Fisheries Management

21 Pages·2012·0.99 MB·English

Fisheries Management:
5
Matching Expectations to Reality
Introduction
Anglers love to tell stories, and
a lake’s reputation is often based
on a few experiences by a limited
number of anglers. U nfortunately,
anecdotal evidence and casual
observations by a few anglers
do not give an accurate picture
of a fshery’s status. If a boat load
of friends went fshing and were
skunked, does that mean that the
lake is empty of fsh? A bad day
of fshing could be due to bad
luck, poor site selection, or poor
technique, and have nothing to
do with fsh abundance. Scientifc
surveys to determine the status of
a fshery are necessary to d evelop Fig. 5–1. Great expectations!
an a ccurate picture of the current Some anglers have unrealistic expectations about the size and availability of fsh
state of a lake’s fshery and to for harvest in small lakes. Acre-for-acre comparisons, however, show that small
develop and evaluate fshery lakes produce more fsh than large lakes. (Credit: John FoSter)
management plans. Without sci-
entifc surveys, fsheries management deteriorates to Ideally, lakes should be surveyed every three to fve
intuition and guesswork. years. Lakes with high socio-economic importance
This chapter introduces fsheries ecology, data- are more frequently surveyed. Small lakes are sur-
gathering tools and management strategies for veyed infrequently, however, because of limited state
managing the fsh, their habitat, and anglers. resources.
The environmental surveys discussed in Chapter
four, “Problem diagnosis,” provide an initial step
Gathering fisheries information
toward identifying the physical, chemical and bio-
The New York State Department of Environmen- logical factors affecting fsheries. This information
tal Conservation (DEC) is responsible for managing is important for understanding how best to manage a
fsheries in New York State. As DEC conducts its own fshery since lake ecology has a tremendous infuence
scientifc surveys on most lakes, their staff may also on fsh populations and the health of fsheries.
gather information in partnership with other groups When environmental survey information is com-
such as universities, conservation organizations, lake bined with fsh, angler and habitat survey information,
associations, and rod and gun clubs. With guidance a fsheries management plan can be developed, or
from a professional fsheries biologist, citizens can an existing plan evaluated. Fish, angler and habitat
gather very accurate fsheries data on their lake. surveys, discussed later in this chapter, give a good
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Diet For A SmAll lAke
indication of how much harvest a lake can support brown-trout production. The management plan would
and the level of angling pressure it receives. It wil then focus on improving the factors of brown-trout
indicate where habitat improvement or stocking can habitat that limit production. Physical limiting fac-
enhance fsh populations and angling opportunities. tors include habitat, temperature, light, elevation,
Before delving into these important data-gathering substrate, depth, structure, shoreline curves, turbidity,
tools, it helps to understand some basic principles and water clarity or transparency. The two most com-
of fsh habitat. mon factors limiting fsh production are temperature
and nutrient availability.
The temperature regime is the primary factor
Habitat limiting factors and
determining what species of fsh a lake can support.
critical parameters Lakes are classifed by their dominant fsh habitat
into three broad categories, optimal temperature of
Eachf shs pecies,a ndl ifes tagesw ithint hes ame 59oF (15oCelsius) for coldwater fsh, 68oF (20oC) for
speciesr, equired iferenth abitatst oc arryo utc ritical coolwater fsh, and 77oF (25oC) for warmwater fsh.
life functions, such as feeding, resting, hiding from Most trout stop feeding, for example, at 36o F (2.5oC),
predatorsa, nds pawningT. hep hysicalc, hemicala nd have optimum growth and survival at 59oF (15oC),
biological components of the habitat affect the popu- and are stressed at temperatures above 68oF (20oC).
