United States
Environmental Protection
Agency
environmental Research
Laboratory
Corvallis OR 97330
EPA-600 3 80 036
Mdri-h 1980
Research and Development
&EPA
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Influence of
Advanced Wastewater
Treatment on the
Fishery Resource of
Shagawa Lake,
Minnesota
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RESEARCH REPORTING SERIES
Research reports of the Office of Resear^n and Development. U S Environmental
Protection Agency have been grouoed into nine series These nine broad cate-
gories were established to facihta e further development and application of en-
vironmental technology Elimination cf traditional grouping was consciously
planned to foster technology transfer and a maximum .ntertace in related fields
The nine series are
I Environmental Health Effects Research
2 Environmental Protection Technology
8 Ecological Research
4 Environmental Monitoring
5 Socioeronomic Environmental Studies
6 Scienti ic and Techn.ca1 Assessment Reports (STAR)
7 Interagoncy Energv-Envircnment Research and Development
8 Special Reports
9 Miscellaneous Reports
This report has been assigned tc the ECGLOGiCAL RESEARCH series This series
describes research en the effects of po.lution on humans p'ant and animal spe-
cies, and materials Problems are assessed Tor their long- a*vJ short-term influ-
ences Investigations inc'ui.T'formation transport ana pcithwj, studies tc deter
mine the fate of pc Mutants and tnei' erect: Tr
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EPA-600/3-80-036
March 1980
INFLUENCE OF ADVANCED WASTEWATER TREATMENT
ON THE FISHERY RESOURCE OF SHAGAWA LAKE, MINNESOTA
by
William A. Swenson
Department of Biology
University of Wisconsin
Superior, Wisconsin 54880
Project Officer
Paul D. Smith
Corvallis Environmental Research Laboratory
Corvallis, Oregon 97330
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floor
Chicago, IL 60604-3590
CORVALLIS ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CORVALLIS, OREGON 97330
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DISCLAIMER
This report has been reviewed by the U.S. Environmental Protection
Agency, Environmental Research Laboratory-Corvallis, and approved for publi-
cation. Approval does not signify that the contents necessarily reflect the
views and policies of the U.S. Environmental Protection Agency, nor does men-
tion of trade names or commercial products constitute endorsement or recommen-
dation for use.
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FOREWORD
Effective regulatory and enforcement actions by the Environmental Pro-
tection Agency would be virtually impossible without sound scientific data on
pollutants and their impact on environmental stability and human health.
Responsibility for building this data base has been assigned to EPA's Office
of Research and Development and its 15 major field installations, one of
which is the Corvallis Environmental Research Laboratory (CERL).
The primary mission of the Corvallis Laboratory is research on the
effects of environmental pollutants on terrestrial, freshwater, and marine
ecosystems; the behavior, effects, and control of pollutants in lakes and
streams; and the development of predictive models on the movement of pollu-
tants in the biosphere.
This study was initiated to define what effects, if any, reduced nutri-
ent loading had on the fish species complex in lakes. This information is
essential to relating cost and benefits of the regulatory and enforcement
activities of EPA as they relate to the national eutrophication problem.
Although quantification of effects proved to be impractical within the time
frame of this study, this report provides some new insights into the subtle
mechanisms by which the fish species complex of temperate lakes is controlled
by changes in their trophic status. A conceptual model developed to define
the changes x\/hich occurred in Shagawa Lake will have application in predict-
ing the general effects of controlling nutrient loading in other lake systems.
Thomas A. Murphy
Director
Environmental Research Laboratory-Corvallis
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ABSTRACT
This project was conducted to measure the response of the warmwater fish
populations in Shagawa Lake, Minnesota, to altered trophic conditions brought
about by phosphate removal from sewage discharges entering the lake. The
project also served to provide basic information on the mechanisms through
which a lake's trophic status controls its fish species complex.
Abundance, distribution, growth, and feeding interrelationships of wall-
eye (Stizostedion yitreum vitreum), northern pike (Esox lucius), yellow perch
(Perca flavescens), and lake herring (Coregonus artedii) were described in
relation to changes in prey density and other environmental factors. A con-
ceptual model based on the "niche concept" was developed to identify the in-
fluence of changes in various parts of the system on individual fish popula-
tions and on the total fish species complex. Information on physical-chemical
parameters and the response of other segments of the lake's community to the
lake restoration process was provided by concurrent investigations conducted
by or coordinated through the EPA Environmental Research Laboratory-Corvallis,
Oregon.
Recycling of nutrients xras found to slow the response of the system to
the restoration program. Zooplankton density was not altered significantly,
and hypolimnetic oxygen remained low throughout the three year period of
study following the initiation of phosphate removal. Transparency and abun-
dance of macrophytes increased. The field data and model suggest that pro-
duction of walleye is being reduced by increased transparency, higher macro-
phyte abundance and continued low hypolimnetic oxygen concentrations. These
factors restrict walleye feeding and their distribution. They also promote
increased cannibalism and northern pike predation. Growth and production of
yellow perch and lake herring appears to be limited by some of the same fac-
tors which control production of walleye and by apparent changes in the zoo-
plankton assemblage. The "niche" and production of the northern pike popula-
tion is expanding in response to changes in several feeding-related "niche
dimensions."
It is recommended that advanced wastewater treatment and the fish studies
be continued to allow for improved precision in measuring the effects of
altered trophic conditions on fish production and to promote higher hypo-
limnetic oxygen concentrations and related fish production in Shagawa Lake.
This report was submitted in fulfillment of grant number R-803673 by
University of Wisconsin-Superior, Center for Lake Superior Environmental
Studies, under the sponsorship of the U.S. Environmental Protection Agency.
This report covers a period from June 1974 to September 1977 and work was
completed as of December 1979.
iv
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TABLE OF CONTENTS
Foreword iii
Abstract iv
Acknowledgements vii
Introduction 1
Conclusions 4
Recommendations 6
Methods
Field Sampling 7
Abundance and Distribution Analysis 11
Growth, Population Structure and Mortality Measurements 13
Feeding Analysis 14
Measurement of Parasite Infestation Rates 14
Results
Prey Abundance and Distribution 16
Abundance and Distribution of Age II and Older Fish 18
Walleye Population Studies 18
Population Structure and Survival 18
Growth 18
Feeding 25
Age 0 Walleye 25
Food Consumption 25
Prey Quality and Feeding 33
Prey Density and Feeding 37
Physical Conditions and Feeding 37
Perch Population Studies 39
Growth 39
Food Habits 43
v
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Lake Herring Population Studies 43
Population Structure 43
Growth 43
Food Habits 52
Triaenophorus sp. Infestation Rates 52
Northern Pike Population Studies 57
Population Structure 57
Growth 57
Food Habits 57
Discussion
Lake Restoration and Fish Habitat 65
The Niche Model 66
Influence of Lake Restoration on Niche Size 67
Prey Populations 70
Transparency 71
Macrophyte Distribution 71
Oxygen Concentration 72
Summation of Effects 72
References 74
Appendix Tables 79
VI
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ACKNOWLEDGEMENTS
Mr. Douglas Standen, Mr. Jack Granquist and Mr. Walter Wasko assisted
in field work and analysis of walleye population data. Mr. Larry Brooke
assisted in studies of lake herring, and Mr. Keith Otis took primary respon-
sibility for analysis of yellow perch population statistics. The special
performance of these project participants is gratefully acknowledged.
The coordinating efforts of Mr. Paul Smith, the EPA project officer,
were significant to efficient operation in the field and in insuring that
information developed through related Shagawa Lake research was available
for our analysis.
vn
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INTRODUCTION
Shagawa Lake, St. Louis County, Minnesota, is a softwater Canadian shield
lake on the edge of the Boundary Waters Canoe Area, where tourism represents
a significant part of the regional economy. Paleolimnological studies indi-
cate Shagawa Lake was mesotrophic prior to the arrival of European settlers
around the turn of the century and subsequently became eutrophic (Bradbury
and Waddington, 1973; Gorham and Sanger, 1976). Municipal effluent from the
city of Ely represents the probable cause for the change in trophic status.
Sediment deposition resulting from inlake disposal of mine tailings and log-
ging in the region may have contributed to the eutrophication process
(Bradbury and Megard, 1972).
Shagawa Lake supports important sport fisheries for walleye (Stizostedion
vitreum vitreum), northern pike (Esox lucius) and lake herring (Coregonus
artedii). Yellow perch (Perca flavescens) are also abundant. Walleye and
northern pike are fished during all seasons. Lake herring are taken during
the autumn spawning period when gill netting is permitted. Importance of the
fisheries is suggested by the presence of four active resorts and by the al-
most continuous line of marker buoys which dot much of the shoreline during
the herring netting season. Herring fishing is stimulated by the relatively
low parasite infestation rates (Triaenophorus sp.) and large size of fish in
the Shagawa Lake population. Fishery surveys conducted by the Minnesota
Department of Natural Resources suggest densities of walleye, northern pike
and lake herring in Shagawa Lake exceed averages for Northern Minnesota
lakes.1
Shagawa Lake was selected by the U.S. Environmental Protection Agency to
demonstrate the effectiveness of advanced wastewater treatment in reducing
nutrient loading and promoting return of culturally enriched waters to their
natural trophic level. An advanced wastewater treatment plant was constructed
and began operation in 1973 reducing total phosphorus loading to the lake by
75% (Larsen et al., 1975). The recovery process was studied extensively by
agency and cooperating scientists to define the response of the system and
the mechanisms that controlled the changes which occurred.
This fish population study was performed to measure the response of the
fish species complex to lake restoration. The primary objective was to
develop a conceptual model that would describe significant changes in the
fish stocks and the mechanisms which control change. Development of the model
-'-Minnesota Department of Natural Resources, Shagawa Lake Survey Reports
for 1959, 1966 and 1967.
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was based on existing information, analysis of habitat alterations in Shagawa
Lake (conducted primarily by cooperating scientists) and on a comprehensive
study of the Shagawa Lake fish species complex. Niche theory served as the
framework for the model. The geographic extent of the cultural eutrophica-
tion problem, the cost of lake restoration and the need for information on
the influence of lake restoration on fishery resources represent the stimuli
which resulted in the study.
Altered hypolimnetic oxygen concentration is frequently identified as
the cause for shifts in the fish species complex of lakes subject to cultural
eutrophication or restoration (Beeton, 1969; Lukowics, 1967; Colby and Brooke,
1969; Colby, 1972; Leach jrt jil_. , 1977). Where anoxic conditions occur or are
approached, production of coldwater species has declined. Population changes
have been identified with two types of stress. Fish inhabiting the hypo-
limnion are stressed directly by low oxygen and may exhibit reduced growth or
survival (Colby, 1972). Reduced growth and survival also occur when oxygen
stress stimulates migration into the epilimnion and results in thermal stress
or increased interaction with warmwater species. These modes of action have
been identified as the mechanisms through which eutrophication results in
reduced production or extirpation of many coldwater species, including the
lake herring (Frey, 1955; Beeton, 1969; Colby and Brooke, 1969; Colby et al.,
1972). Conversely, where lake restoration has resulted in increased hypo-
limnion oxygen concentrations, production of coldwater fish has been stimu-
lated (Lukowics, 1967).
