemouth bass due to decreased predation in their critical first year of life.
Additionally, young-of-the-year largemouth bass would be able to obtain more of the
proper type and size of food, thus enabling them to grow faster to maturity. As a result, a
higher percentage of juvenile largemouth bass would likely be recruited to stock size in
Submersed and emergent aquatic macrophytes provide additional benefits to both
adult and young largemouth bass. They provide spawning substrate (Kramer and Smith
1962, Chew 1974) and protect spawning nests from wind and waves (Kramer and Smith
1962, Holcomb et al. 1975a, b). Aquatic macrophytes are an important component of the
ecosystem of a lake (Durocher et al. 1984, Dibble et al 1996). Plants have the ability to
affect water chemistry. Their abundance is inversely related with phytoplankton biomass
as measured by chlorophyll, and is positively related to water clarity as measured by use
of a Secchi disc (Canfield and Hoyer 1992). However, aquatic plants do not increase the
lake-wide clarity of water nor decrease the phytoplankton biomass until there is 30 to
50% coverage (percent volume inhabited (PVI)) (Canfield and Hoyer 1992). A narrow
belt of aquatic plants will not reduce lake-wide phytoplankton biomass. A 15% coverage
(PVI) could help largemouth bass populations, however, by providing critical habitat
needed by age-0 largemouth bass during their first year of life (Canfield and Hoyer
Aquatic plants could be particularly helpful in solving problems associated with
fluid sediments in Lake Apopka. They would help stabilize the bottom by anchoring the
sediments (Carter and Rybicki 1985) and they would reduce turbidity from wind
resuspension of the sediments (Canfield and Hoyer 1992, James and Barko 1994). Plants
would additionally tend to retard large waves by baffling wave action and thus provide
protection for less well-anchored species such as coontail (Ceratophyllum demersum)
(Carter and Rybicki 1985) and reduce shoreline erosion (Canfield and Hoyer 1992).
Previous studies have indicated that for some lakes, the lack of aquatic vegetation
can be detrimental to fish populations. For example, Canfield and Hoyer (1992)
demonstrated in a 60-lake study of Florida lakes that there exists a potential for depressed
fish populations at low levels of aquatic macrophytes (< 15% coverage (PVI)). Porak et
al. (1999) suggested that a lack of aquatic plants was detrimental to the survival of young
largemouth bass due to decreased shelter and food supply.
In large, shallow Florida lakes without macrophytes, such as Lake Apopka, the
shoreline habitat is the only refuge from predation for juvenile largemouth bass (Hoyer
and Canfield (1996a, b). The limited shoreline habitat in Lake Apopka may not be
sufficient for adequate largemouth bass recruitment. The addition of about 15% coverage
(PVI) of aquatic macrophytes or other structure would allow sufficient young-of-the-year
largemouth bass to recruit into adulthood to allow Lake Apopka and other similar lakes to
reach their carrying capacity (Hoyer and Canfield 1996a, b).
Lake Apopka continues to have a low abundance of aquatic macrophytes (< 0.9
percent area covered (PAC) and < 0.4% PVI). The plants only occupied a narrow (mean
= 19.4 m) belt around the shoreline in 2004, with a few exceptions. There were some
isolated stands of soft-stem bulrush (8.5 ha) that existed around the perimeter and in the
northern portion of the lake (Hog Island), as previously noted by Johnson and Crumpton
(1998). Remnant stands of aquatic grass, primarily Egyptian paspalidium (1.5 ha), also
existed in various places around the perimeter of the lake.
Vegetated areas are important largemouth bass habitat. Johnson and Crumpton
(1998) reported that soft-stem bulrush and aquatic grass (Egyptian paspalidium,
maidencane, and torpedograss) sites contained the largest total mean biomass for all four
principle sportfish species (largemouth bass, bluegill, redear sunfish, and black crappie)
in both 1989 1993 and 1996 1997 sampling periods for Lake Apopka. Total mean
biomass for sportfish was lowest in open sites.
The most commonly occurring plants in 2004 were cattail, duck potato (Sagittaria
lancifolia), and pickerelweed (Pontederia cordata) (Table 9). No submersed vegetation
existed in the open area of the lake outside of the narrow belt of emergent aquatic plants.
Essentially, the rooted aquatic vegetation in Lake Apopka has not expanded in the last
two decades and remains similar to what the lake had after the loss of its 80% coverage
of aquatic plants in 1947 (Clugston 1963, Schneider and Little 1969, Holcomb 1977,
Lake Apopka Restoration Council 1986, Johnson and Crumpton 1998).