lation dynamics of each species. Each of the species Overlap of fsh communities can occur in the same
will have an optimal range and a tolerance range for lake. Temperature optimums differ among life stages
every habitat component. Optimal temperature for within the same species. Many deep lakes can support
brook trout, for example, is around 60ºF (Fahrenheit) coldwater, coolwater and warmwater species in the
although their range of tolerance is from 32ºF to 74ºF. same lake. In deep lakes, however, available habitat
As temperature deviates from optimum, however, will best support one particular fsh community.
it begins to limit brook trout survival, growth, and Understanding the ecological relationships between
production and population size. Many fsheries are the temperature regime of a lake and the temperature
limited by a few obvious habitat parameters, but requirements of fsh provides insight into the possible
fsheries are more often limited by a combination or development of a particular fshery.
interaction of factors. Lake fertility is the primary factor determining the
Habitat analyses focus number and biomass of fsh. The more fertile the lake,
on determining the primary the more fsh-per-acre it will produce. Higher nutrient
limiting factor for a particular levels increase phytoplankton and zooplankton pro-
game fsh. Fisheries biology duction. Predation transfers this energy and biomass
and public interest can result through the food chain, resulting in increased fsh
in a lake being designated for production and their potential harvest.
Fig. 5–2. Generalized energy-fow diagram for a typical small lake or reservoir.
(Credit: John FoSter)
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FiSherieS mAnAgement: mAtching exPectAtionS to reAlity
Many anglers believe that the clear, cold, Adiron- Mean depth is an indicator of the extent of a lake’s
dack lakes provide the best fsheries in New York euphotic-littoral productive zone.
State. Actually, the green, warm, downstate lakes con- TDS provides a crude measure of a lake’s limiting
tain and produce the most fsh-per-acre. The higher nutrients such as phosphorus or nitrogen. Shallow
altitude Adirondack lakes have shorter summer with lakes are more productive than deep lakes. Phospho-
a shorter growing season, but the main difference rus is the most limiting nutrient in standing waters, so
between these crystal-clear mountain lakes and their it is also used to measure fertility. The optimal range
murkier southern counterparts is fertility. is from .01 to 3.0 parts per million (ppm.)
Lake fertility is frequently used by aquatic ecolo-
gists to classify lakes. High elevation and northern
Physical limiting factors
lakes in New York State tend to be either relatively
infertile oligotrophic lakes or dystrophic lakes with Temperature and nutrients are critically important,
rocky shores and few nutrients. Oligotrophic lakes but there are many other factors that also limit a lake’s
have scanty nourishment. Dystrophic lakes tend to be fshery. Fish require adequate space, or habitat, to
shallow, tea-colored, and high in humic matter, with live. Steep-sided lakes provide very limited littoral-
acidic waters and few plants. Oligotrophic lakes are zone habitat, and thus very limited space for fsh such
usually so deep and dystrophic lakes so stained with as pickerel, bass, or bullheads that require a shallow,
tannic acid that sunlight penetrates only the surface inshore, weedy habitat. Other physical limiting factors
waters. Lowland lakes of central and southern New include light, elevation, substrate, depth, structures,
York State tend to be extremely fertile, nutrient-rich, shoreline curves, turbidity and water clarity.
eutrophic lakes, surrounded by rich farmland. They Light is required for primary production and
are usually shallow, and plant growth is supported ultimately affects fsh productivity of all fsh in
because sunlight penetrates throughout the water the lake. Fish generally are more comfortable in
column. Between these two extremes are moderately dim light, but too little light reduces productivity.
nourished mesotrophic lakes. Fish production is signifcantly reduced in areas
Fertility or nutrient level can be used to accurately of prolonged cloud cover, such as the eastern side
predict the yield or standing crop biomass of fsh in of the Great Lakes,
a lake. Lake fertility determines fsh abundance and Elevation can be correlated with a number
the type of fsh species in a lake. Fisheries biologists of limiting factors. In central New York State,
measure fertility by conductivity, a measure of how trout usually do not occur in shallow lakes less
well electricity is conducted through water, or by than 1,500 feet above sea level because of the
Total Dissolved Solids (TDS) a measure of the level correlation between altitude and temperature.
of dissolved particles in the water. Optimal conduc- High-elevation lakes have a shorter growing
tivity ranges from 100 to 300 µmhos. (Electrical season and are fed by rain water which is low in
conductivity, µmho, is the opposite of electrical nutrients. High-elevation lakes, therefore, have
resistance, ohm. Conductivity is resistance spelled low productivity due to colder water, a shorter
backwards and preceded by the Greek letter µ.) growing season, low pH and low nutrients.
