• TABLE OF CONTENTS
HIDE
 Front Cover
 Title Page
 Introduction
 Techniques for estimating fish...
 Trophic state and total fish...
 Trophic state and sportfish...
 Trophic state and species...
 Trophic state and some common Florida...
 Implications for the management...
 Other relevant research and...
 Notes
 Back Cover






Group Title: Information circular - University of Florida LAKEWATCH ; 110
Title: Beginner's guide to water management : fish communities and trophic state in Florida lakes
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Title: Beginner's guide to water management : fish communities and trophic state in Florida lakes
Series Title: Information circular - University of Florida LAKEWATCH ; 110
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Language: English
Creator: LAKEWATCH
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Publication Date: 2007
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Table of Contents
    Front Cover
        Front Cover
    Title Page
        Title Page
    Introduction
        Page i
        Page ii
        Page iii
        Page iv
    Techniques for estimating fish populations in Florida lakes
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
    Trophic state and total fish biomass
        Page 11
        Page 12
        Page 13
        Page 14
    Trophic state and sportfish biomass
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
    Trophic state and species richness
        Page 20
        Page 21
    Trophic state and some common Florida fish species
        Page 22
    Implications for the management of Florida lakes
        Page 23
        Page 24
    Other relevant research and information
        Page 25
        Page 26
    Notes
        Page 27
        Page 28
        Page 29
    Back Cover
        Page 30
Full Text






A Beginner's Guide to Water Management

Fish Communities and Trophic State in Florida Lakes

Information Circular 110


Bowfin
(Amia calva)


Florida LAKEWATCH
Department of Fisheries and Aquatic Sciences
Institute of Food and Agricultural Sciences
University of Florida
Gainesville, Florida
April 2007
UF UNIVERSITY of
UF FLORIDA
IFAS


LA WATCH


X0









A Beginner's Guide to Water Management in Florida Lakes
Fish Communities and Tropic Status
Information Circular 110


Florida LAKEWATCH
Department of Fisheries and Aquatic Sciences
Institute of Food and Agricultural Sciences
University of Florida
Gainesville, Florida

April 2007

UF/IFAS Communications
This publication was produced by Florida LAKEWATCH
University of Florida/Institute of Food and Agricultural Sciences
Department of Fisheries and Aquatic Sciences
7922 NW 71st Street
Gainesville, FL 32653-3071
Phone: (352) 392-4817
Fax: (352) 392-4902

T message line: 1-800-525-3928
E-mail: fl-lakewatch@ufl.edu
Web address: http://lakewatch.ifas.ufl.edu

Copies are available for download from the Florida LAKEWATCH Web site:
http://lakewatch.ifas.ufl.edu/LWcirc.html
or from the
UF/IFAS Electronic Document Information Source (EDIS) Web site:
http://edis.ifas.ufl.edu

Limited reproduction of and/or quotation from this circular is permitted, providing proper
credit is given.

Color illustrations courtesy of the Florida Fish and Wildlife Conservation Commission

Editing and Design: Allison Slavick, www.allisonslavick.com



As always, we welcome your questions and comments.


Copyright 2007

















hen the issue of nutrient enrichment
eutrophicationn) arises at a lake, the
public often fears that increases in
nutrient concentrations will ultimately lead to the
demise of the lake's fish community. This fear is
based in part on past experiences in northern lakes
where nutrient enrichment has been associated
with the loss of fish for two different reasons. First,
many of the lakes where eutrophication has been
cited as the cause for changes in fish communities
are deep, stratified (warmer water layered on top
of colder water) lakes in the northern regions of
the United States, where cold-water fish such as
salmon or trout have been reduced or eliminated.
Increases in nutrients cause increases in algae cells
that often sink into the hypolimnion (the deep cold
water portion of the lake below the thermocline),
where bacteria use oxygen to digest the cells.
Trout and salmon and other coldwater species
require highly oxygenated colder water to survive.
Thus, when the hypolimnion loses oxygen due to
eutrophication, trout and salmon are forced into
upper waters where it is too warm for them to
survive. Therefore, loss of dissolved oxygen in the
hypolimnion of stratified lakes, which is important
habitat, causes a reduction of these species offish.

Secondly, northern nutrient-rich lakes with
thick ice cover often undergo winterkill. During
winter, after oxygen in the water is consumed by
fish, aquatic plants, bacteria and other aquatic
organisms, fish often die. The ice that covers the
lake's surface prevents oxygen from entering the
lake and snow on the surface of the ice prevents
light from entering the lake for plants to use
for photosynthesis, which produces oxygen.
Eutrophication has also been cited as a cause for
the decline in fish species richness.

Florida lakes, however, are shallow, do not
have cold hypolimnia, and do not support cold-
water species such as salmon and trout. As you
might expect, fish species in Florida are well


adapted to shallow, warm water. Ice cover on
lakes in Florida is a rare occurrence, eliminating
the possibility of fish-kills under the ice, which
occasionally happens in some northern lakes during
the winter. Even though these two situations do not
occur in Florida, eutrophication is still a concern
because of the state's rapid population growth as
people flee the cold north to live in sunny Florida.
This growing human population brings changes in
land use that may increase nutrient inputs to many
lakes. The impact of nutrient concentrations on
algal populations (Florida LAKEWATCH Circular
102) and that of algal communities on water
transparency (Florida LAKEWATCH Circular 103)
in Florida lakes are well documented. However,
there is less information available to the public on
the effects of eutrophication on fish populations in
Florida's lakes.

Prior to 1947, Lake Apopka was covered
with aquatic plants maintaining clear water and
an extensive largemouth bass fishery. After 1947,
the lake switched to an algal-dominant system
with turbid water and the largemouth bass fishery
collapsed. The blame for this was placed on
nutrient additions to Lake Apopka from agricultural
activities. Now, Lake Apopka has become Florida's
"poster child" for the potential adverse effects of
nutrient enrichment because it has lost its major
largemouth bass fishery. Because of this, Apopka
is also the target of a massive and expensive
restoration program by the St. Johns River Water
Management District. In fact, the Florida media
once described Lake Apopka as a dead lake, which
contributed to the effort to restore Lake Apopka.
Because of what has occurred at Lake Apopka,
there is special interest concerning eutrophication
that is occurring at Lake Okeechobee, a large,
shallow, eutrophic lake in south Florida that
supports a major recreational fishery. Annual
total phosphorus concentrations in the lake
have increased from 49 gg/L to about 200 gg/L
from 1974 to 2006, and many people fear Lake


Introduction









Okeechobee is headed the way of Lake Apopka.
A 1980 study conducted by the Florida Game and
Freshwater Fish Commission (now the Fish and
Wildlife Conservation Commission) support this
fear because it suggests that sportfish populations
reach maximum biomass and optimal densities in
mesotrophic to eutrophic lakes, but suffer adverse
effects at higher levels of biological productivity. It
is natural that many Floridians are concerned that
the loss of recreational fishing at Lake Apopka will
be the same fate for Lake Okeechobee and for the
local lakes they live on and/or fish!