Although hypolimnetic oxygen depletion may restrict populations from
habitat which would normally be available, a positive relationship between
fish growth and trophic status of lakes appears to be characteristic except
under conditions of advanced eutrophication. The positive relationship is
attributable to increased energy availability (Leach et^ a.1. , 1977; Oglesby,
1977). Analysis of fish populations in several lakes undergoing eutrophica-
tion shows that the optimum trophic condition for yellow perch and walleye is
high with the optimum for yellow perch exceeding that of walleye (Leach et al.,
1977). Eggers _et^ aJ. (1978) found growth and production of yellow perch and
other benthic-littoral species declined as Lake Washington recovered from
advanced cultural eutrophication. Production of pelagic planktivores in Lake
Washington was not influenced by shifts in the plankton community associated
with the recovery process. Eggers et al. (1978) attributed the difference in
responsiveness to the strength of refuges that each habitat provides. Because
benthic-littoral species are provided greater cover and protection from pre-
dation, it is suggested that they are more likely to be resource limited and
responsive to changing trophic status.
Relationships between prey density and feeding rates of walleye (Swenson
and Smith, 1976; Swenson, 1977) show prey densities exceed the levels limiting
to walleye feeding in mesotrophic Lake of the Woods, Minnesota, during most
of the growing season, but are below the limiting level throughout the year
in oligotrophic Lake Superior. These observations and those of Beeton (1969),
who suggests destruction of spawning habitat occurs under eutrophic condi-
tions, indicate the mechanisms which control the relationship between trophic
level and walleye production.
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Destruction of spawning and cover habitat has been suggested as the mech-
anism which controls the relationship between northern pike production and
trophic status of Oneida Lake. On the basis of historic information, Forney
(1977) suggests that production of northern pike was higher during early years
when the lake was mesotrophic and supported an extensive growth of macro-
phytes. With cultural enrichment, abundance of macrophytes and northern pike
declined. Forney's (1977) observations are supported by Makowecki (1973) who
showed northern pike density is positively correlated with abundance of macro-
phytes and by Hurley and Christie (1977) who found macrophyte abundance de-
clines in eutrophic lakes in response to reduced transparency.
Forney (1977) proposes that reduced abundance of northern pike in Oneida
Lake stimulated production of walleye because interspecific food competition
and predation were reduced. Food densities and light levels favorable to
walleye feeding have been found to occur under mesotrophic and moderately
eutrophic conditions (Swenson, 1977) and represent alternate or additional
mechanisms which may have resulted in accelerated walleye production in Oneida
Lake in recent years.
The importance of light intensity as an isolation mechanism between wall-
eye, yellow perch and other species has been considered by Kerr and Ryder
(1977) on the basis of niche theory. They suggest that the reduction in light
intensity which occurs at higher trophic levels in lakes serves to isolate
feeding of yellow perch and walleye in time, expanding the niche of both
species by reducing their feeding interactions. Because most physical and
biological requirements of a population can be identified as individual niche
dimensions and are subject to measurement, they suggest the niche concept pro-
vides a useful framework for development of models which explain the response
of resource populations to changes in lake systems.
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CONCLUSIONS
This study demonstrated that no dramatic changes have occurred in the
fish species complex of Shagawa Lake during the first three years of tertiary
wastewater treatment plant operation. Internal cycling of nutrients within
the lake slowed the response of the entire system to reduced nutrient loading.
The rates at which changes are occurring suggest that internal processes are
effective in maintaining system stability and in controlling the rate at which
lake systems proceed from one trophic equilibrium condition to another. The
rate of system response substantiates the need for extended research in mea-
suring the influence of lake restoration on lake communities. Although the
duration of this project did not provide for quantitative measurements of the
effects of lake restoration in the Shagawa Lake fish species complex, the
general responses of major species populations were described and a conceptual
model useful in defining the changes which occurred and the mechanisms respon-
sible for bringing them about was developed.
The field data and model indicate that increased transparency and ex-
panded distribution of macrophytes are resulting in higher production of light-
tolerant species which associate with macrophytes, particularly northern pike.
Production of walleye and other species not well adapted to high light inten-
sities or the occurrence of macrophytes appear to be declining. This study
indicates that macrophytes are expanding their distribution to water 4 m deep
as a result of increased transparency and that walleye do not distribute near
macrophytes. Feeding success of walleye was reduced during periods of in-
creased light intensity and when age 0 yellow perch concentrated in areas
with macrophytes. Availability of yellow perch appears to control walleye
production through its influence on feeding and growth and by regulating
walleye cannibalism which increases when yellow perch are not available. In
addition to cannibalism, an expanding northern pike population appears to be
increasing predation pressure on the walleye population. Northern pike prey
heavily on walleye during July-August when age 0 walleye concentrate in in-
shore areas. Increased northern pike production is demonstrated by above
average growth during 1974 and 1975. Northern pike appear to be taking advan-
tage of high yellow perch availability in areas with macrophytes.
Depressed oxygen conditions during the 1974-1976 sampling period limited
distribution of walleye and troutperch (Percopsis omiscomaycus), the two most
abundant light-sensitive species, to water less than 5 m deep during much of
the open water season. At the same time, increased light intensity and macro-
phytes restricted their distribution and feeding in the inshore area. If con-
tinued lake restoration results in maintenance of hypolimnetic oxygen concen-
trations above 5 ppm, the habitat available to troutperch and walleye would
expand. Increased production and availability of troutperch and macroinver-
-------
tebrates should result in improved feeding, growth and production of walleye.
Increased availability of alternate foods in the deep water areas should pro-
mote higher walleye survival by limiting walleye cannibalism. Increased
availability of macroinvertebrates in deep water (>5 m) should also stimulate
faster growth and production of the yellow perch and lake herring.
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RECOMMENDATIONS
It is recommended that research on the effects of lake restoration
activities on Shagawa Lake be continued. Although it is recognized that
wastewater treatment plant operation and ongoing research represent sizable
investments, continuation of the project is important from several points of
view. The project is providing information on the benefits and costs of lake
restoration essential to the development of effective national and interna-
tional pollution control programs. At a time when government is requiring
major expenditures for pollution control, the identification of benefits and
costs to the resources that these expenditures are expected to improve must
be a prerequisite. Studies conducted under the Shagawa Lake program repre-
sent a primary source for such information. Shagawa Lake research has demon-
strated that a variety of mechanisms exist which can slow a lake's response
to reduced nutrient loading. Further study should provide an opportunity to
accurately identify the relative influence of these homeostatic mechanisms
and to predict which mechanisms will exert the greatest control in Shagawa
Lake and similar lake systems. Understanding of these mechanisms and the
ability to predict their effects is important in identifying the influence
of pollution control activities on fisheries and to the general area of
fishery management. Because fish represent a primary pollution-related con-
cern of the public, an important recreational resource and an important source
of protein, information gained through continued study of the Shagawa Lake
fish species complex would have broad application.
Under present conditions, changes brought about by lake restoration
measures have resulted in a loss of walleye habitat, the principal species in
the fishery. The results of this study suggest that continued control of
nutrient loading will benefit the Shagawa Lake fish community if higher oxygen
concentrations are maintained in the hypolimnion, thereby enlarging the niches
of walleye, troutperch, yellow perch and lake herring.
Because the study design minimized impact on the species complex and pro-
vided precise measurements of those dynamic characteristics which are most
responsive to system energetics, continued analysis of the fish species com-
plex should follow a similar design. It is recommended that additional work
be initiated during 1981 or 1982. The delay is recommended to insure that the
response of the fish species complex will have progressed enough to be mea-
sured accurately.
Continued work on Shagawa Lake should provide general estimates of pro-
duction for major warmwater species under varying trophic conditions. Pre-
dictive models developed through such studies would find broad application in
designing lake restoration programs.
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METHODS
FIELD SAMPLING
The field sampling program was designed to provide information on abun-
dance, growth, feeding and distribution of resource populations and abundance
and distribution of prey. Fish were collected during 1974-1976 on 52 days
from 25 stations (Table 1, Figure 1).
A 7.6 m (25 ft) headrope semiballoon trawl, 3 cm (1-1/4 in) bar-mesh with
a 0.6 cm (1/4 in) cod liner was the principal gear used in collecting samples.
A total of 337 tows were made at depths from 1.2 to 13 m. The occurrence of
rocks on the bottom made trawling impossible in most inshore areas. Trawling
was limited to three shallow water (<5 m) stations (Stations I, II, III;
Figure 1) and three deep water (>5 m) locations (Stations IV, V, VI; Figure 1)
where the substrate was composed primarily of sand and organic materials.
Sampling was stratified on the basis of fish abundance with most trawling
occurring at inshore stations during June through mid-September and at the
deeper water stations during late-September through November. However, tows
were made in shallow and deep water during most days to provide information
on distribution.
Tow distances were estimated from readings of a Clarke-Bumpus meter which
was towed with the net on selected dates during 1974-1975 and from readings
of a net meter mounted in a Tri-Vane which was towed with each trawl during
1976. Standard 5 min trawls covered 365 m. Average trawling distance was
equivalent to approximately one twentieth of the maximum length of Shagawa
Lake and one eighth of its maximum width. Measurement of the net opening
permitted estimation of the average volume sampled (1,600 nP). Depth was
measured by fathometer (Vexilar Model 510) or by dropping a lead line at one
or two minute intervals during trawling. Temperature was measured at most
trawling locations with a Yellow Springs Instrument Model 43 Telethermometer
or a Model 54A oxygen-temperature system. Oxygen was measured on selected
dates. Presence of aquatic macrophytes or other debris in the net was gen-
erally noted. Gross estimates of the volume of macrophytes in tows made at
different depths served as an index of macrophyte abundance and distribution.
Gill nets and seines were used to sample areas and segments of the fish
species complex not effectively captured by trawl. Nets similar to those
used during the Minnesota Department of Natural Resources surveys were em-
ployed so catches could be compared. Experimental gill nets were 77 m (250
ft) by 1.9 m (6 ft) with 5 meshes of 1.9, 2.5, 3.2, 3.8 and 5.1 cm (0.75, 1,
1.25, 1.5, 2.0 in) bar mesh. Gill nets were used on 15 sampling days and a
total of 41 sets were made (Table 1). Three nets were usually set overnight
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10
-------
(20-25 h) with one suspended near the surface (0.5 to 2.5 m), one suspended
midwater (3-6 m) and one set on the bottom. The gill nets were more effective
in capturing walleye exceeding 400 mm total length (XL) than the trawl
(Swenson and Smith, 1976).
A 15.4 m (50 ft) by 1.9 m (6 ft) seine constructed of 0.6 cm (1/4 in)
bar-mesh with a 1.1 m (3.5 ft) bag was used to sample the nearshore zone.