Eel-grass was the only submersed vascular aquatic macrophyte besides hydrilla
that was observed in 2004. Eel-grass is a versatile submersed aquatic plant that is
capable of surviving in a variety of growing conditions (Jaggers 1994). It can inhabit a
variety of sediment types (sand, silty sand, or mud) (Korschgen and Green 1988, Catling
et al. 1994). It is more shade adapted (Meyer et al. 1943) than several other submersed
aquatic plants (e.g., Myriophyllum spp. and Potamogeton spp.) because of its
physiological adaptability to low (1% Io) light regimes (Titus and Adams 1979, Carter
and Rybicki 1985, Korschgen and Green 1988, Catling et al. 1994, Rybicki and Carter
2002). It is capable of surviving and even flourishing in eutrophic water (Catling et al.
1994, Jaggers 1994). It is also capable of making major comebacks after restoration
(Carter and Rybicki 1990, Kimber et al. 1995a, b).
Studies have reported, however, that eel-grass, grown from tubers, did not survive
in greater than 0.25 m of sediment (Carter et al. 1985, Rybicki and Carter 1986,
Korschgen and Green 1988). Studies also found that there were significant population
declines caused by excessive (numerically undefined) turbidity or highly variable
turbidity (Korschgen and Green 1988, Jaggers 1994). Korschgen and Green (1988) noted
that it was subject to uprooting by tropical storms. For example, Tropical Storm Agnes,
which struck the East Coast in 1972, was cited as a factor in the decrease of eel-grass in
different portions of the Chesapeake Bay (Kerwin et al. 1976, Bayley et al. 1978). Carter
et al. (1985) reported that storm damage was also partially responsible for the decline of
eel-grass in the Potomac River in the 1930s. They indicated that deposition of 0.15 -
0.25 m of sediments from storms were sufficient to completely wipe out populations of
eel-grass, naiad (Najas gracillima and N. guadalupensis), and common elodea (Elodea
canadensis). Years or decades may be required for recovery after exceptionally severe
storms (Carter et al. 1985).
The growth of eel-grass in Lake Apopka has been a special concern to Florida
agencies (Jaggers 1994). It is a valuable plant that is used as refuge and habitat for
invertebrates and fish populations (Hoyer et al. 1996). Eel-grass was planted by the
FFWCC to an unknown degree in the 1990s (William Johnson, administrator, FFWCC,
Eustis, FL, personal communication). It was also planted in plots around the shore of
Lake Apopka by the SJRWMD during drought conditions in 1998 and 1999 (Conrow and
Of the 23 eel-grass sites that were sampled, beds were present at 15 sites (Table
10). Twelve of the sites, where eel-grass was present, were located on the west side of
the lake; two closely spaced sites were located on the south side; and one site was located
on the east side. No plants were found at seven of the sites, and only a single plant was
found at one site. All but two of the 15 beds were small (< 100 m2). There was one
medium (279 m2) bed near Smith Island (on the west shoreline). There was another
medium (536 m2) bed near Monteverde Boat Ramp (also on the west shoreline). No
large (> 1000 m2) plant beds were found during this study.
The density of the plant beds ranged from 1 to 100%. The average density of eel-
grass was estimated to be 50%, at those sites where it was present. Hurricane Jeanne
passed over central Florida on 25 September 2004 (NOAA 2005), one week before eel-
grass was sampled on 1 October 2004. An attempt was made to find eel-grass uprooted
by the recent hurricane, but only a couple of plants were observed to be uprooted.
The total lake bottom colonized by eel-grass in 2004 was approximately 900 m2
(Table 10). The SJRWMD reported an area of 11,032 m2 in 1999 (Conrow and Peterson
2000). Therefore, the observed eel-grass area in 2004 was only 8% of that reported by
the SJRWMD. The drought in 1999 2000, followed by rising waters in 2000 should
have created ideal growing conditions for the expansion of eel-grass. The natural
drawdown would have oxidized and compacted the exposed sediments. Oxidation would
have released nutrients from the soil, stimulating plants to regenerate and expand
(Wegener and Williams 1974, Moyer et al. 1995). Compaction of the soil would have
provided a firmer bottom for rooting and holding plants. However, this study shows that
eel-grass drastically declined during the past five years at Lake Apopka. This suggests
that there is a limiting factor that is preventing eel-grass from surviving and expanding.
Studies in the upper Mississippi River and Chesapeake Bay (Carter and Rybicki
1990, Kimber et al. 1995a, b) reported that eel-grass only grew to 12 15% incident light
(I ) because of turbidity in the water column caused by suspended sediments. Later,
when the turbidity from suspended sediments was reduced through restoration efforts,
eel-grass recolonized those study sites, but only grew down to depths of 5 10% Io.