General ecological principles suggest that, all
other things being equal, an increase in productivity
at the base of the food chain in a lake should lead
to an increase in the abundance offish at higher
trophic levels (see the trophic status sidebar on
page iv). There are many quantitative fisheries
studies that support this. Fish yields in northern
lakes have been positively related to summer
phytoplankton standing crops (the weight of algae
that can be sampled from a given volume of water)
as measured by chlorophyll or annual primary
productivity. Studies of tropical lakes in Africa
and India also found that fish yields increased with
primary production. The overwhelming evidence
from the studies of lakes outside of Florida is
that as lakes become more eutrophic the standing
crops (the weight offish that can be sampled
from a given area), productivity, and yields offish
increase. Furthermore, people who raise fish for a
living aquaculturalists fertilize their waters and
feed to increase fish production; even recreational
pond owners will fertilize and feed their ponds to
have more fish (see University of Florida/IFAS
Fact Fheet FA-13 for more information).

The increase in fish biomass with nutrient
enrichment has been cited by some professional
fisheries biologists as one of the positive
consequences of eutrophication. Yields of sportfish
to angler harvest in the United States have also
been related to phytoplankton standing crop as
measured by chlorophyll. This relationship is the
basis for many fish management agencies, such
as the Florida Fish and Wildlife Conservation
Commission (FFWCC), to intentionally fertilize
some lakes. In Florida, the FFWCC fertilizes Bear


Lake in Santa Rosa County and Karick Lake in
Okaloosa County. In south Florida, nutrient-rich
runoff from agricultural lands is now diverted
into impoundments for the purpose of protecting
Florida's natural waters from eutrophication, yet
these impoundments such as the Stick Marsh in
Indian River County have developed nationally
recognized largemouth bass fisheries. Even
with all of this evidence, people consistently
make the statement that eutrophication will
hurt fish populations. So why does the myth of
eutrophication causing dead lakes persist?

In light of such popular misconceptions
surrounding fish populations and nutrient
enrichment, one thing is clear all Florida
residents and visitors stand to benefit from a greater
understanding of the fish populations in lakes of
different trophic state (productivity) in Florida.
The relationships between fish populations and
trophic state discussed in this circular are based on
a study of many Florida lakes of varying trophic
state rather than on an individual lake undergoing
eutrophication over time. This approach is taken
because there are virtually no long-term studies
offish populations in Florida lakes undergoing
eutrophication. Before you begin, however, we
encourage you to read A Beginner's Guide to
Water Management Nutrients (Circular #102)
and A Beginner's Guide to Water Management
-Water Clarity (Circular #103). It might also be
useful to peruse A Beginner's Guide to Water
Management The ABCs (Circular #101) to
become acquainted with the meaning of some
commonly used words. These publications
can be downloaded for free from the Florida
LAKEWATCH web site at http://lakewatch.ifas.ufl.
edu. 6


Bluegill
(Lepomis macrochirus)











Lake Stratification and Temperature Profiles





Epilimnion
-5

,O -10
Metalimnion
S- -15

O -20
aL
0 -25
I
Hypolimnion i
O -30
S-----Florida
S -0- Iowa -35


5 10 15 20 25 30 35
Temperature (oC)



Figure 1 above compares the relationship between lake depth and temperature
for a lake in Iowa with a lake in Florida. Both temperature profiles shown in the
graph were taken in August.

As illustrated in the graph, the upper-most layer of warmer water is called the
epilimnion. The deeper, relatively undisturbed layer of cooler water is called the
hypolimnion and the water between these two layers is called the metalimnion.
This is the zone where water temperature changes the most rapidly in a vertical
direction; it is also known as the thermocline. The Florida lake does not appear to
stratify as strongly because it is shallower.

Notice, in the Florida lake, that there is a much smaller temperature difference
between the surface and the bottom; temperatures range from about 32C (900F)
down to 23C (730F), a difference of only 90C. In the Iowa lake, the temperature
span is considerably larger, ranging from 25C (770F) to about 10C (500F), a
difference of 15C. This tells us that the stratification in the Florida lake is not as
strong or as stable as the stratification in the Iowa lake. While strong stratification
happens much less frequently in Florida's shallow lakes, it does occur in the
deeper sink hole lakes found throughout the state.











Trophic State
Trophic status is defined as "the
degree of biological productivity
of a waterbody." Scientists debate
exactly what is meant by biological
productivity but, generally, it relates to
the amount of algae, aquatic macrophytes,
fish and wildlife a waterbody can produce
and sustain. Waterbodies are traditionally
classified into four groups according to
their level of biological productivity. The
adjectives denoting each of these trophic
states, from the lowest productivity level to
the highest, are oligotrophic, mesotrophic,
eutrophic, and hypereutrophic.

Aquatic scientists assess trophic state
by using measurements of one or more of
the following:
* total phosphorus concentrations in the
water;
* total nitrogen concentrations in the water;
* total chlorophyll concentrations a
measure of free-floating algae in the water;
* water clarity, measured using a Secchi
disc; and
* aquatic plant abundance.

Florida LAKEWATCH professionals
base trophic state classifications primarily
on the amount of chlorophyll in water
samples. Chlorophyll concentrations
have been selected by LAKEWATCH
as the most direct indicator of biological
productivity, since the amount of algae
actually being produced in a body of water
is reflected in the amount of chlorophyll
present. In addition, Florida LAKEWATCH
professionals may modify their chlorophyll-
based classifications by taking the aquatic
macrophyte abundance into account.


Warmouth
Lepomis gulosus


This circular provides a first step
towards understanding a complex
subject that professionals still
intensely debate. Basic information on how
fish are sampled by professionals as well as
how fish abundance and species composition
are related to trophic state is provided in the
following parts:

1. Techniques for Estimating Fish
Populations in Florida Lakes

2. Trophic State and Total Fish Biomass

3. Trophic State and Sport Fish Biomass

4. Trophic State and Species Richness

5. Trophic State and Some Common Florida
Fish Species










Part 1
Techniques for Estimating Fish Populations
in Florida Lakes


There is a limit as to how useful the published literature can be in determining what effect nutrient

enrichment eutrophicationn) might have on fish populations on Florida lakes. In particular, many
of the studies cited by professionals, even those working in Florida, have been carried out on deep,
northern lakes that support fish communities very different than Florida's. For example, many northern
lakes with their cold, oxygenated water support trout and salmon, whereas warm shallow Florida lakes do
not support these cold-water fish. Florida lakes support a variety of warm-water fish including largemouth
bass, black crappie, redear sunfish and bluegill all major sportfish in Florida. Consequently, the results
from more northern studies may not directly apply to shallower lakes in a warmer climate.

Fisheries biologists face a difficult problem when asked to assess the fish community in a lake. Like
anglers, they must first catch the fish! Unfortunately, there is no one sure-fire technique for catching all
and sizes offish or determining the absolute amount offish in a lake. Biologists in Florida use multiple
techniques such as electrofishing, gillnetting, and rotenone sampling with blocknets to determine fish
abundance and to catch as many fish species as possible. Faced with the reality of limited resources, the
fisheries biologist often must rely on only one technique to meet a project objective.

Florida LAKEWATCH works cooperatively with the Florida Fish and Wildlife Conservation
Commission to provide long-term assessments offish populations in selected Florida lakes. We will start
this circular with a description of electrofishing because Florida LAKEWATCH and most state and federal
agency biologists use electrofishing as the primary technique to sample fish communities. 6


-*^-L-'----
Largemoulh bass
(Micropterus salmoides)












0 0crfihn


.'- .....-.


lectrofishing uses electric current to capture
fish. This sampling technique is called an
"active gear" as the fisheries personnel
motor around a lake to capture fish from different
areas. Electrofishing can be used on virtually all
Florida lakes, although it is less effective in lakes
with low specific conductivity (see box on p. 4).