The area seined was measured and sampling volumes were calculated from esti-
mates of the area seined and average seining depth. Use of the seine was
limited by the occurrence of rocks, depth inshore and dense growths of aquatic
macrophytes which appeared to be more prevalent during 1975 and 1976. A
larger seine (62 m x 1.9 m; constructed of 1.9 cm bar-mesh) was tested on two
days to determine its effectiveness in capturing northern pike. However,
aquatic macrophytes made use of the gear impractical. Ineffectiveness of the
method limited the number of northern pike in the sample (Table 2).
Total catch was recorded for each species. Abundance of age 0 and older
individuals was reported separately for larger species. Length, weight,
scale samples and stomach samples or stomach content were collected in the
field from walleye, northern pike, lake herring, yellow perch and white sucker
(Catostomus commersoni) (Table 2). All walleye and yellow perch stomach
samples were collected from trawl catches. Some northern pike and most lake
herring samples were taken from gill net catches. A stomach pumping system
(Swenson, 1972) was used to remove food from walleye exceeding 125 mm TL
which were then returned to the lake alive. The portion of the digestive
tract between the anterior end of the esophagus and terminus of the pyloric
sphincter was removed from perch, lake herring and northern pike exceeding
125 mm TL. Stomach content from larger walleye, excised stomachs from other
species and intact smaller fish (<125 mm) were preserved in 5 to 10% formalin.
ABUNDANCE AND DISTRIBUTION ANALYSIS
Densities of fish effectively sampled by trawl and seine were estimated
from field counts and sampling volume estimates. Densities were expressed as
number per 100 m^ for both large and small species or size groups. Average
weights of prey size fish were calculated for each sampling day (bimonthly,
for less abundant species). Prey density was calculated as the total catch
of all prey sized fish, expressed as milligrams prey per cubic meter of water
(mg-nT3).
Changes in abundance of various fish species were related to indicies of
primary production (chlorophyll a concentration), oxygen concentration, abun-
dance of macrophytes, time, depth and predation rates to define the potential
influence of these factors on distribution and abundance. Estimates of prey
density (mg-m~3) were used in measuring the influence of food availability on
food consumption by walleye and as a means of comparing energy availability
in Shagawa Lake with other waters.
Catch per gill net set was compared with Minnesota Department of Natural
Resources Fishery Survey catches to identify changes in abundance. Catches
11
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from gill nets set at different depths were used to describe vertical distri-
bution.
GROWTH, POPULATION STRUCTURE AND MORTALITY MEASUREMENTS
Walleye age and growth were estimated from 366 scale samples subsampled
from 1974-1976 collections and 86 scale samples subsampled from collections
made during the 1967 Minnesota Department of Natural Resources Survey.
Scale samples from 331 lake herring, 669 yellow perch and 192 northern pike
subsampled from the 1974-1976 collections and from 69 northern pike collected
during 1967 were used in measuring growth.
Determination of age was made from cellulose acetate impressions with
annuli in walleye and perch distinguished on the basis of criteria for ctenoid
scales (Hile, 1941; Regier, 1962). Lake herring annuli showed accessory
checks after the first year. Checks were distinguished from true annuli by
the degree of lateral cutting and intensity of circuli in the posterior field.
All scales were read at least twice to assure accuracy using an Eberbach pro-
jector which magnified the image 44X (walleye and lake herring) or 100X (perch
and northern pike). Growth to each annulus was back-calculated using body-
scale relationships and corrected nomographs (Carlander and Smith, 1944).
Body-scale relationships were developed from total length measurements
and measurement of anterior scale fields for 137 walleye and 70 northern pike
collected during 1967 by the Minnesota Department of Natural Resources and
from 400 walleye, 259 northern pike, 669 yellow perch and 331 lake herring
collected during this study (1974-1976). Separate body-scale relationships
were developed for the 1967 and 1974-1976 sampling periods to control varia-
tion between the two periods.
Growth of walleye is described for each year-class from 1966 through
1975. Growth of the 1967-1976 yellow perch year-classes, 1966-1974 lake
herring year-classes and 1961-1966 and 1969-1975 northern pike year-classes
was measured. Grand average length was calculated and compared to growth
estimates for other waters. Percentage deviation in mean annual growth (Hile,
1941) and changes in annual growth increments were used to identify potential
effects of lake restoration on growth and production.
Length modes of age 0 walleye and yellow perch were estimated for each
sampling period and used to describe growth variation between 1974, 1975 and
1976. Differences between sampling seasons, comparisons with other waters and
variations between the two species were used to identify the conditions which
control growth during the first year of life.
Age structure of walleye, northern pike and lake herring populations was
described from the selected aged samples and the larger random sample of fish
''"Scale samples collected during 1967 were provided by Dennis Ernst,
Minnesota Department of Natural Resources, Fisheries Manager.
13
-------
measured in the field. Assignment of age to fish measured in the field was
based on the procedure of Ketchen (1950). Population structure was used to
compare the general success of year-classes during years following lake res-
toration and those immediately preceding treatment. Walleye age structure was
used to construct a catch curve and estimate annual mortality (Ricker, 1975).
FEEDING ANALYSIS
Food habits of walleye, yellow perch, lake herring and northern pike were
described through analysis of samples from 2,791 fish. Prey fish were identi-
fied on the basis of anatomical characteristics including features on scales,
structure of vertebrae and structure of the gastrointestinal tract. Inverte-
brates were sorted and identified using keys by Eddy and Hodson (1962), Brooks
(1957) and Edmondson (1959). A dissecting microscope (20-40X) was used in
identifying invertebrates and some fish. Food from lake herring and yellow
perch stomachs consisted primarily of invertebrates and was quantified volu-
metrically. The food of walleye and northern pike consisted primarily of fish
and was quantified gravimetrically. Diets are described as percentage fre-
quency of occurrence and percentage by volume or weight for stomachs contain-
ing food. Diets are reported separately for young-of-the-year walleye and for
yellow perch less than 70 mm TL.
Food consumption rates and diel feeding periodicity were estimated from
1,270 walleye stomach samples taken from fish exceeding 200 mm TL during 30
days. Estimates are based on walleye digestion rates and the technique
described by Swenson and Smith (1973). Food consumption was estimated as
milligrams food consumed by the average individual in each day's catch
(mg. g~l. day "-*-). The technique requires approximately 25 samples from a sta-
tion, collected during a minimum of two sampling periods separated in time by
no more than 12 h. To increase precision, stations were usually visited four
to six times daily (Swenson and Smith, 1973). Estimates of average hourly
consumption (mg.h ) for 2 h periods were used to describe diel periodicity
in feeding. The number of each prey species consumed by all walleye in a
standard volume (walleye per 100 m ) was estimated as the product of the
weight of that species eaten by the average walleye (mg.g~ ) multiplied by
average walleye density (g-100 m ) and divided by the average weight of the
prey species (mg).
MEASUREMENT OF PARASITE INFESTATION RATES
Infestation of lake herring with the pleurocercoid stages of the tape-
worm Triaenophorus sp. was defined by examination of fillets from 156 fish
collected during 1974-1977^ to determine if improved water quality resulted
in changes in infestation rates. Fish were prepared for examination by a
Sampling was conducted September 16, 1977, specifically to collect
herring for cyst analysis. Because of the limited nature of the 1977 sampling
(3 gill net sets and 8 standard trawls), it has not been identified as a rep-
resentative part of the field program.
14
-------
filleting process which removed the backbone, rib bones and skin. Major mus-
cle segments, muscles clinging to the bone and skin were examined visually.
15
-------
RESULTS
PREY ABUNDANCE AND DISTRIBUTION
Analysis of 1974-1976 trawl catches shows density of smaller fish (<10 g)
increases from June through August and then declines during September-October,
in water less than 5 m deep (Table 3). The increase in abundance was associ-
ated with recruitment of age 0 yellow perch to July trawl catches and their
rapid growth during the ensuing period. Comparisons between years show the
highest densities occurred during 1975. Density was lowest during 1974 (Table
3). Comparison of estimated densities for Shagawa Lake (1974-1976) with those
for Lake of the Woods, Minnesota (1969 and 1970) (Swenson and Smith, 1976) ,
shows abundance of small fish in shallow water (<5 m) was consistently higher
in Shagawa Lake. In contrast, density of small fish in deep water (>5 m) was
higher in Lake of the Woods, due to an abundance of troutperch.
Low abundance of small fish in the deeper (>5 m) areas of Shagawa Lake
during July and August (Table 3) reflects the influence of reduced oxygen con-
centrations and the preference of age 0 yellow perch for shallow water. Den-
sity estimates (Table 3) show age 0 yellow perch concentrate in shallow areas
(<5 m) from July through September. A few sampling days occurred after autumn
turnover and indicated yellow perch move into deep water (>5 m) during late
September or early October. Variation in troutperch densities during July and
August in deep water (>5 m) were positively correlated with oxygen concentra-
tions at the sampling depth (r = ±0.85; P <0.01). Regression analysis showed
troutperch densities averaged approximately 0.8-100 m~^ when oxygen fell be-
low 5 ppm but increased to 10.7-100 m~-> at higher oxygen concentrations.
Positive relationships between oxygen concentration and abundance of age 0
and older yellow perch were indicated.
Possible improvement in spawning habitat or increased abundance of macro-
phytes may be responsible for the relatively high abundance of age 0 yellow
perch and black crappie, Pomoxis nigromaculatus, during 1975. Catches of age
0 walleye were higher durirg 1975 and 1976. Abundance of age 0 walleye during
1976 may have been influenced by the planting of 2 million fry by the Minne-
sota Department of Natural Resources during May.
Personal Communication, Mr. John Blakesley, Fisheries Manager, Minne-
sota Department of Natural Resources.
16
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ABUNDANCE AND DISTRIBUTION OF AGE II AND OLDER FISH
Walleye and yellow perch catch per gill net set (CPE) for the 1974-1976
period was lower than during the 1959, 1966 and 1969 Department of Natural
Resources lake surveys (Table 4). Comparison of CPE for 1974-1976 with the
catches made in earlier years suggests abundance of northern pike, lake
herring and white suckers has not changed significantly (Table 4).
Distribution of walleye appeared to be correlated with changes in dis-
tribution and abundance of the smaller fish which they preyed upon. Trawl
catches showed walleye concentrated in shallow water (<5 m) where age 0 yel-
low perch density was high during July and August. Abundance of walleye was
higher in deeper water during early June and late September when abundance of
small fish increased offshore. Gill net catches demonstrated that walleye
occupy the near surface and midwater depths where pelagic age 0 yellow perch
concentrate in addition to areas near the bottom (Table 5). Gill net catches
show lake herring are abundant at all depths (Table 5).
WALLEYE POPULATION STUDIES
Population Structure and Survival
Age structure of the walleye population was estimated from length fre-
quency distribution and aged samples following the method of Ketchen (1950).
The analysis showed that the population was dominated by the 1972 year-class
which was followed by a weak 1973 year-class (Table 6). Abundance of age 0
walleye in the 1975 catches and age I walleye in 1976 catches suggests the
formation of a second strong year-class in 1975. Strength of the 1975 wall-
eye year-class appears to be related to high food availability during 1975,
when age 0 yellow perch densities were high. Abundance of walleye and age 0
yellow perch during 1975 may result from favorable temperatures. Higher than
normal spring water temperatures occurred during 1975. A similar situation
occurred during 1972 (Appendix Table 1).