The observed and theoretical MDC and MDG of eel-grass, in Lake Apopka in
2004, were compared to determine if eel-grass was growing to its potential depth, as
referenced by the primary literature. The observed MDC and MDG of eel-grass in June -
August 2004 were 1.3 and 0.8 m, respectively, at a mean lake surface elevation of 20.3 m
(66.5 ft) National Geodetic Vertical Datum 1929 (NGVD 29). Hydrologic data were
obtained from the SJRWMD (2005). The theoretical MDC and MDG were 1.3 and 1.0 m
based on the mean depths corresponding to light meter measurements of 1 and 8% Io.
Eel-grass, therefore, seems to be growing near its potential MDC and MDG because the
observed values of MDC and MDG agree closely to the theoretical values.
However, the four hurricanes (Charlie, Frances, Ivan, and Jeanne, NOAA 2005),
all of which occurred before my sampling, that passed over Florida in the summer of
2004, dropped > 0.4 m of total precipitation, and undoubtedly influenced the results.
When the hurricanes are accounted for by calculating the observed MDC and MDG at a
lower lake level (20.0 m, NGVD 29), eel-grass would be predominantly growing at 12 -
15% Io and reaching a maximum depth at 5% Io. These results indicate that other
environmental factors are limiting the depth of growth of eel-grass other than light
The area of the lake that is available for eel-grass to colonize and grow at the
observed MDC and MDG (at lake surface elevation 20.3 m, NGVD 29) was calculated to
be 3940 and 848 ha, respectively. As a percent, eel-grass is only inhabiting 0.002% of
the 3940 ha available at MDC and 0.011% of the 848 ha available at MDG under the
current light regime, strongly indicating that other factors are involved in limiting the
growth and survival of eel-grass besides light attenuation.
The 3940 and 848 ha, at MDC and MDG, represents an estimated 32% of the lake
available for eel-grass to colonize and 7% of the lake available to grow in under the
current light regime. This indicates that there is enough irradiance in the water column
for eel-grass to colonize in shallow water around the shoreline of the lake but that the
light conditions in the water column need to significantly improve before eel-grass can be
expected to grow well enough to occupy a larger proportion of the total lake area.
There is a general belief that irradiance primarily controls the depth-distribution
of aquatic macrophytes (Barko and Smart 1981, Chambers and Kalff 1985, Carter and
Rybicki 1990). Studies have recognized, however, that other environmental factors
besides nutrient enrichment and phytoplankton shading are also responsible for limiting
aquatic macrophyte colonization. Chambers and Kalff (1985) stated that wave action and
substrate type also play a major role in determining the distribution and abundance of
aquatic macrophytes. Spence (1972) indicated that turbulence and wave action
influenced aquatic macrophyte colonization. Carter et al. (1985) reported that, other than
phytoplankton, storm pressure, grazing pressure, substrate characteristics, and suspended
sediments are all factors that in different circumstances could be individually or jointly
responsible for limiting the depth-distribution (or spatial extent) of aquatic macrophytes.
Eel-grass in this study was only found at sites having firm sediments and not at
any sites with sediments deeper than 0.25 m. These observations agree with studies that
reported that eel-grass, grown from tubers, did not survive in greater than 0.25 m of
sediment (Carter et al. 1985, Rybicki and Carter 1986, Korschgen and Green 1988). The
depth of sediments (1.5 m mean thickness) covering 90% of Lake Apopka's bottom
(Schneider and Little 1969) is too thick to allow eel-grass to inhabit those areas. The
depth and fluidity of sediments, and wind resuspension of sediments, are probably major
factors that are limiting the growth and survival of eel-grass in Lake Apopka. These
hypotheses are supported by the fact that Lake Apopka is a large, shallow, and nearly
round lake with a long fetch (Bachmann et al. 1999). Because of these combined
attributes, the bottom sediments are often resuspended in the lake (Scheffer et al. 1992,
Bachmann et al. 1999). Carter et al. (1985) stated that where fetches are long, eel-grass
plants are easily uprooted from fine sediments.
Table 4. Common and scientific names of fish collected by individual studies at Lake
Table 5. Water chemistry parameters measured for Lake Apopka, Florida, in June -
Water Quality Parameter Location Samples Average Range
Dissolved oxygen (mg/L) Near shore stations 16 5.9 (0.8 11.9)
Secchi depth (m) Near shore stations &
18 0.37 (0.20 0.49)
center of the lake
Specific conductance Near shore stations 12 453
S/c at 25 C 12 453 (403 506)
([iS/cm at 25 C)
Water temperature (C) Near shore stations 10 31.6 (29.5 33.7)
Table 6. Mean electrofishing catch per unit effort and standard error of fish number
(number/hr) and weight (kg/hr) for Lake Apopka, Florida. Forty 10-min
transects were sampled in June August 2004.