Electrofishing is usually conducted using
aluminum boats carrying portable generators. The
boats are equipped with one or two booms (poles)
extending forward from the front of the boat for the
support of the electrodes. Boats equipped with one
boom typically use the boat as the cathode (-) and
the electrodes on the boom as the anode (+). Boats
with two booms use both booms as the anode in an
attempt to increase the area fished.


Electrofishing is often used by fisheries
biologists because it is a capture technique that
minimizes fish mortality. Four types of electrical
current are typically used: direct current (DC),
pulsed DC, alternating current (AC) and pulsed
AC. Each boat is equipped with a "control box" for
managing the different current types. Electrofishing
effectiveness depends upon the type of lake being
sampled (specifically, the lake's conductivity) as
well as water depth, water clarity and the wattage
of the generator creating the current.

Fish react to electric current in two basic ways.
Fish in the "escape" field of the electrical current,
far away from the boat and electrodes, show a
"fright" response and swim away unaffected. Fish
in the "stun" field, typically between the boat and


<



LL









electrodes, show effects such as forced swimming,
but ultimately become immobilized or stunned
when electrical current and water conditions are
correct. Their reaction is similar to a human being
hit with an electrical stun gun. Fisheries personnel
can capture these fish with long-handled dip nets.
In most cases, the captured fish become mobile in
a few minutes. Once the fish are mobile, they are
typically released back into the lake unless they are
needed in the laboratory for additional research,
such as for age and growth, mercury analysis, and/
or reproductive studies.

Direct current (DC) is often used in turbid
waters because fish tend to move toward the anode
where they roll over and are more easily captured.
This current type is useful in turbid waters because
the fish tend to move from the deeper water to the
surface, near the anode, where they can be seen
and captured with dip nets. Pulsed DC is generally
better than the continuous, unmodified DC because
it requires less voltage and thus causes less harm to
the fish.

Alternating current (AC) is typically used in
Florida's mineral-poor lakes (specific conductance
less than 100 |jS/cm at 250C). In these lakes, high
voltage is needed to stun the fish. Continuous,
unmodified AC is potentially the most damaging
type of current to use as it can cause hemorrhaging,
ruptured swim bladders, and fractured vertebrae.
Pulsed AC may cause similar responses, but
it is not potentially as harmful as continuous,
unmodified AC. The adverse effects typically
occur when the alternating current is suddenly
activated and the fish is near the electrode. Having
the current active before the boat enters the
sampling area can minimize many of the adverse
effects. Regardless of which current is used, as
the generator wattage increases the strength and
size of the stun field increases, increasing the
effectiveness of the unit.

Electrofishing efficiency is influenced by many
factors. Electrofishing is basically a shallow water
(less than 8 feet) fish sampling technique. Fish that
live along the shore of a lake, such as largemouth
bass and bluegill, can be sampled relatively
efficiently but consideration must always be given


to the potential existence of offshore populations.
Electrofishing is not effective for fish such as
gizzard shad and catfish that live offshore or near
the bottom. Fish size also affects electrofishing
efficiency, with larger fish being stunned more
effectively than smaller fish because the larger fish
have more surface area for the current to come in
contact with. Even when small fish are stunned,
there is a tendency for the dip-netters to select the
large fish and it is often difficult to collect all of
the small fish when large numbers of these fish are
simultaneously stunned, especially when a large
school offish is encountered.

Factors such as water temperature, water
transparency, dissolved oxygen concentration,
number and experience or skill of dip-netters,
and weather can all affect fish capture rates.
The bottom line is that the results from any
single day ofelectrofishing need to be examined
cautiously because electrofishing could be either
very effective or less effective on that particular
day resulting in very high or very low capture
rates. The results become useful for assessing
relationships among lakes and for determining
long-term trends within the fish community of a
single lake if factors such as those listed above are
taken into account.

Fish population abundance can be estimated
with electrofishing by calculating Catch-Per-
Unit-Effort (CPUE). CPUE is a useful and easily
obtained index for the abundance of many fish
species. Fisheries personnel electrofish a given
lake for a defined time period (about 10 minutes
in Florida) and collect all stunned fish. The total
number or weight of all fish collected can then be
calculated as number or weight offish per hour
of electrofishing. The timed fishing transects are
conducted at multiple sites and often on multiple
days for a lake to provide a mean estimate of fish
abundance for that specific lake. These estimates
can then be compared with estimates from other
lakes of varying trophic state or compared to
estimates from other years for the same lake, if data
are available. Florida LAKEWATCH and FFWCC
are currently using electrofishing CPUE to monitor
long-term fish abundance trends for 52 Florida
lakes.











Specific Conductance
Specific conductance is a measure of the capacity of water to conduct electricity. A higher value
of conductance means that the water is a better electrical conductor. The unit of measure for
conductance is the microSieman per centimeter of water measured at a temperature of 25
degrees Celsius (abbreviated "pS/cm @ 25oC"). "Micromhos/cm" (abbreviated "pmhos/cm) is also
used. These two units are equal to each other.
Specific conductance increases when more salts, including the most common sodium chloride,
are dissolved in water. For this reason, conductance is often used as an indirect measure of the salt
concentration in a waterbody. In general, lakes with more salts are more productive except of course
where there are limiting nutrients or other limiting factors involved.
The location of a waterbody has a strong influence on its specific conductance. For example,
lakes in the New Hope Ridge/Greenhead Slope lake region in northwestern Florida (Washington,
Bay, Calhoun, and Jackson counties) tend to have conductance values below 20 pS/cm @ 25C
while lakes in the Winter Haven/Lake Henry Ridge lake region in central Florida (Polk County) tend
to have values above 190 pS/cm @ 25C. However, environmental factors also can cause higher
conductance values. For example, drought conditions can increase the salt concentrations in a
waterbody in two ways: 1) drought can cause inflowing waters to have higher salt concentrations
and 2) heat and low humidity can increase the evaporation of water, leaving the waterbody with
higher concentrations of salt. Waterbodies in the Florida LAKEWATCH database, analyzed through
2006, ranged from 11 to more than 5500 pS/cm @ 25C. More than 75% of these waterbodies had
conductance values less than 190 pS/cm @ 25C.

Salinity (ppt) = -0.332 + 0.00063 Conductance (tS/cm2 @ 250C)

Relation between
14 salinity and specific
conductance for
12 d ata collected
from five Florida
coastal rivers. The
S10 regression equation
S -* ~ for calculating
8 salinity (parts
S per thousand)
from specific
6 conductance
c(microSieman per
4 centimeter of water
measured at a
temperature of 25
2 degrees Celsius) is
above the graph.
0
0 5000 10000 15000 20000 25000

Specific Conductance (gS/cm2 @ 250C)









Between 1999 and 2006, Florida
LAKEWATCH and the Florida Fish and Wildlife
Conservation Commission (FFWCC) electrofished
four to six transects from 32 lakes each year,
when access was available. This yielded a total
of 1277 ten-minute transects, capturing a total 56
fish species. The total number of species in a lake
ranged from eight in E Lake, Miami-Dade County
to 34 in Lake Panasoffkee in Sumter County. Table
1 shows summary statistics for those fish species
that were caught in ten or more lakes. The data
show that the three most recreationally sought-
after freshwater fish in Florida are also the most
commonly sampled fish in Florida lakes (bluegill
in 32 lakes, largemouth bass in 32 lakes, and redear
sunfish in 31 lakes). Table 1 also shows the large
range in CPUE by individual species (e.g., bluegill
averaged 150 fish/hr, but ranged from 22 fish/hr to
682 fish/hr, depending on the lake).