A catch curve was constructed by plotting log-e of frequency (ages I-XI)
on age (Figure 2). Instantaneous mortality (i) (Ricker, 1975) was estimated
as the slope of a line describing changes in frequency (log.e) with age, for
age groups V-XI. Frequencies of younger age groups were influenced by the
unusually large 1972 year-class and were omitted to avoid bias. The instan-
taneous mortality rate of 0.58 is equivalent to an annual mortality of 44%
and a survival rate of 56%. The survival rate compares favorably with esti-
mates by Heyerdahl and Smith (1972) for Lake of the Woods, Minnesota (31%),
and by Smith and Pycha (1961) for Red Lakes, Minnesota (30%). Survival of
Shagawa Lake walleye was similar to the rate estimated by Johnson (1975) for
Wilson Lake, Minnesota (62%).
Growth
Shagawa Lake walleye average 132 mm at the end of their first year, 395
mm at age V, and 555 mm at age IX (Table 7). Comparison with other Minnesota
lakes suggests first year growth in Shagawa Lake is slower (Table 8).
18
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TABLE 6. AGE FREQUENCY DISTRIBUTION OF SHAGAWA LAKE WALLEYE
CAPTURED BY SEINE, TRAWL AND GILL NETS DURING 1974,
1975 AND 1976
Age
1974
0
I
II
III
IV
V
VI
VII
VIII
IX
X
1975
0
I
II
III
IV
V
VI
VII
VIII
IX
X
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XII
XIII
1976
0
I
II
III
IV
V
VI
VII
Year
Class
1974
1973
1972
1971
1970
1969
1968
1967
1966
1965
1964
1975
1974
1973
1972
1971
1970
1969
1968
1967
1966
1965
1964
1963
1962
1976
1975
1974
1973
1972
1971
1970
1969
Number in
Age Group
85
5
540
2
14
8
7
4
1
2
1
Total: 669
526
163
11
818
15
35
8
7
8
2
4
1
1
1
Total: 1600
3453
289
62
5
528
10
6
4
Length Range
in
Millimeters
60 - 150
136 - 181
167 - 300
281 - 310
282 - 488
355 - 437
409 - 485
421 - 560
480
593 - 663
529
48 - 165
100 - 228
160 - 239
212 - 367
268 - 388
324 - 445
431 - 518
450 - 503
442 - 538
556 - 660
489 - 605
692
700
759
61 - 187
130 - 268
197 - 297
225 - 339
244 - 454
380 - 500
425 - 512
437 - 572
Total: 4357
21
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However, size attained by age III exceeded that of the Lake Vermilion and
Lower Red Lake, Minnesota, populations. Calculations based on samples col-
lected during 1967 indicate growth in Shagawa Lake has declined. These dif-
ferences may reflect actual changes in growth; however, precision of estimates
based on the 1967 samples is limited due to variability in the body-scale
relationship and small sample size.
Calculated deviations from mean annual growth (Hile, 1941) indicated
growth declined during 1973 and 1974 and then increased during 1975 (Figure
3). Deviations in growth followed a pattern similar to chlorophyll a concen-
trations (CERL, 1977). The coefficient of correlation between average annual
chlorophyll a concentration and annual growth deviations was not significant
(r = +0.69; P> 0.1). Annual increments of age groups I through VI (Table 9)
accrued during the pretreatment period (1965-1972) were compared with those
accrued during years after operation of the tertiary treatment plant had
resulted in reduced nutrient levels in Shagawa Lake (1974-1975). Two way
analysis of variance demonstrated that growth during 1974 was significantly
lower (P<0.01) than the average for the pretreatment period (1965-1972).
However, increments accrued during 1975 and the average for the 1974-1975
post treatment period did not differ significantly from the average for the
pretreatment period (P>0.1; Table 9).
Comparison of age 0 walleye growth during 1974, 1975 and 1976 showed the
highest rates occurred during 1975 (Figure 4). Growth differences appear to
be related to abundance of age 0 yellow perch during the three growing sea-
sons (Table 3). Morsell (1970) showed that the availability of yellow perch
is a factor controlling growth in age 0 walleye.
Feeding
Age 0 Walleye
Relative length of age 0 walleye and yellow perch had a major influence
on the percentage occurrence of yellow perch in the diet of age 0 walleye
(Figure 5; Appendix Table 2). Occurrence was highest (60 to 80%) when length
of age 0 yellow perch did not exceed 45% of age 0 walleye length. Occurrence
was low (0 to 55%) when age 0 yellow perch length exceeded 50% of age 0 wall-
eye length. Changes in relative length were associated with changes in age 0
walleye growth. During September 1975 when growth of age 0 walleye was rapid,
age 0 yellow perch length was less than 45% of age 0 walleye length. During
September 1974 and 1976 yellow perch length exceeded 45% of age 0 walleye
length and both occurrence of perch and growth of age 0 walleye were rela-
tively low. During periods of reduced age 0 yellow perch availability,
occurrence of johnny darters (Etheostoma nigrum) and invertebrates increased
in age 0 walleye diets (Appendix Table 2).
Food Consumption
Daily food consumption of walleye exceeding 200 mm TL was estimated for
30 sampling days and for five prey species or major food groups (Table 10).
Total daily food consumption in terms of body weight (mg-g -day ) was high-
est during 1975 whereas consumption expressed independent of walleye weight
(g.individual ) was highest during 1976. Differences between the two years
may reflect the larger average size of walleye in 1976 samples (Table 10).
25
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Time
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Figure 4. Growth of young-of-the-year walleye in Shagawa
Lake during 1974 ( ), 1975 ( ) and 1976 (--)
28
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Comparison of 1975-1976 consumption estimates with those for 1974 supports the
conclusion that feeding and growth conditions improved during 1975 and 1976.
Differences in daily consumption were greatest during June. Yearling yellow
perch were not available or eaten during June 1974 but comprised 56 and 45%
of the 1975 and 1976 daily meals (Table 10).
The highest consumption occurred during July of all three years when over
90% of walleye daily meals were comprised of age 0 yellow perch under 40 mm.
During July most of the meal was consumed at night or under low light condi-
tions of dawn and dusk (Figure 6). The high ration size and feeding time
demonstrates availability is highest at night when age 0 yellow perch under
40 mm are pelagic (Noble, 1975). Daily consumption was greatly reduced
during late September 1975 and 1976 when yellow perch disperse into deep water
or concentrated near aquatic macrophytes, indicating that food availability is
reduced under these conditions. Low late September feeding rates were not
associated with reduced temperatures. Temperatures during late September were
similar to those of early September (Table 1) when walleye feeding rates were
higher (Table 10).
Prey Quality and Feeding
Electivity indices (Table 11) were calculated from the percentage of
yellow perch, shiners (Notropis sp.), troutperch and young black crappie in
walleye meals and the percentages of these prey in trawl catches. Electivity
indices suggested yellow perch were relatively more available or were pre-
ferred by walleye in both Shagawa Lake and Lake of the Woods, Minnesota
(Swenson and Smith, 1976). Although troutperch were the second most abundant
prey in both waters (Table 3; Swenson and Smith, 1976) they comprised a very
small part of the walleye daily meal (Table 10; Swenson and Smith, 1976).
For both Lake of the Woods and Shagawa Lake, electivity of troutperch was
strongly negative during all sampling months suggesting that troutperch are
avoided or are not available (Table 11). Electivity indices for Notropis sp.
were positive or zero during periods of reduced age 0 yellow perch abundance
but became negative during periods of high yellow perch abundance (Table 11).
Variability in electivity indices for age 0 black crappie may be attributed
to small sample size and the fact that young black crappie congregate near
vegetation making sampling difficult. The results suggest relatively high
availability or some preference for this species by walleye. The relatively
low electivity for troutperch, a species associated with the bottom and high
electivity of species which locate away from the bottom suggests that pelagic
prey are more available to walleye and are required to maintain high feeding
and growth rates.
The number of yellow perch consumed daily by walleye occupying a stan-
dard volume of 100 m-> was estimated as the product of: the average walleye
biomass captured g-100 m~ , multiplied by an estimate of yellow perch con-
sumed per gram of walleye, divided by the average weight of young (age 0-1)
yellow perch in trawl catches. A similar procedure was used in estimating
the number of troutperch and spottail shiner eaten by walleye. The analysis
showed large numbers of yellow perch are consumed in July when night feeding
is most pronounced and perch are small (Table 10). Predation rates were
lower during August and September when age 0 yellow perch are larger and
total walleye food consumption was slightly lower. The data shows the
33
-------
Jun« 874
Figure 6. Hourly food consumption of invertebrates (stippled),
age 0 yellow perch (open) and other fish (barred) by
walleye.
(continued)
34
-------
June 1976
S 16
Time (hours)
24
Figure 6. (continued)
35
-------
TABLE 11. ELECTIVITY INDICES (Ivlev 1961)d OF MAJOR PREY CONSUMED BY
WALLEYE OVER 200 mm IN SHAGAWA LAKE AND LAKE OF THE WOODS,
MINNESOTA
Month
June
July
Aug.
Sept.
Perca
flavescens
0.0
+0.3
+0.3
+0.3
Percopsis
omiscomaycus
Shagawa Lake,
-0.7
-0.8
-0.9
-1.0
Notropis
sp.
Minnesota
0.0
-0.9
-0.4
-0.2
Pomoxis
nigromaculatus
+1.0
-0.6
+0.7
Lake of the Woods, Minnesota
June
July
Aug.
Sept.
0.0
+0.2
+0.3
+0.2
-0.6
-1.0
-1.0
-1.0
+0.6
+0.2
-0.1
-0.7
The electivity index is a convenient method for estimating the avail-
ability of, or preference for, specific prey species and is calculated
by the formula:
E =
+
where: r = is the relative importance of a prey
species in the predator's daily meal and;
P = is the relative abundance of the prey
species in the environment.
Values may range from +1, indicating the prey species is selected or
highly available to -1, indicating the prey species is avoided or
unavailable.
36
-------
greatest pressure on age 0 yellow perch occurs during July when a small re-
duction in age 0 yellow perch growth could result in a large increase in the
percentage of the total number consumed.
Comparison of the average concentration of yellow perch in trawl catches
(Table 3) with estimates of daily consumption (Table 10) suggests the average
available stock is consumed over a several day period during July. Because
yellow perch are pelagic during early July, prey density was underestimated
by bottom trawl catches. A better estimate of young (age 0-1) perch abundance
for July sampling days was calculated from the average number of age 0 yellow
perch captured during the first two sampling days in August (when they are
demersal) and the total number consumed by walleye-100 m~3 for the period from
the July sampling date to the August sampling period.