Number Standard Weight Standard
Common Name (number/hr) Error (kg/hr) Error
Florida gar 7.4 1.28 5.6 0.99
Longnose gar 0.6 0.36 0.2 0.11
Bowfin 0.8 0.32 0.9 0.43
Gizzard shad 6.6 2.15 0.8 0.29
Threadfin shad 1.7 0.74 0.0 0.00
Black crappie 6.9 1.30 1.2 0.31
Blue tilapia 0.8 0.38 0.4 0.17
Yellow bullhead 0.2 0.15 0.0 0.03
Brown bullhead 1.7 0.61 0.6 0.22
Channel catfish 0.6 0.47 0.1 0.07
Tadpole madtom 0.5 0.33 0.0 0.00
Seminole killifish 2.7 0.86 0.0 0.01
Eastern mosquitofish 3.5 1.34 0.0 0.00
Brook silverside 0.2 0.15 0.0 0.00
Inland silverside 0.3 0.21 0.0 0.00
Warmouth 12.6 3.85 0.8 0.24
Bluegill 86.4 20.52 4.1 1.05
Redear sunfish 16.4 5.05 1.6 0.47
Largemouth bass 6.9 1.60 4.1 1.11
Atlantic needlefish 0.2 0.15 0.0 0.01
Total 156.5 20.3
Table 7. Mean yearly and combined electrofishing catch per unit effort and standard
error of fish number (number/hr) for Lake Apopka, Florida, by Johnson and
Crumpton (1998). Eight hundred seventy 5-min transects were sampled
bimonthly from March 1989 to November 1993.
Common Name 1989 1990 1991 1992 1993 1989-1993 Errora
Florida gar 3.9 3.6 5.1 6.1 8.3 5.4 0.85
Longnosegar 0.3 0.5 1.7 0.7 1.1 0.9 0.24
Bowfin 0.8 0.7 0.6 0.7 0.8 0.7 0.04
Gizzard shad 101.8 71.8 59.3 43.4 42.5 63.8 10.96
Threadfin shad 49.4 47.1 107.5 56.0 37.3 59.6 12.38
Black crappie 26.2 33.7 34.8 52.3 68.8 43.2 7.71
Blue tilapia 7.1 7.6 8.5 15.3 17.7 11.2 2.18
Golden shiner 9.2 7.9 7.1 5.2 6.5 7.2 0.67
Taillight shiner 2.6 6.8 4.6 6.5 8.2 5.7 0.98
Yellow bullhead 2.6 1.4 1.4 1.8 2.9 2.0 0.31
Brown bullhead 45.0 25.7 24.8 14.4 22.5 26.5 5.04
Channel catfish 0.1 0.0 0.0 0.0 0.0 0.0 0.0
White catfish 0.6 0.9 1.8 3.8 16.5 4.7 3.00
Tadpole madtom 0.0 0.0 0.0 0.1 0.0 0.0 0.01
Seminole killifish 3.6 7.4 12.8 10.8 9.3 8.8 1.57
Eastern mosquitofish 4.2 10.6 7.3 6.1 7.4 7.1 1.05
Sailfin molly 0.1 0.7 0.5 0.3 0.6 0.4 0.12
Pugnose minnow 0.0 0.0 0.1 0.0 0.0 0.0 0.01
Brook silverside 0.0 0.0 0.0 1.9 0.0 0.4 0.37
Inland silverside 12.3 15.7 15.9 6.1 3.5 10.7 2.53
Warmouth 5.6 6.0 4.3 9.5 11.7 7.4 1.37
Bluegill 62.0 67.3 55.3 97.6 83.2 73.1 7.67
Redear sunfish 11.1 18.1 15.3 21.1 24.6 18.1 2.33
Spotted sunfish 0.5 0.2 0.1 0.0 0.0 0.1 0.09
Largemouth bass 5.6 6.5 5.9 7.9 6.2 6.5 0.41
Atlantic needlefish 0.1 0.1 0.1 0.2 0.3 0.2 0.03
Total 356.0 341.0 375.0 369.0 381.0 364.0 7.17
aThe combined catch per unit effort number (number/hr) was averaged by month and then by year
to obtain the standard error.
Table 8. Annual mean electrofishing catch per unit effort number (number/hr) of two
size groups of largemouth bass, bluegill, and redear sunfish collected in Lake
Apopka, Florida, by Johnson and Crumpton (1998) for June May of 1989 -
1990, 1990 1991, and 1991 1992, and November 1996 September 1997
(bimonthly samples), and by this study for June August 2004 (once monthly
Common Name 1989-1990a 1990-1991a 1991-1992a 1996-1997 2004
Largemouth bass (> 24 cm) 4.8 4.8 6.0 2.4 6.0
Largemouth bass (< 24 cm) 2.4 1.2 1.2 1.2 0.9
Bluegill (> 14 cm) 22.8 18.0 20.4 15.6 20.0
Bluegill (< 14 cm) 40.8 54.0 66.0 31.2 66.5
Redear sunfish (> 14 cm) 9.6 9.6 9.6 1.2 13.8
Redear sunfish (< 14 cm) 4.8 6.0 12.0 3.6 2.6
a Summary data from 1989 1992 and 1996 1997 were taken from Johnson and Crumpton
(1998), Table 5, which did not report the catch rate of size groups in 1993.