Electrofishing can also be used in mark-
recapture studies to try to provide a more definitive
estimate of the abundance of one or more fish
species in a lake. There are many types of mark-
recapture methodologies and some are extremely
complicated. All of the methods, however, involve
capturing the fish, giving the fish a mark that will
be recognized at a later date (for example, colored
tags or fin clips), releasing the fish alive, and later
sampling the fish population to look for the marks.

A simple estimate of abundance can be
obtained using the Petersen mark-recapture
method. In this approach, electrofishing is used
to capture fish for a period of time. During this
"marking" phase, a substantial proportion of the
fish population is marked in an attempt to gain
better confidence in the final estimate. After
a sufficient number offish have been marked
(usually 10% or more of the fish species being
estimated), sampling is stopped to allow the
marked fish to mix with the unmarked fish. After
this mixing period (usually at least a week), the
census period begins. During this census period,
electrofishing is again used to capture fish.
Fisheries personnel record the number of marked
fish and the total number offish collected. All fish
are given a distinctive second mark prior to their
release to prevent the fish from being counted twice


during the census period. The abundance estimate
is calculated using the following formula:


N =M*C
R

Where:

N = the number of estimated fish in the population;

M = the number offish marked during the marking
period;

C = the number offish caught during the census
period; and

R = the number offish, marked during the marking
period that were recaptured during the census
period.

Electrofishing can also provide useful
information on the size composition of harvestable
sportfish such as the largemouth bass. In other
words, what proportion of a fish species is
comprised of a given length fish in the lake? This
information is crucial for evaluating if regulations
such as size limits are working. Electrofishing
is also useful for species detection. In Florida,
electrofishing will capture the most common
species. However, a complete listing of all
species in a water body can only be obtained if
other sampling techniques are employed. This is
necessary because electrofishing has sampling
limitations, such as the water depth at which it will
effectively sample fish. 6


Spotted sunfish
Lepomis punctatus


I











Mean Minimum Maximum
Common Name Lakes (fish/hour) (fish/hour) (fish/hour)

Bluegill 32 150.3 22.0 682.3
Largemouth bass 32 52.7 1.8 214.3
Lake chubsucker 31 12.6 0.1 65.0
Redear sunfish 31 23.7 0.3 71.5
Warmouth 31 7.7 0.5 34.9
Brown bullhead 28 3.7 0.1 27.2
Florida gar 28 11.8 2.4 34.8
Golden shiner 28 26.1 0.1 273.6
Black crappie 27 5.1 0.1 30.6
Bowfin 27 3.3 0.2 14.6
Brook silverside 27 30.1 0.2 156.5
Eastern mosquitofish 26 16.5 0.1 196.1
Seminole killifish 25 16.6 0.1 117.4
Chain pickerel 24 3.2 0.2 9.4
Taillight shiner 24 18.5 0.1 178.9
Dollar sunfish 22 4.7 0.1 58.5
Bluespotted sunfish 19 1.5 0.1 6.8
Gizzard shad 19 12.4 0.1 120.0
Threadfin shad 18 23.0 0.1 151.0
Spotted sunfish 17 1.8 0.1 11.5
Yellow bullhead 17 0.8 0.1 5.1
Bluefin killifish 14 3.7 0.2 34.4
Golden topminnow 12 0.3 0.1 0.8
Blue tilapia 11 1.8 0.1 5.4
Swamp darter 11 0.3 0.1 0.6
Inland silverside 10 7.4 0.4 16.1
Redfin pickerel 10 0.8 0.1 2.5


Table 1. Overall species mean, minimum, and maximum electrofishing CPUE
(number of fish/hr) for all species captured in at least 10 lakes. Individual
fish species statistics were calculated first for each lake by year including any
zero CPUE values if the species was not caught in an individual transect but
was captured at least once in all transects over all years. Then all years were
averaged among all lakes by individual species.











Gilnea


Gillnets are vertical walls of netting set out
in the open-water in a straight line. Gillnets
provide a "passive" capture method that
works by entanglement in the net. Fish are caught
as they attempt to swim through the opening in the
mesh and get stuck. The gear is called "passive"
because fisheries personnel do not actively move
the nets once they are placed into the water.

Gillnets, like most passive gear, have an
advantage because they are simple in their design
and construction. They can be repaired relatively
easily and at a low cost; this is all important when
used in lakes where large numbers of alligators
live. Alligators can make large holes in the nets,
destroying the net's effectiveness; nothing is worse
than trying to remove a live, mad alligator from a
net!


The catch offish in a gillnet (assuming other
variables are equal) is often proportional to the
abundance offish in a lake. Gillnet CPUE (number
offish caught per gillnet per day) is especially
helpful in determining the relative abundance
of fish in waters where electrofishing is not that
effective. Gillnets can effectively sample open-
water oriented species such as black crappie,
gizzard shad, lake chubsucker, and sunshine bass.
Fish such as catfish are often caught by the spines,
making it difficult to remove them from the net.
In Florida lakes, "experimental gillnets" are often
used because these types of nets have five or six
sections, each with a different size mesh. This
allows a single net to capture different size fish. It
is important to remember that mesh size generally
determines the size offish captured and catch
requires an encounter between the fish and the
gillnet.










Because most Florida lakes are shallow, nets
are typically positioned along the bottom of the
lakes. The typical experimental gillnet used by
the Department of Fisheries and Aquatic Sciences
of the University of Florida is approximately 165
feet long and 8 feet tall (50 m x 2.4 m). The nets
generally have five 33-ft (10-m) panels of different
mesh size. The measurement of one side of an
opening in the mesh is call "bar mesh" and the bar
sizes are generally as follows: 3/4, 1, 1.5, 2.0, 2.5
inch (19, 25, 38, 51, 63 mm). The nets are typically


fished for 24 hours during the summer. Although
the captured fish are destroyed, only a small
number offish relative to the size of the lake's fish
community are captured, so scientific gillnetting
has little effect on the fish community. During
the summer, entangled fish may die due to stress
in the warm water; the Florida Fish and Wildlife
Conservation Commission requires any fish
captured by a gillnet to be disposed of properly. 6


Florida's Fish


Common name

American eel
Atlantic needlefish
Banded sunfish
Black crappie
Blackbanded sunfish
Blue tilapia
Bluefin killifish
Bluegill
Bluespotted sunfish
Bowfin
Brook silverside
Brown bullhead
Chain pickerel
Channel catfish
Coastal shiner
Dollar sunfish
Eastern mosquitofish
Florida gar
Gizzard shad
Golden shiner
Golden topminnow
Grass carp
Inland silverside
Lake chubsucker


Scientific name

Anguilla rostrata
Strongylura marina
Enneacanthus obesus
Pomoxis nigromaculatus
Enneacanthus chaetodon
Oreochromis aureus
Lucania goodei
Lepomis macrochirus
Enneacanthus glorious
Amia calva
Labidesthes sicculus
Ameiurus nebulosus
Esox niger
Ictalurus punctatus
Notropis petersoni
Lepomis marginatus
Gambusia holbrooki
Lepisosteus platyrhincus
Dorosoma cepedianum
Notemigonus crysoleucas
Fundulus chrysotus
Ctenopharyngodon idella
Menidia beryllina
Erimyzon sucetta


Common name

Largemouth bass
Least killifish
Lined topminnow
Longnose gar
Pugnose minnow
Pygmy killifish
Pygmy sunfish
Redbreast sunfish
Redear sunfish
Redfin pickerel
Sailfin molly
Seminole killifish
Spotted sunfish
Swamp darter
Tadpole madtom
Taillight shiner
Threadfin shad
Tidewater silverside
Walking catfish
Warmouth
White catfish
Yellow bullhead


Scientific name

Micropterus salmoides
Heterandria formosa
Fundulus lineolatus
Lepisosteus osseus
Opsopoeodus emiliae
Leptolucania ommata
Elassoma sp.
Lepomis auritus
Lepomis microlophus
Esox americanus americanus
Poecilia latipinna
Fundulus seminolis
Lepomis punctatus
Etheostoma fusiforme
Noturus gyrinus
Notropis maculatus
Dorosoma petenense
Menidia peninsula
Clarias batrachus
Lepomis gulosus
Ameiurus catus
Ameiurus natalis


Table 2. Common and scientific names (genus and species) for the most prevalent fish in Florida
lakes.