The morphoedaphic index is used by fsheries Substrate has multiple effects on the distribu-
biologists use to predict fsh yield per-acre-per-year. tion and production of fsh. The availability and
It is used to estimate the pounds of fsh available for amount of spawning gravel often limits trout
harvest and is calculated by multiplying the yield- production. Fine sediments such as sand and silt
per-acre by the surface area of the lake. This widely provide habitat for bullheads. Coarse sediments
used index is derived by dividing a lake’s TDS mg/l such as cobbles and boulders are favored by
(milligrams per liter), by its mean or average depth sculpins, rock bass and smallmouth bass.
in meters. The mean depth of a lake can be derived Depth can be a limiting factor, since many
by dividing the lake’s volume by its surface area. aquatic organisms are only found at specific
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depths. Sunfsh and pickerel are only found in Oxygen is required for respiration and
shallow water. In general, as depth increases, bio- m etabolism. While some species can survive at
mass and productivity decreases. In the northeast, very low levels of oxygen, production for most
however, shallow lakes less than 10 feet deep are fsh begins to drop when oxygen levels fall below
often subjected to winter-kill, reducing both plant fve ppm.
biomass and fsh productivity.
Carbon dioxide is optimal at levels from 5 to
Structures such as shoals and artifcial reefs
10 ppm, but becomes limiting at levels above 10
provide substrate for algae and invertebrates
ppm. High carbon-dioxide levels can occur in the
whose presence increases food available to fsh
deeper waters of highly productive lakes.
populations. Rocky shoals also provide shelter
pH is a measure of hydrogen ion concentration.
for structure-loving fsh such as rock bass and
Optimal pH for most aquatic organisms is from
smallmouth bass.
6.5 to 8.5. Brief exposure to higher and lower pH
Shoreline curves increase the ecotone or
may not be limiting.
transition zone effect and, therefore, increase
Alkalinity is a measure of carbonate and bi-
productivity. Round lakes are less productive than
carbonate ions, which buffer the changes in pH.
lakes with multiple embayments and shoreline
Optimal alkalinity is from 50 to 400 ppm. Levels
curves.
below 50 ppm often result in rapid fuctuations
Turbidity is a measure of particulates suspended
in pH levels that are detrimental to most aquatic
in the water column. High turbidity reduces water
clarity or transparency, reducing photosynthesis, organisms.
and making it diffcult for fsh and other aquatic Hardness is a measure of magnesium and cal-
animals to fnd food and oxygen. Optimal turbid- cium ions in the water. Optimal levels of hardness
ity for standing waters is 0 to 2 Nephelometric are from 50 to 400 ppm; levels below 50 ppm are
Turbidity Units (NTU), the standard unit for limiting. A low calcium level is often a limiting
measuring turbidity. (NTU replaces JTU, Jackson, factor in dystrophic lakes.
and FTU, Formazin, as turbidity measurement Ammonia is toxic to fsh and other aquatic
units.) organisms. In water, it occurs in two forms, the
Water clarity is the opposite of turbidity. It
toxic deionized form (NH ) and the non-toxic
3
is measured by lowering and retrieving a round, +
ionized form (NH ). The amount of ammonia
4
black-and-white plate called a Secchi disk into
in the toxic form is greater at higher temperatures
the lake (see Fig. 4-1, Chapter four “Problem
and pH. Total ammonia starts to become limiting
diagnosis”). Water clarity or transparency is mea-
around one ppm. High levels of ammonia can
sured by calculating the average depth at which
occur in the deeper waters of highly productive
the Secchi disk disappears and then reappears in
lakes in late winter or summer.