Table 9. Occurrence of plant species in twenty evenly-spaced transects around Lake
Apopka, Florida, in 2004.
Common Name Scientific Name Percent of Transects
Table 10. Location, area (m2), and maximum depth (m) of eel-grass beds in Lake
Apopka, Florida, in 2004.
Area (m2) Reported by
the SJRWMD in 1999 Area (m2) Measured in Maximum Depth
GPS Locations (unpublished data") 2004 of Plant Bed (m)
Twenty three sites where eel-grass beds were sampled on 1 October 2004
X:-81.66268 Y:28.59395 1465.0 None sighted -
X:-81.66999 Y:28.60700 1277.3 (one plant) 1.0 0.9
X:-81.65463 Y:28.59138 885.3 None sighted -
X:-81.67006 Y:28.60724 517.9 Includedb 0.9
X:-81.65359 Y:28.68176 510.9 None sighted -
X:-81.67829 Y:28.61896 494.4 278.9 0.9
X:-81.67803 Y:28.61925 355.7 29.2 0.9
X:-81.60593 Y:28.56912 333.0 2.6 0.9
X:-81.67317 Y:28.61035 310.5 536.0 1.4
X:-81.54957 Y:28.62397 308.0 8.3 0.5
X:-81.65332 Y:28.68184 281.9 None sighted -
X:-81.68426 Y:28.66171 240.8 None sighted -
X:-81.67196 Y:28.60970 237.4 Includedb 1.4
X:-81.66989 Y:28.60720 201.8 Includedb 0.9
X:-81.67787 Y:28.62014 199.5 1.8 0.9
X:-81.65359 Y:28.68210 174.0 None sighted -
X:-81.60584 Y:28.56924 165.2 Includedb 0.9
X:-81.69429 Y:28.64223 93.5 None sighted -
X:-81.68031 Y:28.62056 55.5 10.5 0.9
X:-81.68025 Y:28.62054 54.5 Includedb 0.9
X:-81.66973 Y:28.60674 41.6 Includedb 0.9
X:-81.58700 Y:28.57731 27.0 None sighted -
X:-81.68030 Y:28.62049 12.1 21.0 0.9
Total 8242.7 889.3
Six sites where eel-grass beds were observed while electrofishing in June August 2004
X:-81.64903 Y:28.58658 1.8 0.9
X:-81.64948 Y:28.58717 1.8 0.9
X:-81.64950 Y:28.58781 1.8 0.9
X:-81.67127 Y:28.60920 1.8 0.9
X:-81.67183 Y:28.60972 1.8 0.9
X:-81.66442 Y:28.59677 1.8 0.9
aThe data shown here for 23 sampling sites are a subset of the unpublished data set obtained from the
SJRWMD. The total area for all the eel-grass beds in the data set is 11032.1 m2, which includes only the
area of the plant beds in the major part of the lake and not the area of those in the gourd neck area, Marsh
Flow-Way, or canals.
b The SJRWMD reported beds that were near each other as separate beds, identified with separate GPS
coordinates. At some of the sites reported by the SJRWMD to have multiple beds, only one bed was found,
or in some instances the boundary separating multiple beds was not distinguishable. In these instances, the
total area (m2) of closely spaced bed locations was recorded only once at the GPS coordinates of one bed.
The area (m2) of any nearby beds was annotated as included.
"The six eel-grass beds observed while electrofishing were nearly identical in area and maximum depth.
Figure 2. Mean electrofishing catch per unit effort estimates of total fish abundance
(number/hr) for Lake Apopka, Florida, sampled by Canfield and Hoyer (1992)
(*), Hoyer (UF, unpublished data) (m), Johnson and Crumpton (1998) (A),
and this study (e). Data were not available for Johnson and Crumpton (1998)
for 1996 1997.
data) (for 1987 -
(for 1989 1993)
This Study (for
Figure 3. Mean electrofishing catch per unit effort estimates of total fish abundance
(number/hr) for the combined data of different studies for Lake Apopka,
Figure 4. Individual fish species percent of total number collected by electrofishing for
Lake Apopka, Florida, by Johnson and Crumpton (1998) for 1989 1993.
Figure 5. Individual fish species percent of total number collected by electrofishing for
Lake Apopka, Florida, in June August 2004.