Ro Si w Bme


Hook and line fishing is one of the oldest and
most enjoyable methods for capturing fish.
Sampling fish with toxicants, however, is
also one of the oldest and most reliable methods
for capturing fish. In some communities in Asia,
sodium cyanide is used to kill fish for commercial
harvest. The most commonly used EPA-approved
fish toxicant in the United States is rotenone. In
Florida, fisheries personnel typically use rotenone
in conjunction with nets (called blocknets) to
limit the size of the sampling area. This type of
sampling, despite the number offish killed during
the sampling operation, does not seriously harm the
lake's fish community and provides good estimates
of fish abundance.


Rotenone is a naturally occurring toxicant
extracted from plants of the Fabaceae (bean)
family. In Florida, rotenone is generally applied
in a liquid form containing from two to five
percent of the active ingredient. Rotenone kills
fish by interfering with a mechanism required
for respiration; the fish essentially suffocates.
Rotenone applied at two to five mg/L generally
insures a complete kill of all sizes and species of
fish. Rotenone is sometimes used at concentrations
less than 1 mg/L to selectively remove fish like
grass carp and shad without harming desirable fish
like largemouth bass.









The duration of rotenone toxicity is highly
dependent on water temperature and the
concentration of suspended solids. Rotenone
toxicity lasts longer in cold water that has few
suspended solids. Rotenone sampling in Florida
is generally conducted during the summer when
water is warm. Many Florida lakes have high
concentrations of suspended solids. Consequently,
rotenone toxicity generally lasts for less than two
days in Florida lakes. Rotenone is not considered
toxic to most mammals and birds, although swine
have been affected at the concentrations used to
kill fish. Rotenone will kill zooplankton and other
aquatic invertebrates such as crawfish and grass
shrimp. The effects are short term and affect only a
very small percentage of the lake-wide populations
of these animals.

Blocknets are fine mesh nets that are used by
fisheries personnel to cordon off specific areas of
a lake for fish sampling. The nets are often about
3 to 4 m (9.8 13.1 ft) deep and made of netting
with 3-mm (1/8 in) wide openings. The length of
the nets varies depending on the study's objectives,
but typically 0.08 ha (1/5-acre) to 0.4 ha (one-
acre) areas are sampled in Florida. The primary
purpose of the net is to block fish movement out of
the sampling area. One advantage of using several
small blocknets rather than one large blocknet to
sample an area such as a cove is that more habitats
can be sampled in a lake and far less rotenone is
used.

Sampling with rotenone typically takes place
over three days. Shortly after the net is set and
rotenone is applied, the field crew begins to collect
the fish. As the rotenone begins to effect the fish,
the fish swim to the surface. It is at this time that
a frantic effort is made to capture as many fish as
possible. Working with fresh fish is easier than
working with fish that have been dead for one
or two days. Depending on the fish species and
their size, some fish may dive to the bottom and
bury themselves in the mud. With Florida's warm
water, the dead fish will quickly bloat as a result of
bacterial decay and float to the surface. Biologists
remove the dead fish from the water surface for the
next two days.


Generally, fish outside the blocknets can detect
the rotenone and swim to safety. During most
sampling programs, few fish outside the net are
killed. However, sometimes fish kills do occur.
Quite often these kills are limited to rotenone-
sensitive fish like gizzard shad, threadfin shad
and grass carp thus these types of kills do little
to adversely affect the recreationally-important
sportfish, but they are noticeable to the public
and can cause great concern if not addressed
immediately. Regardless of what appears to be a
large number offish, these "outside the net" kills
affect only a very small percentage of the total fish
community.

Sampling fish with rotenone and multiple
blocknets is one of the best methods for obtaining
an estimate of total standing crop or fish biomass
(per area) at a specific time. The key phrase here is
"at a specific time." Fish communities are highly
dynamic. Fish abundance can change significantly
from year to year. Any sampling, however, will
produce results for a specific lake and time that
may be highly variable. Recovery offish from the
sampling area may be incomplete. Fish can and do
move past the nets. Birds and alligators may eat
some of the fish that are killed before biologists can
collect them. Some fisheries personnel question
the value of rotenone sampling. However, rotenone
sampling remains a reliable method for assessing
patterns offish response to differences in lake
trophic state within a lake over time or among lakes
of varying trophic state. 6


Black crappie
(Pomoxis nigromaculatus)






































Florida has over 7,700 lakes greater than ten
acres in size. Some lakes are biologically
unproductive, whereas others support
a tremendous amount of fish and wildlife.
Professionals often classify lakes according to their
biological productivity using one of several trophic
state classifications systems. In these systems,
the least productive lakes are called oligotrophic
lakes. Moderately productive lakes are classified
as mesotrophic lakes. Productive lakes are termed
eutrophic and the most productive lakes are called
hypereutrophic. Lake trophic state is further
discussed in greater detail in A Beginner's Guide
to Water Management Nutrients (Circular
#102).

Professionals generally use four measurements
to assess lake trophic state: total phosphorus, total
nitrogen, chlorophyll, and Secchi depth. When
available, chlorophyll measurements are the best
for assigning trophic state because chlorophyll is
the most direct measure of biological productivity.
Florida LAKEWATCH has shown that average


lake chlorophyll concentrations in Florida lakes,
participating in the LAKEWATCH program,
range from less than one .ig/L to over 400 .ig/L.
Using only Florida LAKEWATCH chlorophyll
data, about ten percent of the 1,600 sampled
lakes would be classified as oligotrophic (those
with chlorophyll values less than or equal to 3
.ig/L) and approximately 32% of the lakes (those
with chlorophyll values greater than 3 ig/L and
less than or equal to 7 .ig/L) would be classified
as mesotrophic. Eutrophic lakes (those with
chlorophyll values greater than 7 .ig/L and less than
or equal to 40 .ig/L) would represent about 410%
of the lakes; nearly 17% of the lakes (those with
chlorophyll values greater than 40 .ig/L) would be
classified as hypereutrophic.

Studies offish populations in Florida lakes
have shown that total fish abundance (expressed
by weight) increases with lake trophic state (as
indicated by chlorophyll concentrations). This trend
is found regardless of whether fish abundance is
estimated by use of either rotenone, electrofishing,


PART 2


Trophic State and Total Fish Biomass









or gillnet sampling (Figure 2, A-C). On average,
fish abundance increases from oligotrophic to
hypereutrophic in Florida lakes with no sign of a
decrease in the most hypereutrophic lakes. These
relationships are consistent with conventional
wisdom and with numerous published scientific
relations between fish standing crop or yield and
several different measures of lake trophic state.