Prey Density and Feeding
To determine the influence of food availability on walleye production the
relationship between prey density and food consumption was estimated by com-
paring average daily consumption for each month of sampling with density of
small fish (<10 g) using data from Shagawa Lake gathered during this study and
information from Lake of the Woods, Minnesota, and western Lake Superior pre-
sented by Swenson (1977). Estimates of prey density and consumption were
based on catches from water less than 5 m deep except during late September
1975-1976 when abundance of walleye and prey in Shagawa Lake was higher off-
shore (Table 3) and estimates for water exceeding 3 m were used. Troutperch
abundance was not included in estimating prey density due to low utilization
by walleye. The analysis demonstrated a single relationship exists between
walleye food consumption and prey density in the three waters and that both
prey density and food consumption are highest in Shagawa Lake (Figure 7).
Consumption in all three waters increased rapidly with prey densities from
10 to 400 mg«m . At higher prey densities consumption appeared to increase
at a slow rate or to vary independent of prey density. Multiple regression
analysis demonstrated the relationship was significant and curvilinear
(P<0.01). The analysis showed that during most months prey density in
Shagawa Lake greatly exceeds 400 mg-m~3 ancj major reduction in prey abundance
would be required to significantly limit walleye food consumption and growth.
Low prey abundance or availability during June and after mid-September
appears to be responsible for increased walleye cannibalism. Cannibalism was
found to occur during June and September 1975 and August, September and
November 1976. Analysis of walleye food habits (Appendix Table 3) shows the
number of prey species in the diet increases during autumn when yellow perch
availability declines, due to lower abundance, offshore movement and their
association with aquatic macrophytes in shallow water (<5 m). Comparison of
rates of cannibalism (Table 10) with abundance of age 0 walleye (Table 3)
indicates that cannibalism may be a significant source of mortality. Swenson
and Smith (1976) showed that walleye cannibalism did not occur in the Lake of
the Woods, Minnesota, where abundance of yellow perch and troutperch was
higher during late September in water exceeding 5 m deep.
Physical Conditions and Feeding
Walleye consumption of age 0 yellow perch was greatly restricted during
late September 1975 (Table 10, Figure 6) when density of small yellow perch
37
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in open water was less than 200 mg-m 3 but exceeded 1000 mg-m -> in areas of
aquatic macrophytes. Macrophyte growths appear to reduce food availability
to walleye by providing cover for prey. Walleye catches in areas with aquatic
macrophytes were low, but catches of age 0 Centrarchidae (primarily black
crappie) were highest in these areas throughout most of the growing season.
Daily food consumption rate did not change with water temperature during
periods when food abundance exceeded 400 mg-nT^ in Shagawa Lake or Lake of
the Woods, Minnesota (Swenson and Smith, 1976). Daily food consumption at
14-15.5°C (11 days during June and late September) and at 20-22°C (10 days
during August and early September) averaged 30.0 and 29.1 mg-g day ,
respectively. Higher stomach volumes occurred during the lower temperature
periods (t-test; P<0.01) and promoted more efficient digestion (Swenson,
1977).
Predominance of night and twilight feeding by Shagawa Lake and Lake of
the Woods, Minnesota, walleye (Swenson and Smith, 1976) indicates high light
intensity may be a significant factor influencing daily food consumption.
Midday feeding was greatest during June and September when walleye ate more
demersal prey (Figure 6). Light intensity and the percentage of the total
meal consumed between 08:00-18:00 CST by Shagawa Lake walleye were ranked for
six periods during which a minimum of two estimates of food consumption were
available for days characterized by similar prey densities. The ranks xvere
negatively correlated (rc = -0.89; P<0.05) demonstrating walleye food con-
sumption during daylight hours is limited by high light intensity.
PERCH POPULATION STUDIES
Growth
Modal lengths of age 0 yellow perch captured by trawl during each sam-
pling period were used to calculate growth during 1974 through 1976 in
Shagawa Lake and during 1969 and 1970 for Lake of the Woods, Minnesota. Data
for Lake of the Woods were taken from Swenson (1972). Relationships between
length and time show growth is faster in Shagawa Lake (Figure 8) and was
highest during 1976 among the five years compared. Growth in Shagawa Lake
during 1974 was slower in comparison with 1975 and 1976 (Figure 8).
Analysis of 669 scale samples showed Shagawa Lake yellow perch average
63 mm TL at age I and reach 216 mm at age IV (Table 12). Comparison with
yellow perch in Wilson Lake (Johnson, 1975) and Red Lakes (Heyerdahl and
Smith, 1971) shows growth is similar in the three waters (Table 12).
Calculation of percentage deviation from mean annual growth for the
eight year 1968-1975 period indicated growth was slowest during 1974 and
1975, the two years following initiation of advanced wastewater treatment
(Figure 9). Growth during 1974 and 1975 was more than 10% below the eight
year average. Deviations in yellow perch growth were positively correlated
with chlorophyll a concentrations (CERL, 1977) during the 1971-1975 period
(r = +0.85; P<0.05). Mean length increments of age groups I-VI for the
1967-1972 pretreatment period and for the 1974-1975 period, when tertiary
39
-------
80
60
E
E
o>
c
0)
40
20
July
August
Time
September
Figure 8. Growth of young-of-the-year yellow perch in Shagawa
Lake during 1974 ( ), 1975 ( ) and 1976 (--)
and in Lake of the Woods, Minnesota, during 1969
( ) and 1970 ( ).
40
-------
TABLE 12. CALCULATED TL (mm) OF THE 1967 THROUGH 1975 YELLOW PERCH YEAR-
CLASSES IN SHAGAWA LAKE WITH GRAND AVERAGE LENGTH FOR SHAGAWA
LAKE AND OTHER WATERS
No. in
Year Class Sample
1967
1968
1969
1970
1971
1972
1973
1974
1975
Shagawa Lake
Grand Average
Wilson Lake, Minn.
(Johnson, 1975)
Red Lakes, Minn.
(Heyerdahl and
Smith, 1971)
Mill Lake, Minn.
(Schneider, 1971)
7
11
13
76
53
239
54
161
55
669
Red Deer Lake, Ontario
(Chadwick, 1976)
I
65
62
65
66
66
65
61
59
61
63
67
74
76
53
II
115
109
116
108
121
118
107
103
112
116
132
119
85
III
169
168
179
170
175
166
157
169
155
173
150
112
Age
IV
221
214
228
215
213
206
216
176
201
170
144
Group
V VI
248 268
246 269
255 273
237 251
238
245 265
240 268
221 234
196 226
176 178
VII VIII
282
290 309
286
286 309
241
West Okoboji Lake,
Iowa (Moen, 1964)
53 127
183
216
244
264
41
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wastewater treatment had resulted in reduced primary production, were calcu-
lated. Comparison of pretreatment and post-treatment mean growth increments,
for age groups I-VI, by txro-way analysis of variance showed growth was signif-
icantly lower during the 1974-1975 post-treatment period (Table 13; P<0.01).
Food Habits
Analysis of perch food habits showed dependence on zooplankton and macro-
invertebrates during the first year of life (<70 mm TL; Table 14) and on
macroinvertebrates and fish thereafter (Table 15). Young perch diets were
more diverse during 1976 and included a number of items not encountered in
stomachs collected during 1975 (Table 14). Higher diversity was accompanied
by increased food volumes. Faster growth, increased food diversity and larger
stomach volumes during 1976 suggest that prey population densities were higher
or that the larger size of age 0 yellow perch permitted feeding on more food
types. Mouth size has been found to be a major factor limiting predation by
age 0 yellow perch (Wong and Ward, 1972).
Food volumes found in yellow perch exceeding 71 mm TL were highest during
1976. Yellow perch contained a higher percentage of age 0 yellow perch in
their diet during 1976. Increased cannibalism during 1976 does not appear
related to differences in age 0 yellow perch densities which were similar
during 1975 and 1976 (Table 3).
LAKE HERRING POPULATION STUDIES
Population Structure
Information on age from 331 herring was applied to measurements of 567
fish to describe population structure (Table 16). Low abundance of age 0 and
age I herring may indicate failure of recent year-classes. However, rela-
tively high abundance of age II fish in 1975 and 1976 catches indicates the
sampling methods were not efficient in capturing small herring.
Growth
Accessory growth checks were found on scales of all year-classes but
could not be positively attributed to environmental conditions such as high
mid-summer temperatures or spawning activity. The accessory checks made
accurate aging of samples extremely difficult. Although the scales were read
three times by two experienced project staff, age assignments may not be com-
pletely valid. However, because potential errors resulting from inaccurate
age assignment should be distributed between pre- and post-treatment periods,
comparisons of growth between these periods should be valid.
Growth estimates show herring average 174 mm at age I, 290 mm at age III
and 408 mm at age VI (Table 17). Comparison .with other waters indicates
growth is excellent in Shagawa Lake (Table 17). However, comparison of devi-
ations in mean annual growth during the 1967-1975 period suggests growth is
declining (Figure 10). Growth has been below average since 1973 (Figure 10).
Herring growth was not correlated with chlorophyll a concentration (r=-0.29;
43
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TABLE 14. FOOD OF SHAGAWA LAKE YELLOW PERCH <71 mm TL, EXPRESSED AS
PERCENTAGE FREQUENCY OF OCCURRENCE FOR STOMACHS CONTAINING
FOOD, WITH PERCENTAGE OF TOTAL FOOD VOLUME IN PARENTHESIS
Collection Date
Stomachs with Food
Stomachs Empty
CRUSTACEA
Cladocera
Daphnia
Leptodora
Copepoda
Amphipoda
Hyalella
Gammarus
Ostracoda
Decapoda
INSECTA
Diptera
Chironomidae
larvae
pupae
Chaoboridae
larvae
pupae
Ephemeroptera
Hexagenia
Other
Trichoptera
Odonata
Hemiptera
OTHER
Hydracarina
Nematoda
Pelecypoda
Annelida
Hirudinea
Gastropoda
Unidentified
FISH
Yellow Perch
White Sucker
Johnny Darter
Spottail Shiner
Troutperch
Unidentified
PLANT
Average Volume
July
30
1
83(29)
20( 8)
53( 5)
13(12)
27( 2)
17(11)
7( 2)
3(<1)
10(16)
7(14)
3( 1)
.021
Aug.
14
0
79(46)
50(22)
64(19)
36 ( 3)
7( 7)
21 ( 2)
7( 1)
.010
1975
Sept, Oct. Nov.
28 12 10
024
86(25) 100(60) 80(47)
75(25) 50( 9) 10( 1)
82(33) 58( 6) 70(34)
4( i)
36 ( 1)
21(11) 8( 5)
25(20) 20(13)
4( 1)
4( 2)
4( 1) 10( 5)
.022 .015 .015
All
Dates
94
7
85(33)
44(15)
66(19)
5( 5)
24( 1)
14( 9)
2( 1)
6( 3)
K<1)
4( 7)
2( 5)
6( 2)
1(
-------
TABLE 14 (continued)
Collection Date
Stomachs with Food
Stomachs Empty
CRUSTACEA
Cladocera
Daphnia
Leptodora
Copepoda
Amphipoda
Hyalella
Gammarus
Ostracoda
Decapoda
INSECTA
Diptera
Chironomidae
larvae
pupae
Chaoboridae
larvae
pupae
Ephemeroptera
Hexagenia
Other
Trichoptera
Odonata
Hemiptera
OTHER
Hydracarina
Nematoda
Pelecypoda
Annelida
Hirudinea
Gastropoda
Unidentified
FISH
Yellow Perch
White Sucker
Johnny Darter
Spottail Shiner
Troutperch
Unidentified
PLANT
Average Volume
June July
30 18
0 3
57(18) 100(50)
20( 1) 33(10)
20( 4) 89(26)
27( 4) 6( 2)
3(<1) 28( 3)
37( 6) 6( 2)
37(51) 6( 2)
7( 1) 17( 5)
3( 1)
3( 1)
17 ( 8)
3(<1)
3(<1)
7(<1)
3(<1)
3( 3)
0.101 .014
1976
Aug.