0.0-3.9 4.0-7.9 8.0-11.9 12.0-15.9 16.0-19.9 20.0-23.9 24.0-27.9
Total Length (cm)
Figure 6. Length frequency distribution of bluegill collected
Apopka, Florida, in June August 2004.
by electrofishing for Lake
0.0-3.9 4.0-7.9 8.0-11.9 12.0-15.9 16.0-19.9 20.0-23.9
Total Length (cm)
Figure 7. Length frequency distribution of redear sunfish collected by electrofishing for
Lake Apopka, Florida, in June August 2004.
Figure 8. Mean electrofishing catch per unit effort estimates of largemouth bass
abundance (number/hr) for Lake Apopka, Florida, sampled by Canfield and
Hoyer (1992) (+), Hoyer (UF, unpublished data) (m), Johnson and Crumpton
(1998) (A), and this study (e).
SCanfield and Hoyer Johnson and Johnson and This Study
Hoyer (1992) (unpubl. data) Crumpton Crumpton (for 2004)
(for 1986) (for 1987- (1998) (for (1998) (for
1989, 1991- 1989- 1993) 1996-1997)
Figure 9. Mean electrofishing catch per unit effort estimates of largemouth bass
abundance (number/hr) for the data sets of combined years from different
studies for Lake Apopka, Florida.
0.0-7.9 8.0-15.9 16.0-23.9 24.0-31.9 32.0-39.9 40.0-47.9 48.0-53.9
Total Length (cm)
Figure 10. Length frequency distribution of largemouth bass collected by electrofishing
for Lake Apopka, Florida, in June August 2004.
0 .2 *
C S* *
0.0 0.5 1.0 1.5 2.0 2.5
Log Chlorophyll a (ug/L)
Figure 11. Electrofishing catch per unit effort of largemouth bass (kg/hr) versus total
chlorophyll a ([tg/L) for 60 Florida lakes sampled by Canfield and Hoyer
(1992) and Lake Apopka, Florida, sampled by Canfield and Hoyer (1992) (0),
Hoyer (UF, unpublished data) (o), Johnson and Crumpton (1998) (A), and this
study (o). A regression line is included.
study (o). A regression line is included.
LAKE MANAGEMENT RECOMMENDATIONS
Restoration efforts have not yet been successful in increasing the largemouth bass
population or expanding the area occupied by aquatic vegetation in Lake Apopka. It is
up to lake managers to come up with ideas to improve the largemouth bass fishery so that
anglers can enjoy recreational fishing in the lake. A drawdown was proposed by the
USEPA as part of its environmental impact statement in 1978 as a way to improve the
largemouth bass fishery (USEPA 1979). Downdraw has been a successful tool by
increasing the water clarity and littoral vegetation in several Florida lakes including
Lakes Tohopekaliga, Trafford, Hancock, and Griffin (USEPA 1979). However, a
drawdown was not recommended by the SJRWMD for Lake Apopka for several reasons
including the high cost (of at least $20 million) and increased nutrient flow to
downstream lakes (Lowe et al. 1992). Additionally, there were concerns over the
quantity of water to be pumped, the amount of unconsolidated muck, the type of
vegetation that would grow, and the restrictive time schedule (USEPA 1979).
Conditions in Lake Apopka have changed since 1979. Over $100,000,000 in
Federal and State funds was spent to buy out farms, to reduce or eliminate nutrient-rich
pumpage back into the lake, and to construct a marsh flow-way, to remove phosphorus-
rich particles from the lake water (Conrow et al. 1993, Hoge et al. 2003). Mean
phosphorus levels dropped from over 200 [g/L, in 1979, to about 100 [g/L, in 2004. The
Lake County Water Authority also recently proposed building an alum treatment plant for
Lake Apopka that would help reduce phosphorus levels even more. In light of these
improvements, it would be a good idea to reconsider drawdown. The concern over
increased nutrient flow to downstream lakes is no longer as big of an issue in 2004, as it
was in 1979, because nutrient levels are lower now, than they were before.
Since a drawdown was not approved and may never be approved, the next step for
lake managers is to look for other viable alternative strategies that could potentially
improve the largemouth bass fishery in Lake Apopka. Other alternative ideas have been
proposed that could potentially improve the habitat for largemouth bass in Lake Apopka,
but have not been acted upon by the state agencies. For example, artificial reefs could be
placed along the shoreline as habitat for largemouth bass and other littoral fish (Canfield
et al. 2000). Canfield et al. (2000) also suggested allowing hydrilla, a fast-growing exotic
submersed aquatic plant, to grow behind the protective barriers to provide habitat for
largemouth bass. If at least 50% coverage (PVI) of submersed aquatic vegetation
occurred, it would be enough for the water to start to clear in those local areas and
provide additional benefits (Canfield and Hoyer 1992).