When examining changes in the standing crop
of individual fish species, increases in biomass for
gizzard shad and threadfin shad are particularly
noteworthy. Both fish species are practically absent
in oligotrophic-mesotrophic lakes, but increase in
both frequency of occurrence and standing crop
based on blocknet rotenone sampling in eutrophic
and especially hypereutrophic lakes. The average
standing crop for gizzard shad, for lakes in which
this species is found, is about 66 kg/ha (59 lbs/
acre). This is the highest mean biomass for all fish


species encountered in studies of Florida lakes. The
standing crop for threadfin shad is 21 kg/ha (18.7
lbs/acre) and ranks third behind bluegill (38 kg/ha
or 33.8 lbs/acre).

Although there are significant differences in the
mean values among the four trophic levels, there
is tremendous variability in total fish abundance
within any given trophic level (Figure 3). This
variability is not unique to Florida lakes, but
reflects the importance of other environmental
factors such as a lake morphometry, presence
of aquatic plants, clay turbidity and population
dynamics. There are also practical sampling
problems associated with estimating the biomass of
wild fish populations. For these reasons, predictions
based on the relationships are imprecise; however,
holding all other things constant, the total standing
crop offish in a Florida lake should change in the
direction that the lake's trophic state changes. 6


Redear sunfish
(Lepomis microlophus)











1000




S100
- ** 1
100 0



- *
0 10
-*


A


1 10 100 1000
Chlorophyll (pg/L)

1000



O 100 *

.1- *
- *
S100



0 1
Uo A



B
0.1
1 10 100 1000
Chlorophyll (pg/L)










1000



LU 100


_* **





CC


0.1
1 10 100 1000
Chlorophyll (pg/L)


Figure 2 (left and above). Relationships between lake trophic state (chlorophyll concentration)
and total fish abundance as estimated with blocknet and rotenone sampling (A), electrofishing
(B), and gillnets (C) for 60 Florida lakes. Data for both axes were changed with a logarithmic
(base 10) transformation to normalize extreme measurements for comparisons.


Florida gar
(Lepisosteus platyrhincus)








































An enduring belief from northern lakes
associated with lake eutrophication is that
a change to a higher trophic state, resulting
from nutrient enrichment of the water body, causes
the loss of sportfish populations. This association
is untrue for Florida lakes. Blocknet with rotenone,
electrofishing, and gillnet sampling all demonstrate
that more harvestable sportfish are generally
captured in eutrophic and hypereutrophic lakes
than oligotrophic lakes (Figure 4, A-C).
The belief that sportfish are lost with increases
in trophic state is based in part on the fact
that piscivorus fish (fish-eating fish), such as
largemouth bass, comprise a smaller percentage
of the total fish biomass as lakes become more
eutrophic. In Florida, studies have shown that
piscivorus species as a group averaged about 22%
of the total fish biomass, but can range from 0%
to 73%. The absolute weights ofpiscivores do not
decrease with increasing trophic state. However,
when expressed as a percent of the total biomass,


the relative importance (percent abundance) of this
group declines as lakes became more productive.
On average, the percentage of piscivores for
oligotrophic, mesotrophic, eutrophic, and
hypereutrophic lakes are 25%, 28%, 21%, and 11%
of the total fish biomass, respectively.

One of the most important piscivores, the
largemouth bass, on average makes up 15% of the
total fish biomass in Florida lakes, but can range
as low as 0% to over 69%. As with the biomass
ofpiscivores, the absolute biomass of largemouth
bass population does not decrease with an increase
in trophic state though their percentage of the
total fish biomass becomes smaller at higher
trophic state. On average, the percentages of
largemouth bass by weight of the total fish biomass
in oligotrophic, mesotrophic, eutrophic, and
hypereutrophic lakes are approximately 20%, 17%,
16%, and 4%, respectively. Mark-recapture studies
show that oligotrophic lakes support just under ten


PART 3


Trophic State and Sportfish Biomass









harvestable (>10 inches or 250 mm total length)
largemouth bass/ha and eutrophic Florida lakes
support between 25 and 30 harvestable largemouth
bass/ha. This is why Florida's eutrophic lakes are
some of the State's best fishing lakes. Although
hypereutrophic lakes have largemouth bass
populations that make up only about four percent
of the total fish biomass, these lakes support nearly
20 largemouth bass/ha (8 bass/acre), which is a
greater abundance than that supported by Florida's
oligotrophic and mesotrophic lakes.

Most sportfish in Florida belong to the
family of fish called Centrarchidae. This
family of fishes includes the largemouth bass
(Micropterus salmoidesfloridanus), bluegill
(Lepomis machrochirus), redear sunfish (Lepomis
microlophus), and black crappie (Pomoxis
nigromaculatus), which are the primary freshwater
sportfish in Florida. On average, studies of 60
Florida lakes indicate that Centrarchids make up
66% of the total fish biomass. Centrarchid biomass,
however, ranges from 15% to 99% across the
60 lakes. This group of fish also shows a pattern
of increasing absolute biomass with increase in
trophic state and a decrease in percent biomass in
lakes of higher trophic state.


Bluegill show no changes in mean standing
crops or in proportions of larger fish with increases
in trophic status. The redear sunfish, also known as
the shell-cracker, generally increases in biomass
with increases in lake trophic state. Another
important Centrarchid, the black crappie or
speckled perch also becomes more common and
has higher standing crops in more eutrophic lakes.
While black crappie biomass increases, there is a
tendency for a smaller proportion of quality-sized
(larger) fish in more eutrophic lakes.

Redear sunfish (shell-cracker) are not
commonly found in Florida's oligotrophic lakes.
This is probably the result of the low pH, total
alkalinity, and hardness associated with many of
the State's oligotrophic lakes, which limits the
preferred food (snails) of the shell-cracker. Black
crappie are also not found in abundance in Florida's
oligotrophic lakes because low lake productivity
limits the food supply for this species. One type of
Centrarchid commonly found in the oligotrophic
lakes is the warmouth. Warmouth, unlike other
Centrarchids, shows increases in standing crop with
decreases in trophic state. 6

-^^..q '*^ ^ '- .
6Y- ,-


~~1..-









* Gillnet (kg/net/24 hr)
Electrofishing (kg/hr)
Blocknet (kg/ha)


1000


100






10


Oligotrophic


Mesotrophic


Eutr c
Eutrophic Hypereutrophic


Trophic State

Figure 3. Mean total fish abundance estimated with gillnets, electrofishing, and
blocknets (with rotenone) by lake trophic state for 60 Florida lakes. The bars indicate
means and lines represent the 95% confidence intervals. There were 8, 7, 25, and 20
lakes for oligotrophic, mesotrophic, eutrophic and hyperuetrophic groups, respectively.










1000

*
0)




--
I100- *








A
10 10 1000




Chlorophyll (lg/L)

100
( *





LS
c ,
A








0 1






1 10 100 1000



Chlorophyll (pg/L)
100 i----------


















Chlorophyll (pg/L)















D 0
**


O ()


0 C: e



C
0.1
_c c




1 10 100 1000
Chlorophyll (pg/L)


Figure 4 (above and at left). Relationships between lake trophic state (chlorophyll
concentration) and harvestable sportfish abundance as estimated with blocknet and
rotenone sampling (A), electrofishing (B), and gillnets (C) for 60 Florida lakes. Data for
both axes were changed with a logarithmic (base 10) transformation to normalize the
extreme measurements for comparisons.