17
3
59(15)
65(20)
53(12)
6( 1)
24 ( 3)
29( 9)
24(26)
12( 1)
12(13)
--
.014
Sept.
19
2
53(28)
47(23)
32 ( 9)
5( 1)
74(27)
IK 2)
16 ( 3)
5( 5)
5( 1)
5( 1)
.042
All
Oct. Dates
12 94
1 9
83(62) 68(24)
17( 6) 35( 7)
42( 2) 44( 7)
10 ( 3)
8(<1) 13( 1)
50(27) 39(11)
8( 3) 20(34)
10( 2)
1(<1)
4( 3)
6( 5)
1(<1)
1(<1)
1(<1)
2(<1)
1(<1)
K 2)
.032 .055
46
-------
TABLE 15. FOOD OF SHAGAWA LAKE YELLOW PERCH >70 mm TL, EXPRESSED AS
PERCENTAGE FREQUENCY OF OCCURRENCE FOR STOMACHS CONTAINING
FOOD, WITH PERCENTAGE OF TOTAL FOOD VOLUME IN PARENTHESIS
Collection Date
Stomachs with Food
Stomachs Empty
CRUSTACEA
Cladocera
Daphnia
Leptodora
May
3
1
July
8
0
13<<1)
Aug.
22
4
14 ( 1)
1975
Sept.
64
9
3( 1)
20( 2)
Oct. Nov.
7 12
1 2
29( 1)
29 ( 3)
All
Dates
116
17
16( 1)
Copepoda
Amphipoda
Hyalella
Gammarus
Ostracoda
Decapoda
INSECTA
Diptera
Chironomidae
larvae
pupae
Chaoboridae
larvae
pupae
Ephemeroptera
Hexagenia
Other
Trichoptera
Odonata
Hemiptera
OTHER
Hydracarina
Nematoda
Pelecypoda
Annelida
Hirudinea
Gastropoda
Unidentified
FISH
100(56) 38( 2)
2)
8( 2)
67( 2) 63(28)
33( 3)
33( 3)
100(36)
59(17)
55(16)
47(26) 57(40) 75( 9) 54(22)
9( 1) 14( 2) 17( 3)
9( 1) 43( 2)
2)
63(23)
38( 9)
25(15)
32(36)
2)
56(40) 29(33) 50(89) 51(46)
3( 1)
14(15) 9( 2)
Yellow Perch
White Sucker
Johnny Darter
Spottail Shiner
Troutperch
Unidentified
PLANT
Average Volume
25(19)
1.01 0.89
9(13)
5(15)
0.75
5(11)
2(10)
5( 3)
3( 2)
2( 1)
3( 1)
0.78 0.38
6(10)
2( 8)
4( 2)
2( 1)
1(<1)
2(<1)
1.25 0.81
(continued)
47
-------
TABLE 15 (continued)
Collection Date
Stomachs with Food
Stomachs Empty
CRUSTACEA
Cladocera
Daphnia
Leptodora
Copepoda
Amphipoda
Hyalella
Gamma rus
Ostracoda
Decapoda
INSECTA
Diptera
Chironomidae
larvae
pupae
Chaoboridae
larvae
pupae
Ephemeroptera
Hexagenia
Other
Trichoptera
Odonata
Hemiptera
OTHER
Hydracarina
Nematoda
Pelecypoda
Annelida
Hirudinea
Gastropoda
Unidentified
FISH
Yellow Perch
White Sucker
Johnny Darter
Spottail Shiner
Troutperch
Unidentified
PLANT
Average Volume (ml)
1975
June July Aug. Sept. Oct.
65 48 48 58 36
5 13 16 6 8
17 ( 3) -- 8( 1)
2(<1) 2(<1) 6(<1) 21(<1) 42( 5)
6(<1) 2(<1)
48( 6) 6( 1) 6(<1) 7(<1) 3(<1)
2(<1)
9(20) 2(<1) 2( 1) 3( 5)
65( 8) 33( 3) 31( 7) 90(41) 58(16)
45( 2) 15(14) 27( 2) 41( 4) 8( 1)
17( 1) 4(<1) 2(<1) 3(<1) 11(<1)
15 ( 1) 6(<1)
62(50) 23( 3) 65(60) 55(27) 56(54)
11(<1) 2(<1)
28( 3) 4(<1) 15( 2) 5(<1) 22( 5)
8(<1) -- 2(<1) 3( 1)
5(<1) 4(<1)
14(<1) 4(<1)
11(<1) 2(<1) 10(<1) 3(<1) 3( 1)
5(<1)
6(<1) 2( 1)
6(<1)
5(<1) 4(<1) 2(<1)
3( 1) 58(68) 23(23) 10(22) 3(10)
2( 5)
2( 4) 6( 5)
2( 2)
2(<1) 2(<1) -- 2(<1) 3( 1)
3(<1) 2(<1) 12(<1) 2(<1)
1.03 0.90 0.72 0.92 0.47
All
Dates
255
48
6( 1)
13(<1)
2(<1)
17( 2)
1(<1)
4( 8)
57(16)
30( 5)
Q f jf" ~l \
O ^ -L /
5(<1)
53(37)
3(<1)
15 ( 2)
3(<1)
2(<1)
4(<1)
6(<1)
1(<1)
2(<1)
2(<1)
2(<1)
19(25)
K 1)
K 1)
i(
-------
TABLE 16. AGE FREQUENCY DISTRIBUTION OF SHAGAWA LAKE HERRING
CAPTURED BY GILL NET AiMD TRAWL DURING 1974, 1975
AND 1976
Age
1974
0
I
II
III
IV
V
VI
Year
Class
1974
1973
1972
1971
1970
1969
1968
Number in
Age Group
0
0
17
0
10
24
12
Length Range
in
Millimeters
221 - 240
317 - 402
376 - 435
405 - 464
Total: 63,
1975
0 1975 3 147 - 160
I 1974 0
II 1973 213 160 - 281
III 1972 18 310 - 360
IV 1971 56 316 - 401
V 1970 74 361 - 453
VI 1969 74 365 - 479
VII 1968 23 405 - 480
VIII 1967 3 457 - 464
Total: 464
1976
0 1976 21
I 1975 0
II 1974 6 221 - 337
III 1973 33 290 - 360
IV 1972 7 388 - 420
V 1971 12 420 - 464
VI 1970 13 410 - 457
VII 1969 10 448 - 490
VIII 1968 1 457
Total: 103
49
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P = >0.1). Mean length increments of age groups I-VII were calculated for
the 1968-1972 pretreatment period and 1974-1975 period when nutrient levels
and primary production were reduced as a result of advanced wastewater treat-
ment (Table 18). Comparison of pretreatment and post-treatment means for age
groups I-V showed growth did not change significantly (P>0.1).
Food Habits
The percentage of stomachs containing food was highest (>85.5%) during
May-June and October-November of 1975 and 1976. Empty stomachs were most fre-
quent (32.0-74.1%) during July-September of both years (Table 19).
Dipteran larvae (Chironomids and Chaoborus sp.) represented a higher per-
centage of the diet early in the spring (Table 19) and declined later in the
sampling season. Cladocerans, principally Daphnia sp. and Leptodora kindtii,
increased in importance in the diet during the sampling season. Copepods and
related organisms were found in many stomachs and usually comprised between
10 and 30% of the total volume.
Zooplankton abundance during the ice-free seasons of 1974 and 1975 was
studied by Richter (1975) and Piragis (1976) and was not correlated with the
food habits of lake herring. Cladocerans (Daphnia sp. and Leptodora kindtii)
showed progressive importance in the diet of lake herring through each sam-
pling season while their abundance (Richter, 1975; Piragis, 1976) remained
high through the summer and early autumn. Abundance of Leptodora kindtii
peaked during early July and late August in Shagawa Lake and during mid-
September in lake herring stomachs. Copepods constituted 6% or less of the
stomach volume which did not reflect fluctuations in their populations.
Copepoda exhibited population maximas during mid-June through early July, the
month of August and again in late September.
Occurrence of Chironomids and Chaoborus sp. in the diet of lake herring
suggests an association with the bottom during spring and early summer months.
A shift to pelagic feeding during the summer and autumn is suggested by the
increased occurrence of zooplankton. This change in distribution may be asso-
ciated with hypolimnetic oxygen depletion during the summer.
Triaenophorus sp. Infestation Rates
Occurrence of Triaenophorus sp. plerocercoids in the flesh of lake her-
ring represents a major factor influencing economic value of lake herring
stocks in northern lakes. The adult cestodian parasite occurs in the intes-
tine of northern pike. The mature tapeworm lays eggs when northern pike
spawn. The eggs hatch into coracidium larvae which exist for one to two days
unless injested by a Cyclops, primarily Cyclops bicuspidatus thomasi (Miller,
1945). A few procercoids have been observed in C^. vernalis brevispinosus
(Watson and Lawler, 1965) and C^. strenuus (Lawler and Watson, 1963). After a
10- to 14-day development period the procercoid will infect lake herring or
other fish (Table 20) if infected Cyclops are ingested. The pleurocercoids
usually are located in the muscles around the back and are enclosed by a cyst
formed by the irritated tissues of the infected fish.
52
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53
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TABLE 19. FOOD HABITS OF SHAGAWA LAKE LAKE HERRING DESCRIBED AS
PERCENTAGE FREQUENCY OF OCCURRENCE WITH MEAN PERCENTAGE OF FOOD
VOLUME IN PARENTHESIS
Number of Stomachs
Percentage Empty
Insecta
Chaoborus sp.
Chironoinidae
Ephemera sp.
Hexagenia sp.
Megaloptera
Zygoptera
Tricoptera
May June
1 36
0 14
100( 2) 50(14)
100(93) 44(15)
100 ( 1)
3(<1)
1975
July Aug. Sept. Oct. Nov.
13 25 37 15 35
69 32 35 13 6
28( 7) 16( 1) 7(<1) 9(<1)
23(50) 44(33) 30 ( 8) 7( 8) 6(<1)
13( 6)
Arachnidae
Hydracarina sp.
Crustacea
Copepoda
Daphnia sp.
Leptodora sp.