Hydrilla is an exotic aquatic plant that has invaded many lakes. It will often
outcompete native aquatic macrophyte species and expand to fill in an entire water body,
and interfere with navigation and other uses of a water body (Hoyer et al. 2005). Control
of hydrilla across lakes has been unsuccessful (Hoyer et al. 2005). It spreads from one
water body to another when fragments and tubers, left attached to vehicles, trailers, or
propellers, are transported from one water body to another. It also spreads when
fragments and tubers, from one water body, drift into another attached water body. It
costs a tremendous amount of money, to purchase herbicides and equipment, and to
provide manpower to control it. Allowing hydrilla to grow anywhere in Florida lakes,
such as suggested by Canfield et al. (2000), is controversial. However, Lake Apopka
historically had about 80% aquatic macrophyte coverage (PVI). So it should not be a big
issue if hydrilla were to expand into the open area of this lake.
Recent studies have stated that in certain situations, native aquatic macrophytes
will grow in close association with hydrilla. For example, Rybicki and Carter (2002)
found populations of eel-grass, Eurasian water milfoil (Myriophyllum spicatum), and
coontail growing within hydrilla and along the edges, in portions of the Potomac River
recovering from poor water transparency and high turbidity. Smart (1992), in
experimental plots, that were monitored for a duration of one year, showed that Illinois
pondweed, eel-grass, and hydrilla, that were planted in randomly assigned plots, could
grow together for one year, as long as Illinois pondweed and eel-grass were planted
before or at the same time as hydrilla. Illinois pondweed and hydrilla grew in the canopy
and eelgrass grew in the understory.
Smart (1992) stated that eel-grass is considered to be a later successional species
than hydrilla. In the long run, eel-grass should theoretically be able to recolonize areas
that were formerly dominated by hydrilla. There is evidence in Florida lakes that native
aquatic macrophyte species recolonized areas that were dominated by hydrilla for several
years. For example, Lake Okahumpka, Florida, was dominated by hydrilla for several
years, in the late 1990s and early 2000s (Mark Hoyer, research manager, UF, personal
communication). A regular maintenance control program, in the early 2000s, using
contact herbicides, put hydrilla into submission. Native submersed aquatic macrophyte
species (e.g., Illinois pondweed and eel-grass) recolonized areas previously dominated by
Hydrilla is already present in small quantities in Lake Apopka. Erich Marzolf
(environmental scientist, SJRWMD, Palatka, FL, personal communication) stated that
contact herbicide is applied regularly by the SJRWMD to keep it from expanding. Eel-
grass and hydrilla beds were observed near each other during this study at Lake Apopka
in 2004. Eel-grass plants were also observed growing within hydrilla beds and along the
Evidence from Lake Okahumpka, Florida (a hypereutrophic lake similar to Lake
Apopka), and from Lake Apopka itself, suggests that if hydrilla were allowed to grow in
small quantities (< 15% PVI) around the periphery of Lake Apopka, to provide habitat
for largemouth bass, that no more than the already existing maintenance control program
using contact herbicides would be required to keep hydrilla in check, and that native
submersed aquatic macrophyte species (e.g., Illinois pondweed and eel-grass) could
coexist with hydrilla.
Erich Marzolf also raised concerns about not being able to control hydrilla and it
spreading to downstream lakes, if it were allowed to grow. Hydrilla is already in the
lakes that are downstream of Lake Apopka. Maintenance control of hydrilla, with
contact herbicides, is already needed as well, in those lakes. Lakes Beauclair and Dora,
the nearest downstream lakes from Lake Apopka, are separated from Lake Apopka by the
Apopka-Beauclair Canal that is about 48 km long, and about 10 m average width.
Should hydrilla be allowed to grow in limited areas in Lake Apopka, the likelihood of
invasion by hydrilla, by fragments and tubers drifting to those downstream lakes would
be reduced to minimal risk because the Apopka-Beauclair Canal is a long and narrow
canal. Additional expenditures might be needed to control hydrilla in Lake Apopka and
in downstream lakes, if hydrilla were allowed to grow in limited areas in Lake Apopka.
It might be appropriate for lake managers to perform a cost assessment to balance the
income from a largemouth bass fishery to the surrounding community against the
additional expenditures needed to control hydrilla.