the water column. This depth provides a crude
Hydrogen sulfide is very toxic to aquatic
measure of the zone where plant photosynthesis
organisms, and continuous exposure to levels as
equals respiration. In lakes with low levels of
low as .002 ppm is lethal to most fsh. Hydrogen
inorganic suspended solids, it provides a relative
sulfde is more toxic at low temperatures and pH.
measure of the density of phytoplankton.
High levels of hydrogen sulfde can occur in the
deeper waters of highly productive lakes, also in
Chemical limiting factors
late winter or summer.
Chemical limiting factors include fertility, oxy- Heavy metals such as cadmium, copper, lead,
gen, carbon dioxide, pH, ammonia, hydrogen sulfde, mercury and zinc become toxic at levels as low as
heavy metals, alkalinity and hardness. See also the .002 ppm. Acidic lakes leach heavy metals from
discussion in Chapter four, “Problem diagnosis.” their substrates.
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FiSherieS mAnAgement: mAtching exPectAtionS to reAlity
reducing the size of the littoral zone and interfer-
Biological limiting factors
ing with littoral-zone fsh.
Biological limiting factors include benthic inver- Woody debris or deadfalls provide shelter and
tebrate production, zooplankton, the fsh community, feeding stations for many fsh species. The presence
woody debris, submergent rooted vegetation, and and density of many pan fsh (sunfsh, rock bass,
shoreline vegetation. bullheads and yellow perch) and game fsh (pike
Benthic, or bottom-dwelling, invertebrate pro- and smallmouth bass) are highly dependent upon
duction is consumed by benthic (or demersal) fsh the amount of deadfalls in a lake. Deadfalls also
such as lake trout, whitefsh, sculpins, suckers, provide substrate for algae and invertebrates, which
bullheads and carp, which are allfree-swimming increases overall lake productivity. In spite of these
species living close to the bottom. The size and positive attributes, deadfalls are the most frequent
diversity of the benthic community has a strong thing people remove on “lake clean-up days.”
infuence on the fsh community. A large biomass
of benthic invertebrates does not necessarily
result in a large biomass of fsh. Zebra mussels
(Dreissena polymorpha) can have a tremendous
biomass, but relatively little of it is converted into
fsh biomass. Zebra mussels strain plankton out
of the water column, reducing the food available
for planktivorous fsh, thus reducing the overall
fsh production of lakes they invade. A survey of
benthic invertebrates should focus on standing
crop biomass and species diversity. The presence
of an indicator species, such as blood worms
and tubifcid, or tube worms, is indicative of past
Fig. 5–3. Deadfalls and woody debris attract fsh by
episodes of oxygen depletion that could be limit-
providing structure and substrate for their food, but they
ing the biomass of benthic fsh.
are often removed during shoreline development.
Zooplankton are the main source of food for
(Credit: WAyne WurtSBAugh)
the fry stage of virtually all fsh. The fry stage
extends from yolk absorption by the larva to the Submergent rooted vegetation delineates the
fsh reaching approximately one gram in weight. littoral zone of a lake, the most productive of the
Zooplankton also feed many juvenile and adult three lake zones. Submergent rooted vegetation
fsh. The size and species diversity of the zoo- provides habitat for littoral zone fsh and food
plankton community infuence survival, growth for aquatic invertebrates and fsh. When the size
and abundance of the early life stages of most of the littoral zone is limited, species, population
lake fsh. Dystrophic lakes produce relatively few sizes and the production of the lake will also be
large zooplankton species, thus limiting the food limited.