1 i -.l^ ^ --,' -,
4-! 40 100 10















The number of different species of fish
(species richness) inhabiting a lake is often
used as an indicator offish community
quality and lake quality. Florida lakes support more
than 100 species offish, but all of these species
are rarely, if ever, collected in a single lake. The


large number of species encountered reflects the
fact that Florida is at the center of some major
biogeographical (animal dispersal) movements.
Non-native fish are colonizing lakes in south
Florida. Waters in the panhandle of Florida support
species that have evolved in the central continental


20 -


15 I


10 -


5 k


Oligotrophic Mesotrophic


Eutrophic Hypereutrophic


Lake Trophic Status

Figure 5. Relationship between lake trophic state (estimated with chlorophyll
concentration) and mean fish species richness for 60 Florida lakes.


PART 4

Trophic State and Species Richness









United States. Waters in northeast Florida support
fish species that have evolved along the Atlantic
coast. Some lakes, like Lake Okeechobee, even
support marine fishes because there are often no
physical barriers preventing the inland movement
of these fishes.

The number offish species per lake does not
decrease in eutrophic and hypereutrophic Florida
lakes (Figure 5). The most important determinant
offish species richness is lake surface area;
larger lakes tend to have higher species richness
than smaller lakes (Figure 6). For example, Lake


Okeechobee, a large lake in south Florida, has 41
species offish, while Lake Lawbreaker, a small
lake (5 ha or 12 acres) in central Florida, maintains
only four species offish. Surface area explains
about 70% of the variation in species richness.
Species richness is only weakly related to the
commonly measured trophic state variables of total
phosphorus, total nitrogen, chlorophyll, and Secchi
depth. However, there is no correlation between
the number offish species per lake and the various
trophic state indices, after first accounting for lake
surface area. 6


100









S10








Florida lakes.
i-i)


S0 0 1000 10000 100000





Lake Surface Area (ha)


Figure 6. Relationship between lake surface area and fish species richness for 60
Florida lakes.

















The public is often concerned about specific
fish species rather than all species of fish.
As of 2007, it is difficult to make statements
regarding the pattern of response of many fish
species to lake eutrophication. This situation exists
because many fish species are extremely low in
abundance and occur in only a small percentage of
Florida lakes; there is therefore, little information
on many of the fish species.

The Department of Fisheries and Aquatic
Sciences at the University of Florida collected
more than sixty different fish species during a five-
year study of 60 Florida lakes. Table 2 on page
7 shows the most prevalent fish in Florida lakes.
Some species, such as bluegill, largemouth bass,
warmouth, mosquitofish, and redear sunfish, were
commonly encountered and found in greater than
75% of the study lakes. Fifteen of the fish species
collected during this study were found in only a
single lake.

Redfin pickerel were found to be most common
in less productive lakes, while channel catfish were
more likely to be in lakes of higher productivity.
However, of the 29 species with sufficient
information for statistical tests, only 3 species, the
lake chubsucker, the golden topminnow, and the


lined topminnow showed decreases in frequency
of occurrence (number of lakes within a given
trophic status) with increasing trophic state. All
other species either stayed the same or increased
in frequency of occurrence in the eutrophic and
hypereutrophic lakes.

A similar pattern was found for average
standing crops of individual species in lakes of
differing trophic status. Most species showed no
significant difference in biomass (kg/ha) among
lakes of different trophic state. However, warmouth
decreased while five additional species, the gizzard
shad, threadfin shad, black crappie, redear sunfish,
and blue tilapia showed increases in standing
crops with increasing trophic state. Again, for the
recreationally important Centrarchids, the only
negative changes noted with increasing trophic
status were a decrease in the standing crops of
warmouth and a decrease in the proportion of
larger-sized black crappie. On the positive side,
the Centrarchids as a group showed an increased
biomass in the more eutrophic lakes. The redear
sunfish and black crappie increased in average
standing crop and the largemouth bass had a higher
proportion of larger fish in the more eutrophic
lakes. 6


Brown bullhead
(Ameiurus nebulosus)


PART 5

Trophic State and Some Common Florida Fish Species






















4)I~



-r- -
P,,
AI
-hturwe_


Many Floridians have come to Florida
from northern states. The same is true
for many of the professionals involved
in Florida lake management. The beliefs of many
citizens and professionals are based on experiences
and information derived from northern lakes.
Thus, it is important to consider the similarities
and differences between Florida and northern lakes
when evaluating the effects of changing trophic
state on fish communities and the ecology of the
lakes in general.

The fish population trends discussed in
this circular for Florida lakes fit the patterns
found for other lakes discussed in the published
fisheries literature. There is an increase in the total
standing crops offish as the concentrations of
total phosphorus, total nitrogen, and chlorophyll
increase and as the Secchi depth decreases. On
average, fish standing crops increase about ten-fold
from the oligotrophic to the hypereutrophic Florida
lakes, with no sign of a decrease in the most
hypereutrophic lakes.

There is considerable unexplained variation
(approximately 75%) in these relationships due
in part to other factors influencing fish standing
crops and the practical sampling problems


associated with estimating the biomass of wild fish
populations. This is true for both Florida lakes and
northern lakes. For this reason, predictions based
on these relationships are imprecise; however,
holding all other things constant, the standing crop
offish in a Florida lake or northern lake should
increase as the nutrient concentrations increase.

This is not surprising considering that the
abundance of many aquatic organisms in Florida
lakes as well as other lakes around the world have
been shown to be positively related to lake trophic
state, which is generally defined by the limiting
nutrient concentrations, primarily phosphorus.
Chlorophyll concentrations (Canfield 1983),
zooplankton abundance (Canfield and Watkins
1984), fish biomass (Jones and Hoyer 1982),
bird abundance (Nilsson and Nilsson 1978), and
even the abundance of top predators, such as the
alligator (Evert 1999), have all been shown to be
positively related to the trophic status of lakes.
The bottom line is that when the base productivity
of a system is increased, the biomass of aquatic
organisms will likely increase.

The relationships discussed in this circular
between fish populations and trophic status are
based on the study of many Florida lakes of


Implications for the Management of Florida Lakes









varying trophic status rather than individual lakes
that have undergone eutrophication over time.
This places a limit on predictions that can be made
for any single lake. Because of the scatter in the
original data, due in part to the many unexplained
factors influencing fish populations and the
problems associated with obtaining accurate
standing crop information for fish, there would
be problems in making precise predictions offish
standing crops for a specific lake that only has data
on basic water chemistry.

On the other hand, there are some general
patterns in the relationships between lake trophic
state and the total biomass, species richness,
species composition, and species standing crops
offish in Florida lakes that should be useful in a
predictive manner. First, the number offish species
in a lake seems to be determined primarily by
the area of the lake and not its trophic status, so
one should not expect dramatic changes in total
species numbers as a lake becomes more or less
eutrophic. Second, there might be shifts in species
composition with changes in trophic state, though
only a few species show significant changes in
their standing crops. In particular, the recreational
sportfish do not show large changes over the
trophic spectrum. Finally, no critical points on
the trophic spectrum can be identified that cause
dramatic changes in fish abundance and standing
crops; there is nothing comparable in Florida lakes
to the loss of dissolved oxygen in the hypolimnion
of eutrophic northern lakes.