Hypereutrophic lakes function differently than other lakes. Authors have
suggested in recent years that hydrilla can benefit hypereutrophic lakes such as Lake
Apopka where both light attenuation and deep sediments prevent native vegetation from
colonizing major parts of the lake (Moxley and Langford 1982, Canfield and Hoyer 1992,
Porak et al. 1999, Hoyer et al. 2005) by providing habitat, food resources, and refugia for
fish and wildlife (Aggus and Elliot 1975, Durocher et al. 1984, Dibble and Kilgore 1994,
Dibble et al. 1996, Porak et al. 1999). Canfield et al. (2000) suggested that the
combination of light attenuation and wind resuspension of sediments in Lake Apopka
should prevent hydrilla from filling in the open area of the lake in the same way that
native plants are now being prevented from colonizing that area. In a hydrilla workshop
(Hoyer et al. 2005), it was recommended for hypereutrophic lakes like Lake Apopka that
a regular control program such as the program now in affect in Lake Apopka, using
contact herbicides to control hydrilla, in keeping with the management objectives aimed
at improving the sports fishery, may represent a more practical and viable alternative than
completely eliminating hydrilla.
There is controversy between aquatic weed specialists, that recommend not letting
hydrilla grow in any Florida lakes, and fishermen and UF biologists that say that a small
amount (<15% coverage (PVI)) of hydrilla should improve the largemouth bass fishery.
Perhaps allowing hydrilla to grow in limited areas in Lake Apopka should be considered
again, despite the debate.
A key lake management action should be to balance the needs of anglers with
other members of the community that enjoy looking at or boating in an open lake
(Canfield et al. 2000). If the entire lake were to be completely covered with aquatic
macrophytes, largemouth bass anglers would enjoy fishing there, but the general public
would likely perceive the profuse growth of aquatic macrophytes as an aquatic weed
problem. If lake managers allowed the lake to have about 15% coverage (PVI) of rooted
aquatic vegetation, they could make anglers happy by improving the sport fishing and
also keep other members of the community happy by keeping most of the lake open
(Canfield et al. 2000).
If the expansion of hydrilla is not to be allowed, then another alternative to
mitigate the loss of largemouth bass would be stocking. Management efforts have been
conducted at Lake Apopka for several decades to improve the environmental conditions
in the lake (Conrow et al. 1993, Hoge et al. 2003). In lieu of the recent improvements, it
is a good time to stock largemouth bass into Lake Apopka.
Previous stocking by the FFWCC in 1990, using small fry, did not result in
adequate survival to produce a fishery (Johnson and Crumpton 1998). Because that
stocking was not successful in producing a fishery, the FFWCC could use fingerlings or
advance fingerlings. The use of advanced fingerlings to stock Lake Talquin, Florida, has
been more successful than stocking projects at Lake Apopka, which used smaller fish
(Dr. Daniel E. Canfield, Jr., professor, UF, personal communication).
Stocking larger (> 24 cm) wild largemouth bass into Lake Griffin enhanced the
economic activity in the surrounding community (Canfield et al. 2005). If fingerlings or
advanced fingerlings were stocked into Lake Apopka, and fishermen caught them,
anglers would come back. It could improve the economic viability of the largemouth
bass fishery in Lake Apopka. Of course, it would be more viable to stock largemouth
bass into Lake Apopka once protected, vegetated areas were established. The additional
plants would also make it easier for anglers to locate the fish.
LIST OF REFERENCES
Aggus, L. R. and G. V. Elliott. 1975. Effects of cover and food on year-class strength of
largemouth bass. P. 317-322. In R. H. Stroud and H. Clepper (eds.). Black bass
biology and management. Sport Fishing Institute, Wash., DC.
Andrus, C. 2000. Experiences from the field: alcove sampling on the Willamette River,
OR. P. 43-46. In S. M. Allen-Gil (ed.). New perspectives in electrofishing.
EPA/600/R-99/108. USEPA, Wash., DC.
Bachmann, R. W., B. L. Jones, D. D. Fox, M. V. Hoyer, L. A. Bull and D. E. Canfield, Jr.
1996. Relations between trophic state indicators and fish in Florida (U.S.A.) lakes.
Can. J. Aquat. Sci. 53:842-855.
Bachmann, R. W., M. V. Hoyer and D. E. Canfield, Jr. 1999. The restoration of Lake
Apopka in relation to alternative stable states. Hydrobiol. 394:219-232.
Bachmann, R. W., M. V. Hoyer and D. E. Canfield, Jr. 2000a. The potential for wave
disturbance in shallow Florida lakes. Lake and Reserv. Manage. 16(4):281-291.
Bachmann, R. W., M. V. Hoyer and D. E. Canfield, Jr. 2000b. Internal heterotrophy
following the switch from macrophytes to algae in Lake Apopka, Florida.
Bachmann, R. W., M. V. Hoyer and D. E. Canfield, Jr. 2001a. Evaluation of recent
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Stephen J. Murphy earned an Associate of Arts degree at Valencia Community
College in Orlando, Florida, in 2001. He went on to earn a Bachelor of Science degree
with a major in wildlife, ecology, and conservation (WEC) at the UF, College of
Agricultural and Life Sciences (CALS) in August 2003. He also completed the CALS
upper division honors program in August 2003.