base for planktivorous fsh. Shoreline vegetation, such as bulrushes, pro-
The fsh community limits the production of vide habitat for aquatic invertebrates that fsh eat,
some species through predation, competition or nesting areas for crappies, sunfsh and bass, and
interference. Alewives (Alosa sapidissima) can spawning habitat for northern pike. Emergent,
limit walleye production by predation on larval or above the surface vegetation also protects
walleye (Sander vitreus). They can also limit windswept shorelines from erosion by waves
production of ciscoes by out-competing them for and boat wakes. Shoreline wetlands can provide
food. Large populations of carp and bullheads a signifcant source of decaying organic material
uproot aquatic vegetation and cloud the water, to fuel the food chain of a lake.
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A tremendous amount of environmental data can
Scientific techniques for
be colected during one or two sample dates, but
conducting fisheries surveys
some limiting factors might be missed. Additional
dataa ndc arefulys electeds amplingd atesa ndt imes
may be necessary to gain a more complete picture. Successfulym anaginga f sheryr equiresk nowing
Seasonaly,l ates ummera ndl atew intera rem ore the habitat needs of the fsh. Scientifc surveys are
likely to have limiting conditions such as dissolved needed to determine existing fsh populations, fsh-
oxygena, mmoniah, ydrogens ulfdea ndte mperature. ing activity and fsh habitat. Information from these
During the day, early morning is likely to have low surveys is then used in the three major management
pHa ndd issolvedo xygenp roblems,w hilel atea fter - approaches: managing the fsh, the habitat, and the
noon may have temperature problems. angler.
Critical environmental conditions happen spo-
radicaly. Hot dry summers and severe winters may
Fsih surveys
occur infrequently. When a fshkil is the result of
a transient event, conditions can be back to normal During fsh surveys, fsheries biologists capture,
beforea s amplingt eamc ani solatet hep roblem.I ti s
identify, weigh, measure, and remove scales to
important to remember, however, that although fsh
check for age and growth determinations. These
abundance, survival and productivity can be limited
measurements provide valuable information about
bytr ansienet ventsp, hysicalc, hemicaal ndb iological
fsh population dynamics including population size,
conditions support fsh 99.9 percent of the time.
communitys tructurea, ge/sizes tructurer, eproductive
success, and growth. Information on habitat utiliza -
tion and movement paterns can also be gathered.
Fish-survey techniques are adapted to difer -
ent species and sizes. Fish in lakes are most often
surveyed with gil nets, trap nets, seine nets and
electro-fshing, although towed nets and angling are
sometimes used.
Gil nets used in fsh surveys are usualy 300
feet long, and consist of six, 50-foot panels of
diferent-sized mesh. Most fsh are vulnerable to
gil nets, and they are highly efective in both
deep and shalow water. Gil nets should be used
sparingly since the mortality of captured fsh is
high.L ivef shc anb er eleaseda fterw orkerst ake
measurementsa nds cales amplest hata rea nalyzed
later to determine fsh age and growth. Fish that
die in the net can stil be used for analysis of
their reproductive condition, stomach contents
and parasites.
Trap nets are most effective for fsh that live in
Fig. 5–4. Late-winter water-quality sampling may
shallow water at particular times of the year. Fish
indicate limiting factors, such as low oxygen levels, that
that migrate along the shore are vulnerable to trap
will reduce fsh survival and production. The device
shown can collect samples at various depths beneath the nets. These nets are very gentle on the captured
ice. (Credit: John FoSter) fsh and most can be released unharmed.
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FiSherieS mAnAgement: mAtching exPectAtionS to reAlity
specifc species. It is most effective in sampling
adult or sub-adult fsh. For many species, angling
is the most effective way of capturing fsh for
stomach analysis.
Fish surveys should be extended to lake tributary
streams if the species of interest uses streams as part
of their life cycle. Walleye and many trout species,
for example, use streams for spawning and nursery
areas.
Angler surveys
The goal of an angler survey is to collect informa-
tion about the fshery from the angler’s perspective.