Florida lakes, with up to 240 |tg/L of algal
chlorophyll (hypereutrophic), show no decrease
in biomass of important sport fish, except the
warmouth. Although some people believe that Lake
Okeechobee is undergoing eutrophication and the
lake's fisheries are threatened, there is no evidence
of a declining fishery. Many fisheries biologists
believe that the loss of submersed macrophytes due
to high water levels is an issue of greater concern.
Even Lake Apopka, Florida's most talked about
hypereutrophic lake, is not a "dead" lake. The lake
supports many fish including the recreationally
important black crappie and commercially valuable
catfish. What the lake does not support is a large
population of largemouth bass, which is the fish


that once made Lake Apopka a world-class fishing
lake. Why the largemouth bass is virtually absent
from the lake is debated, but what is known is
that largemouth bass live in Lake Apopka and the
ones that survive are some of the fastest growing
largemouth bass in Florida. It seems the biggest
problem is the lack of habitat for young fish,
particularly submersed macrophytes. Submersed
plants were lost in this lake during the late 1940s.

These studies illustrate a paradox in lake
management. For many purposes, the public
would prefer to have less productive lakes where
low nutrient concentrations result in low plankton
productivity and high water clarity. These lakes
are most suitable for water supply and contact
(swimming, water skiing, etc.) recreation and also
have pleasing aesthetic properties. On the other
hand, the more eutrophic lakes have larger fish
populations and a potential for higher yields for
sport fisheries. This basic fact has caused some
professionals to worry not about eutrophication, but
cultural oligotrophication the reduction of nutrient
inputs due to human management activities. Thus,
if a nutrient reduction program is successful and
reduces algal populations, it will benefit one group
of lake users, but at the same time, there is the
potential to reduce fish abundance to the detriment
of other users. 6















Bachmann R. W., B. L Jones, D. D. Fox, M. Hoyer, L. A., and D. E. Canfield, Jr. 1996. Relations
between trophic state indicators and fish in Florida (U.S.A.) lakes. Canadian Journal of Fisheries
Aquatic Sciences 53: 842-855.

Bays, J.S., and T.L. Crisman. 1983. Zooplankton and trophic state relationships in Florida lakes.
Canadian Journal of Fisheries and Aquatic Sciences 40: 1813-1819.

Canfield, D.E., Jr. 1983. Prediction of chlorophyll a concentrations in Florida lakes: the
importance of phosphorus and nitrogen. Water Resources Bulletin 19: 255-262.

Canfield, D.E., Jr., and C.E. Watkins, II. 1984. Relationships between zooplankton abundance
and chlorophyll a concentrations in Florida lakes. Journal of Freshwater Ecology 2: 335-344.

Canfield D.E., Jr., and M.V. Hoyer. 1992. Aquatic macrophytes and their relation to limnology
to Florida lakes. Final Report submitted to the Bureau of Aquatic Plant Management, Florida
Department of Natural Resources. Tallahassee, Florida. University of Florida, Gainesville,
Florida. 598 pp.,

Colby, P.J., G.R. Spangler, D.A. Hurley, and A.M. McCombie. 1972. Effects of
eutrophication on salmonid communities in oligotrophic lakes. Journal of the Fisheries Research
Board of Canada 29: 975-983.

Evert, J.D. 1999. Relationships of alligator (Alligator mississippiensis) population density to
environmental factors in Florida Lakes. M. S. Thesis. University of Florida. Gainesville, Florida.
88 pp.

Hanson, J.M., and W.C. Leggett. 1982. Empirical prediction offish biomass and yield. Canadian
Journal of Fisheries and Aquatic Sciences 39: 257-263.

Hoyer, M.V., and D.E. Canfield, Jr. 1996. Largemouth bass abundance and aquatic vegetation in
Florida lakes: an empirical analysis. Journal of Aquatic Plant Management 34: 23-32.

Jones, J.R., and M.V. Hoyer. 1982. Sportfish harvest predicted by summer chlorophyll a
concentration in midwestern lakes and reservoirs. Transactions of the American Fisheries Society
111: 176-79.

Kautz, R.S. 1980. Effects of eutrophication on the fish communities of Florida lakes. Proceedings
of the Annual Conference of the Southeastern Association of Fish and Wildlife Agencies 34: 67-
80.


Other Relevant Research and Information










Keller, A.E. and T.L. Crisman. 1990. Factors influencing fish assemblages and species richness in
subtropical Florida lakes and a comparison with temperate lakes. Canadian Journal of Fisheries
and Aquatic Sciences 47: 2137-2146.

Lee, G.F., P.E. Jones, and R.A. Jones. 1991. Effects of eutrophication on fisheries. Reviews in
Aquatic Sciences 5: 287-305.

Nilsson, S.G., and I.N. Nilsson. 1978. Breeding bird community densities and species richness in
lakes. Oikos 31: 214-221.

Oglesby, R.T. 1977. Relationships of fish yield to lake phytoplankton standing crop, production,
and morphoedaphic factors. Journal of the Fisheries Research Board of Canada 34: 2271-79.

Schultz E.J., M.V. Hoyer, and D.E. Canfield, Jr. 1999. An index of biotic integrity: a test with
limnological and fish data from sixty Florida lakes. Transactions of the American Fisheries
Society 128: 564-577.

Stocker J.G., E. Rydin, and P. Hyenstrand. 2000. Cultural oligotrophication causes and
consequences for fisheries resources. Fisheries 25: 7-14.

Yurk, J.J., and J.J. Ney. 1989. Phosphorus-fish community biomass relationships in southern
Appalachian reservoirs: Can lakes be too clean for fish? Lake and Reservoir Management 5: 83-
90.














Notes:


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LAKWATCH





Florida LAKEWATCH


Florida LAKEWATCH (FLW) is one of the
largest citizen-based volunteer monitoring
endeavors in the country with more than 1,500
individuals monitoring more than 700 lakes
and other bodies of water in more than 50
Florida counties. Staff from the University of
Florida's Department of Fisheries and Aquatic
Sciences train volunteers throughout the
state to conduct monthly long-term monitoring
of both fresh and saline waterbodies.
LAKEWATCH uses the long-term data to
provide citizens, agencies and researchers
with scientifically-sound water management
information and educational outreach.

To become part of the Florida LAKEWATCH
team, volunteers are required to have access
to a boat and complete a two-hour training
session. During the session, volunteers learn
to collect water samples, take water clarity
measurements, and prepare algae samples
for laboratory analysis. Once a volunteer
is certified by a regional coordinator and
sampling sites are established, he or she will
sample the designated stations once a month.
Samples are frozen immediately upon being
collected and are later delivered to a collection
center, where they are stored until they can be
picked up by Florida LAKEWATCH staff and
delivered to the Univerity of Florida IFAS water
chemistry laboratory at the Department of
Fisheries and Aquatic Sciences.


In return for participation, volunteers
receive:

* Personalized training in water monitoring
techniques;
* Use of lake sampling materials and water
chemistry analysis;
* Periodic data reports, including an annual
data packet regarding their waterbody;
* Invitations to meetings where FLW staff
provides an interpretation of the findings as
well as general information about aquatic
habitats and water management;
* Access to freshwater and coastal marine
experts;
* Free newsletter subscription and educational
materials regarding lake ecology and water
management.

For more information, contact:
Florida LAKEWATCH
UF/IFAS
Department of Fisheries & Aquatic Sciences
7922 NW 71st Street
Gainesville, FL 32653-3071
Phone: (352) 392-4817
Toll-free: 1-800-LAKEWATCH (1-800-525-
3928)
E-mail: lakewatch@ufl.edu
Web-site: http://lakewatch.ifas.ufl.edu/




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