Daphnia ephippium
Ostracoda
Hyalella
Annelida
Oligochaeta
Nematoda
Fish
Etheostoma nigrum
Unidentified
100(<1) 36(<1) 8( 6) 16( 2) 8(<1)
100(^1)
25 ( 4)
83(60)
6)
8( 6)
15(31)
15 ( 7)
12 ( 3)
36(25)
44(23)
22( 9)
49(41)
46(32)
87(62)
31 ( 6)
86(50)
46(27) 71(43)
100 ( 3) 3(<1)
5( 2)
3( 1)
8( 7) 20( 3)
(continued)
54
-------
TABLE 19 (continued)
Number of Stomachs
Percentage Empty
Insecta
Chaoborus sp.
Chironomidae
Ephemera sp.
Hexagenia sp.
June
16
6
50( 2)
56(21)
12 ( 8)
July
7
57
14 ( 2)
43(98)
1976
Aug.
18
39
44(15)
39(22)
6(<1)
Sept. Oct.
27 3
74 0
11(13) 33 ( 4)
4( 7)
Megaloptera
Zygoptera
Tricoptera
Arachnidae
Hydracarina sp. 44( 1)
Crustacea
Copepoda 38( 4)
Daphnia sp. 75(45)
Leptodora sp.
Daphnia ephippium 6(<1)
Ostracoda
Hyalella 6(<1)
Annelida
Oligochaeta 19(<1)
Nematoda 6( 7)
Fish
Etheostoma nigrum
Unidentified 38(12)
44( 2)
56(58)
44( 2)
15(12)
7( 3)
22(60)
100( 2)
100(81)
100( 3)
100(10)
4( 5)
55
-------
TABLE 20. SPECIES OF INTERMEDIATE HOST FISH FOUND IN SHAGAWA LAKE FROM WHICH
VARIOUS SPECIES OF TRIAENOPHORUS HAVE BEEN REPORTED IN NORTH
AMERICA (modified from Lawler and Scott, 1954)
Lake herring
White sucker
Bullhead
Troutperch
Yellow perch
Smallmouth bass
Black crappie
Slimy sculpin
Brook stickleback
Burbot
Walleye
(Coregonus artedii)
(Catostomus commersoni)
(Ictalurus sp.)
(Percopsis omi s comaycus)
(Perca flavescens)
(Micropterus dolomieui)
(Pomoxis nigromaculatus)
(Cottus cognatus)
(Culaea inconstans)
(Lo_ta. lota)
(Stizostedion vitreum vitreum)
X
X
X
to
3
tO
o
tH
3
T3
O
C
X
X
X
X
X
X
X
X
X
X
56
-------
The grubby appearance of fish flesh infected with numerous cysts reduces
acceptability. Shagawa Lake herring are particularly desirable because of
low levels of parasite infestation and large size whereas herring from many
lakes of the region carry numerous cysts.
Changes in Shagawa Lake zooplankton populations suggest lake restoration
is creating conditions favoring the parasite. Available EPA project data sug-
gest Cyclops and related genera (Copepoda) have doubled (Table 21) since ini-
tiation of advanced wastewater treatment. Cyst counts on 156 fish show the
level of infestation has remained low (Table 22). Failure to identify cysts
in fish captured during 1974 or June 1975 and their occurrence thereafter
suggests infestation rates may be increasing.
NORTHERN PIKE POPULATION STUDIES
Population Structure
Information on age from 192 northern pike samples was applied to measure-
ments of 251 fish to describe population structure (Table 23). Low abundance
of age 0 and I fish in catches for all three sampling seasons could indicate
failure of recent year-classes. However, abundance of age II fish in 1976
catches demonstrates that the 1974 year-class was abundant during 1974 and
1975 and low occurrence resulted from sampling bias.
Growth
Estimates of growth in length of northern pike suggest growth of the
1962-1966 year-classes exceeded that for the 1970-1975 year-classes (Table
24). However, data for both periods are limited and variable and provide
little assurance that differences are real. Growth of northern pike in
Shagawa Lake was comparable or slower than growth in other waters (Table 24).
Growth of Shagawa Lake northern pike during 1973-1975 was above the
average for the 1970-1975 period (Figure 11). Deviations in growth appeared
inversely related to chlorophyll a concentration (r = -0.24; P> 0.1). Mean
length increments of age groups I-IV were calculated for the 1969-1972 pre-
treatment period and the 1974-1975 period when nutrient levels and primary
production were reduced as a result of advanced wastewater treatment (Table
25). Comparison of pretreatment and post-treatment means for age groups I-IV
by two-way analysis of variance showed growth of northern did not change sig-
nificantly (P > 0.1).
Food Habits
Analysis of northern pike food habits was limited to 44 samples collected
during 1976 (Table 26). Few of the 1974 and 1975 samples, which were col-
lected primarily by gill net, contained food. More intensive trawling in
areas with macrophytes during 1976 provided a better sample for use in food
habit analysis. The limited sample showed yellow perch were the primary food
of northern pike. High occurrence of age 0 walleye in northern stomachs
(Table 26) suggests that predation by nortnern pike may be a major factor
limiting walleye survival and production.
57
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58
-------
TABLE 22. COUNTS OF TRIAENOPHORUS SP. CYSTS IN LAKE HERRING FROM SHAGAWA
LAKE
Mean Total
Date
1974
6/12/75
9/12/75
9/27/75
10/28/75
11/14/75
7/23/76
9/18/76
9/16/77
Number
of Fish
Examined
12
13
27
5
19
31
8
18
23
Mean Total
Length
(m) of
All Fish
408
417
372
375
335
379
395
382
420
No. of
Infected
Fish
0
0
2
2
1
7
0
5
4
Length
(mm) of
Infected
Fish
445
471
429
442
442
442
Mean No .
of Cysts
Per
Infected
Fish
1.0
1.0
2.0
1.6
1.2
1.0
Mean No .
of Cysts
Per Fish
>400 mm
0.2
0.1
2.0
0.6
1.0
0.3
156
59
-------
TABLE 23. AGE FREQUENCY DISTRIBUTION OF SHAGAWA LAKE NORTHERN PIKE
CAPTURED BY SEINE, TRAWL AND GILL NET DURING 1974, 1975
AND 1976
Age
1974
0
I
II
III
IV
Year
Class
1974
1973
1972
1971
1970
Number in
Age Group
0
11
30
32
15
Length Range
in
Millimeters
240 - 312
405 - 555
464 - 669
548 - 734
Total: 88
1975
0 1975
I 1974 2 246 - 267
II 1973 16 330 - 450
III 1972 23 418 - 562
IV 1971 33 433 - 721
V 1970 16 590 - 718
VI 1969 7 667 - 800
Total: 97
1976
0 1976 1 119
I 1975 4 292 - 335
II 1974 30 320 - 546
III 1973 28 416 - 636
IV 1972 2 607 - 620
V 1971 1 696
Total: 66
60
-------
TABLE 24. CALCULATED TOTAL LENGTH OF NORTHERN PIKE YEAR-CLASSES EXPRESSED
IN MILLIMETERS
Year Class
1961
1962
1963
1964
1965
1966
1969
1970
1971
1972
1973
1974
1975
1961-1966
Grand Average
1969-1975
Grand Average
Gilbert and Big Cedar
Lake, Wis. (Priegel
and Krohn, 1975)t
Eight Wisconsin Lakes
(Ave.)f (Snow, 1969)
No. in
Sample
2
3
9
20
28
7
7
22
47
37
44
31
4
69
192
173
Age Group
I
205
227
228
235
249
268
196
191
197
204
217
222
249
235
211
256
216
II
258
235
308
345
390
256
240
285
299
330
363
307
296
455
351
III
383
536
462
470
370
355
419
427
430
463
400
551
452
IV V VI
535 653 748
680 754
580
471 568 639
468 577
512 660
571
598 704 748
506 602 639
629 662 680
503 561 612
'Values are averages for female and male.
61
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DISCUSSION
The tertiary waste treatment facility constructed by the U.S. Environ-
mental Protection Agency has operated successfully since 1973, reducing the
estimated external total phosphorus (TP) supply to Shagawa Lake by 70-80%
(CERL, 1977). Measurement of TP and soluble reactive phosphorus (SRP) from
1971 to 1976 indicates that both forms were significantly reduced during
1974-1976. Annual average TP and SRP were reduced from approximately 50 and
20 vg-XT1 during 1971-1973 to 30 and 6.6 yg-£-1 respectively during 1974-
1976. Although the change was significant, earlier estimates indicated TP
would be reduced to less than 15 yg«& within the 1974-1976 period and would
result in a major response in the trophic character of the system (Larsen et
al., 1973). Internal cycling of TP resulted in a less dramatic response
(Larsen et al., MS).
LAKE RESTORATION AND FISH HABITAT
Phosphate reduction was expected to stimulate changes in production of
major fish species populations and the fish species complex through its in-
fluence on the following general habitat conditions:
1. Prey Populations
2. Light
3. Macrophyte Abundance
4. Hypolimnetic Oxygen Concentration
Information from this and cooperating projects suggests that minor
changes in prey populations, light penetration and macrophyte production have
occurred; however, available evidence fails to show improvement in hypolim-
netic oxygen concentration.
Reduction in primary production and increased light penetration were
demonstrated by a 29% drop in annual average chlorophyll a concentration and
a 22% increase in secchi disc readings between the pretreatment (1971-1973)
and post-treatment (1974-1976) study periods (CERL, 1977). The lowest annual
average chlorophyll a concentrations (6.2 yg- £"-"-) and the highest secchi disc
readings (3.2 m) occurred during 1974. During 1976 chlorophyll a was 96% of
the pretreatment period average and secchi disc readings declined from 3.2 m
(1974) to 2.6 m (1976).
Although not verified by direct observation during pre- and post-treat-
ment years, core analysis shows diatom diversity will increase as the lake
progresses to a mesotrophic state and abundance of pollution intolerant
65
-------
zooplankton such as Bosmina longirostris and Cyclops bicuspidatus will also
Increase (Bradbury and Megard, 1972). Response of bacteria and rotifer popu-
lations was not studied in Shagawa Lake; however, increased transparency and
other studies (Lynch and Smith, 1931) suggest their production probably de-
clined.
Growth and abundance of age 0 yellow perch were reduced during 1974 when
light penetration increased (CERL, 1977) and May Copepoda abundance was low
(Table 21). These observations indicate that Shagawa Lake reached its lowest
trophic level during 1974 and changes in primary production indirectly re-
stricted the amount of energy, in the form of small fish, which was available
to major predator populations.
Increased light transmission appears to have caused a shift from phyto-
plankton to macrophyte production. Although not substantiated by critical
mapping, abundance of aquatic vegetation in the trawl indicated distribution
of macrophytes was extended from a maximum depth of 2 to 3 m during 1974, the
first year of increased transparency (CERL, 1977), to 4 m during 1975 and
1976. The trawl data are supported by other observations showing macrophyte
distribution is influenced by transparency (Forney, 1977; Hurley and Christie,
1977). Because trawling was restricted to fairly protected areas with sub-
strates conducive to macrophyte development, changes in macrophyte production
may have been less extensive than suggested by the observations.