To be effective, angler surveys must be conducted
throughout the fshing season. Five to ten percent of
the available fshing days in the season should be
covered, including both weekdays and weekends. A
good survey of anglers will include an angler census,
creel surveys, and angler interviews.
Angler censuses are usually conducted without
direct contact. Small lakes can be surveyed from
Fig. 5–5. Gill nets are used sparingly by professional
a lookout point, by boating around the lake, or
fsheries biologists to sample fsh populations
at a boat launch if lake access is restricted. The
throughout a lake. (Credit: John FoSter)
purpose of the census is to develop counts of how
Seine nets are typically small-meshed nets many anglers are fshing the lake each day and
that are used to capture young fsh, and are only whether they are shore fshing, trolling, or still-
effective when used close to shore. fshing. The census should also note boat size,
Electro-fshing is done from a boat using equip- angler age, and whether the angler is with other
ment that sends a non-lethal electrical charge into adults, families with children or is alone.
the water, temporarily stunning fsh so they can be Creel surveys focus on the fsh harvested by
easily netted at the surface. Electro-fshers work surveying the angler’s creel or basket used to hold
best in water less than six feet deep and, therefore, the day’s catch as a measure of angler success.
are most effective on shallow-water species such The species, number, and length of fsh harvested,
as bass and sunfsh. Boat electro-fshers are often and the number kept or released are recorded.
used at night since many fsh move to the surface During a creel survey, fsh are examined for fn
after dark. clips and disease, and scales are often removed to
Towed nets can be used to sample fsh eggs determine age. Anglers are often asked the length
and larvae. Trawls can be used to survey small of time they have been fshing, and the equipment
or young-of-the-year fsh on the bottom or in the and techniques they employed. Fishing success
water column of the lake. Towed nets in small is defned as catch-per-unit­effort, or the number
lakes typically use fne-mesh sizes. of fsh caught per hour of fshing. Data collected
Angling or fshing with a hook and line is an during creel surveys can be used to monitor fsh
effective method of surveying a wide range of populations, fshing techniques, and the effective-
species. It can be used at any depth and can target ness of management regulations.
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Diet For A SmAll lAke
Angler interviews assess the socio-economic effectiveness of fsh stocking using data from fn clips,
aspects of a fshery.T he focus of the interview is whether regulations are effective or necessary, and
to assess the angler’s preferences. Data colected how anglers are impacting fsh populations. Follow-
includeth eo rigino fth ea nglert,h em oneys penitn up surveys measure whether the management plan is
pursuito f shi nt hef sherya, ndt hes peciesb eing working or if it should be modifed.
soughtA. nglers are also asked to rate the fshery A bias may be inadvertently introduced into the
and management techniques or regulations. survey data if special interest groups conduct the
angler surveys. To evaluate the surveys, professional
The combination of the above components of a fshery biologists independently check the fshery by
good angler survey will provide signifcant information making a site visit to observe the fshery and perhaps
on the state of the fshery. Important data on fshing do some experimental fshing as a frst-hand assess-
pressure, the size and total number of fsh harvested, ment of fshing success.
and the recreational value of the fshery can only be Another approach to conducting angler surveys
determined by combining the three different data and monitoring a fshery is the Angler Diary Program
sets. Survey information can be used to determine the conducted annually by DEC. Anglers are asked to
record the date, where they were fshing, and number
and type of fsh caught in a diary. This approach
is loosely based on the observation that 20 percent
of anglers catch 80 percent of the fsh. By asking
the e xpert angler and the major users of a fsheries
resource to maintain a diary of their activities, DEC
fsheries biologists can effciently monitor the fshery.
The diary is mailed to DEC at the end of the fshing
season, and is a useful method of monitoring the fsh-
eries of small lakes that are infrequently surveyed.
Habitat surveys
The goal of habitat surveys is to characterize the
physical, chemical and biological environment of the
lake as they relate to fsh production. Most game fsh
have critical habitat requirements that are essential
for their existence and well-being. Periodic habitat
surveys, coupled with fsh surveys, indicate whether
changing water chemistry or habitat conditions are
causing changes in fsh populations.
Many aspects of habitat surveys can be conducted
at any time of year, but some surveys are better
conducted at specifc times. Physical and biological
surveys are usually done during the summer. Water-
chemistry surveys are generally conducted at the end
of summer and in the late winter, when environmen-
tal conditions such as dissolved oxygen, ammonia,
hydrogen sulfde and temperature are limiting fac-
Fig. 5–6. The DEC Angler Diary is a useful tool for tors. Water chemistry is discussed in more detail in
monitoring fsheries. (Credit: deC) Chapter four, “Problem diagnosis.”
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FiSherieS mAnAgement: mAtching exPectAtionS to reAlity
Physical habitat surveys are focused on mea- Fish community structure
sures of size, depth, temperature and turbidity or
suspended solids. Other measures of the physical The frst step in characterizing the fsh commu-
environment include fushing rate, water clarity, nity of a lake is to conduct a survey on the relative
and substrate type. Initial habitat surveys involve abundance of the different species living in the lake.
the development of a hydrographic map. This The next step is to characterize and group the fsh
will assist in determining the extent of the lit- species by their habitat, niche and value to the fsh-
toral, limnetic and profundal zones in a lake, and ery. Habitat describes where in the lake the fsh is
help classify whether a lake is best suited for a found (residence), and niche describes what it does
warmwater, coolwater or coldwater fshery. there (occupation) or its role in lake ecology, such
Water-chemistry surveys are conducted to as planktivore. The value to the fshery is assessed
determine whether water-quality parameters are in terms of whether a fsh is a game fsh, pan fsh,
limiting fsh populations. Common limiting factors or forage fsh. Forage fsh, such as minnows, darters
are dissolved oxygen, pH, alkalinity, hardness, and and sculpins are preyed upon by game fsh, pan fsh,
fertility or available food supply. While profes- or other competitors.
sional fsheries biologists rely on electronic meters Fisheries biologists seek specifc data to analyze
to make these measurements, relatively inexpen- the fsh fauna in a lake. Do warmwater, coolwater
sive and accurate chemical test kits can also be or coldwater fsh predominate? Is there a good
used to conduct water-chemistry surveys. balance between species of the littoral zone with
Biological habitat surveys focus in three weedy inshore species, and species that can feed on
a reas: aquatic plants, benthic or bottom-dwelling plankton in open water such as yellow perch, crap-
invertebrates, and plankton. Fisheries biologists pie and golden shiner? Are benthic feeders such as
survey the biological environment to determine bullheads, suckers, or whitefsh in low density? What
if adequate resources are available to support the percentage of the fsh community consists of species
fshery. Biological surveys of lower trophic levels of interest to anglers?
can also be used to assess the relative abundance of What constitutes a good fsh community structure
fsh. The size and species of plankton, for example, is often open to interpretation. Fisheries biologists
are a good indication of the relative abundance of and anglers are usually focused on flling most
planktivorous species such as alewife, compared niches with fshable species. In a coldwater lake, for
to predatory species such as lake trout. example, is it better to have alewives or ciscoes as
the forage species for lake trout? Alewives appear
to support larger populations of lake trout, but are
Analysis of fish populations
themselves not a fshable species. Ciscoes, on the
Surveys provide a tremendous amount of data other hand, are quite tasty, and can support their own
defning the state of the fsh population. Fish commu- fshery. With ciscoes and lake trout, therefore, there
nity structure and population dynamics are important is a fshery for both species, but with alewives and
components of these data, which are then analyzed lake trout there is only one fshable species.
by professional fsheries biologists. Data required to analyze community structure are
Analysis of community structure provides an often diffcult to collect in complex lakes. Surveys
overall assessment of the lake’s fsh population, but usually require the use of seines, experimental gill
most fsheries biologists and anglers focus on col- nets, trap nets and electro-fshing.
lecting data about game-fsh populations. Common Relative fsh abundance is the most widely
measures of game fsh include population size, and used measure of community structure, comparing
physical size or age structure, as well as measures the proportional representation of each species in
of population well being, including diet, condition the fsh community and providing an overall view
and growth. of community structure. Relative abundance data
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Diet For A SmAll lAke
indicate the “type” of fsh community. If bass, the fsh community. Diversity increases either
sunfsh and bulheads dominate the fsh commu - by having more species or by having species
nity, the lake is probably eutrophic and best as a numbers in balance. Having large numbers of
warmwaterf shery.R elativea bundanced ataa lso species, without a dominant species, indicates a
permit calculations on the proportion of fshable more diverse fsh community. Fish diversity is
speciest og ames peciesR. elativea bundanced ata often strongly correlated with habitat diversity.
alsoi ndicatet her elativeb alanceo ft hef shc om- Many aquatic ecologists use the Shannon-Wiener
munity, and whether al niches are fled. Diversity Index as one of several measures of fsh
The ratio of predators to available prey is biodiversity. The fsh community of most small
another way to measure the balance within a fsh lakes would have an expected diversity in the
community. Predators are carnivores that feed on range of one to four species.
other fsh. Prey fsh, also called forage fsh, can Richness is a similar, but much simpler mea-
be divided into carnivores that feed on inverte- sure of diversity. It is a measure of the number of
brates and zooplankton and herbivores that feed species in a lake or community. Current theory is
on plants and detritus. Predator­to­prey ratio is that lakes with a larger number of species have a
calculated by dividing the combined weight of all more stable fsh community than lakes with fewer
prey by the combined weight of all predators in a species.
lake. The ratio can be determined by comparison
of predators and prey in a sample collected dur-
Population structure
ing an electro-fshing survey. Fish production is
most effcient when there exists a proper balance Fish population structure describes the different
between predator and prey. The most desirable sizes or ages of fsh that make up a specifc species.
predator-to-prey ratio is 1:4-6 by weight. For each An accurate assessment of the population structure
pound of predator, there should be four to six
would require length or age measurements of a
pounds of prey.
random sample of at least 100 fsh. Care should be
Relative balance of the fsh community in a
taken that the sampling gear is not overly selective
lake is another measure of community structure.
for either smaller or larger fsh. Electro-fshing gear,
The amount of game fsh in most well-managed
for example, is selective for larger fsh, but a trap net
small lakes typically represents 15 to 25 percent of
is not size-selective.
the fsh community. This measure is very similar
Length-to-frequency distributions plot the number
to the predator-to-prey ratio since game species,
of fsh in a particular species into different length
such as bass, walleye and musky, are predators.
groups. These distributions refect an interaction of
There are some predatory fsh such as gar and
rates of reproduction, growth, and mortality of the age
bowfn, however, that are not considered game
groups present. The length-to-frequency distribution
species, and thus are not considered desirable
is used to assess fsheries-ecology interactions, and
predators by fsheries managers.
can be used to identify problems such as year-class
strength failure, high fshing pressure, habitat or life-
Diversity and richness are other measures of
stage bottlenecks, low recruitment, and stunting or
the fsh community that are used when an aquatic
slow growth.
ecologist surveys a lake. Aquatic ecologists do not
Year-class strength is a measure of the propor-
have the bias towards fshable species that fsherman
and fsheries biologists do. Ecologists value all fsh tion of a fsh population born in a particular year,
equally, while fsheries biologists consider game fsh the interaction between birth rate and survival.
and pan fsh to be more valuable than other fsh. Year-class failure, therefore, could be due to
Diversity measures the number of fsh species either a high mortality rate or a reproductive
in a lake and their proportional representation in failure rate.
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