Anoxic conditions continued to occur in the hypolimnion when the lake
stratified during post-treatment years. Oxygen concentration fell below 1 ppm
at depths exceeding 5 m during all three post-treatment years (unpublished EPA
data). Improvement in hypolimnetic oxygen was not suggested by either in-
creased oxygen concentrations or reduced oxygen demand. Experience with other
waters, however, indicates that hypolimnetic oxygen will increase as lake
recovery progresses (Edmondson, 1977).
THE NICHE MODEL
Kerr and Ryder (1977) demonstrated that the "niche concept" has practical
application in assessing the influence of habitat change on individual stocks
and the fish species complex of lakes. By adopting Hutchinson's (1965)
abstract hypervolume concept of a "niche" and the "zone of tolerance" of Fry
(1947), they redefine "niche" as a hypervolume consisting of multiple measur-
able dimensions. Under this definition, any condition which has a measurable
influence on the dynamics of a species and can be measured qualifies as a
niche dimension. Provided the capacity to measure changes in significant
niche dimensions and the response of populations to changes in these dimen-
sions, Kerr and Ryder (1977) show the concept could have broad application in
measuring the effects of habitat changes on individual populations and the
total fish species complex in lakes. Precision of predictions from models
employing the concept is dependent upon the number of niche dimensions and
population responses which can be accurately measured.
The limited and variable response of Shagawa Lake to reduced nutrient
loading and the scope of the total Shagawa Lake research program make it
66
-------
impractical to quantitatively define the influence of lake restoration on
production of any species population. However, the available data and the
"niche concept" were applied in developing a conceptual model which describes
whether production of major stocks is being negatively or positively in-
fluenced and the mechanism through which the effects are induced.
In developing the model it was concluded that primary production, trans-
parency, macrophyte production and hypolimnetic oxygen concentration should
not be directly identified as "niche dimensions" as their influence on fish
populations are induced through their effects on spawning, space and predator-
prey relationships. Therefore, the influence of each habitat condition on
spawning, space and predator-prey relationships were analyzed. The number of
niche dimensions negatively and positively affected for each species popula-
tion was considered in describing changes in relative niche size. Predator-
prey relationships were considered on the basis of changes in seven separate
variables (dimensions) including prey distribution, prey species, prey size,
prey density, walleye predation, yellow perch predation and northern pike
predation.
Changes in the size of each niche dimension were described on the basis
of information developed through this and other studies. Changes in the
dimensions were identified by interpreting the effects of changes in prey
populations, transparency, macrophyte distribution and oxygen concentration
on niche size of walleye, yellow perch, northern pike, lake herring, trout-
perch and the Centrarchidae (sunfish) group. Age 0 walleye and yellow perch
were considered separately from older age classes.
INFLUENCE OF LAKE RESTORATION ON NICHE SIZE
Changes in the niche dimensions were identified, organized and summa-
rized using a niche dimension response matrix (Table 27) which consists of a
listing of the nine niche dimensions with each general habitat condition on
the vertical axis. The fish species and groups are identified on the hori-
zontal axis. A positive sign was entered where changes in general habitat
conditions were interpreted from existing data to result in an increase in
the size of a specific niche dimension for a given species or group. Nega-
tive signs signify reduction in the size of a given dimension. Zeroes were
entered where no meaningful data were available or no change was suggested.
Effects of change in general habitat conditions on niche size of individual
species or groups were interpreted by summation (Table 27). Relative change
in niche size was used to interpret gross changes in the species complex.
Accuracy of the analysis is based on the assumption that effects on each
dimension were correctly interpreted and that changes in the size of each
dimension have an equal influence on niche size and production of a species
or group. The first assumption which concerns the qualitative aspects of the
study is supported by considerable information. Due to the constraints
alluded to previously, quantitative information was limited and, therefore,
the assumption of equal effects was adopted.
67
-------
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69
-------
Prey Populations
Available data on prey populations and fish feeding suggest that lower
primary production has had limited influence on the niche of most fish spe-
cies. In lakes where extensive spawning gravel or stream spawning habitat
are less available, reduced plankton production and related decomposition in
the system may promote spawning of walleye and lake herring. However, the
available data did not indicate that lower plankton production would influ-
ence space or spawning in Shagawa Lake (Table 27).
Reduction in plankton populations resulting from reduced phosphorus load-
ing should result in reduction in the size of some predator-prey dimensions
in the niche of walleye, yellow perch and lake herring (Table 27). The limit-
ing effects on walleye and yellow perch identified in Table 27 are based on
the assumption that zooplankton density will decline. Under reduced zoo-
plankton densities, growth of age 0 yellow perch will be slower and mortality
will increase because walleye will require larger numbers of smaller perch to
reach preferred feeding levels. Because yellow perch year-class strength is
dependent on walleye predation (Forney, 1974) abundance of yellow perch will
decline. Production of adult yellow perch may also be limited as a result of
reduced density of benthic macroinvertebrates (Stewart and Rohlich, 1967;
Eggers et_ ai^. , 1978). Growth of adult yellow perch was significantly slower
during post-treatment years, and growth of age 0 yellow perch was slow during
1974, supporting a conclusion that yellow perch production is declining as a
result of reduced food availability. Growth of age 0 and older yellow perch
was equal to or exceeded growth rates of populations in other waters suggest-
ing that effects of reduced prey production on the niche of yellow perch are
limited to date. The yellow perch niche may also be restricted by species
shifts in zooplankton which result in lower rotifer densities. Rotifers
should be favored in systems sponsoring high bacteria production (Lynch and
Smith, 1931) and represent the first food of young yellow perch (Siefert,
1972).
Zooplankton shifts should favor survival of young walleye which are
dependent on larger zooplankton than yellow perch. Increased availability of
zooplankton for walleye and lower availability of small zooplankton utilized
by yellow perch could result in larger size differences and higher avail-
ability of age 0 yellow perch as food for age 0 walleye. However, expansions
of the walleye niche accrued through zooplankton species changes may be off-
set by lower yellow perch densities resulting from walleye predation.
Actually, lower walleye survival could occur as a result of increased canni-
balism at lower prey densities (Table 27). This study and others (Swenson
and Smith, 1976; Forney, 1976) indicate prey density is limited by walleye
predation and is a primary factor controlling cannibalism in the species.
Cannibalism did increase when yellow perch availability was low in Shagawa
Lake and represents a mechanism which limits production of the species.
Because yellow perch density greatly exceeded the level limiting to walleye
feeding (Figure 7) during most of the open water period, reductions in wall-
eye food availability and effects on growth, cannibalism and abundance (Table
27) should be minor and are dependent on continued reduction in the trophic
status of Shagawa Lake.
70
-------
Prey species and prey density dimensions of the lake herring niche are
probably being reduced. Food habit analysis shows lake herring are dependent
on zooplankton in open water as an energy source. Reduced growth of lake
herring during post-treatment years was suggested and supports a conclusion
that the food density dimension of their niche is being restricted. Abundance
of cyclops (Table 21) and the number of Triaenophorus crassus in lake herring
appears to be increasing slightly. Although the apparent changes are minor
and fail to demonstrate significant trends, they support the hypothesis that
changes in the prey species component of the lake herring niche are having a
minor negative impact on the population and on the fishery by resulting in
increased parasite burdens. Distribution of Triaenophorus indicates occur-
rence is higher in clear water lakes (Lawler and Scott, 1954).
Transparency
Increased transparency decreases the space dimension of walleye and
troutperch niches but increases the space dimensions of yellow perch, northern
pike and Centrarchidae niches (Table 27). Data from this study show walleye
exceeding 200 ml TL are crepuscular and night active predators, and Ryder
(1977) found cover and offshore movement are used to reduce light intensity
in clear water. Ryder (1977) and Swenson (1977) found activity of age II and
older walleye increases under reduced light. Swenson (1978) and Ryder (1977)
found distribution of age 0 walleye is not limited by high light intensity in
shallow (1-5 m) depths of clear lakes. Distribution (Swenson and Smith, 1976)
and movements of troutperch (McPhail and Lindsay, 1970) indicate a preference
for low light intensity. Distribution and information on feeding behavior of
yellow perch, northern pike and some Centrarchidae (Keast, 1970) indicate an
affinity for high light intensities typical of the littoral areas in clear
water lakes. On the basis of the above it was concluded that increased trans-
parency limited the niche of walleye through reductions in suitable feeding
space. The time of maximum prey availability was also restricted (Figure 6)
as a result of distributional shifts. Increased transparency has the oppo-
site effect on yellow perch, northern pike and Centrarchidae (Table 27).
Lower walleye feeding success should result in lower predation related mor-
tality in yellow perch; however, mortality associated with increased northern
pike predation could take the place of that imposed by walleye. The effect
of increased light therefore appears to be a transfer of more energy from the
less light-tolerant walleye to the more tolerant northern pike. Increased
northern pike feeding success and related production could stimulate higher
mortality in age 0 walleye as a result of increased predation by northern,
further limiting walleye abundance (Table 27). Movement of walleye into
deeper water increases the niche of the Centrarchidae group by limiting wall-
eye predation but could result in a reduction of the troutperch niche because
walleye and troutperch are placed in the same zone increasing the probability
of walleye predation on troutperch.
Macrophyte Distribution
Increased macrophyte production and distribution appear to be resulting
in a reduction of several walleye niche dimensions and increased size of
several northern pike niche dimensions (Table 27). Macrophytes limit space
availability for walleye which trawl catches indicate avoid areas near
71
-------
macrophytes. Macrophytes reduce six feeding related dimensions of the wall-
eye niche (Table 27). The results of this study suggest age 0 yellow perch
may congregate near macrophytes reducing their availability to walleye and
prey density in open water. This results in reduced walleye feeding rates.
Some evidence was developed by this study and Forney (1976) to show that
cannibalism increases when alternate prey is less available. Because macro-
phytes are distributed over a larger area resulting in more space for northern
pike (Makowecki, 1973), northern pike-induced mortality on age 0 walleye
probably has increased during the July through mid-September period when age
0 walleye concentrate in the littoral area. The first evidence of substantial
predation on walleye by northern pike in Shagawa Lake was provided during
1976. Changes in macrophyte distribution and northern pike abundance should
result in increased predation by northern pike on yellow perch. The influence
of northern pike predation on yellow perch mortality should replace that im-
posed by walleye predation. Thus, the effect of increased macrophyte abun-
dance as well as transparency would appear to be a transfer of available
energy away from xjalleye to northern pike with the total effect, on yellow
perch being limited. A similar change in energy transfer is identified for
relationships between walleye and northern pike predation on lake herring.
Increased walleye predation on troutperch would be stimulated by offshore
movements of walleye. Change in macrophyte abundance should improve space,
spawning and walleye predation dimensions of the Centrarchidae "niche."
Oxygen Concentration
The influence of lower plankton production, increased transparency and
macrophyte distribution all appear to be resulting in a shift from walleye to
northern pike production because they reduce important dimensions of the wall-
eye niche and increase or do not influence the dimensions of the northern pike
niche (Table 27). No improvements in hypolimnetic oxygen concentration
occurred as a result of the first three years of nutrient removal. However,
continued treatment should eventually result in higher hypolimnetic oxygen
concentrations. Change in oxygen supplies could dramatically increase the
niche size of walleye, yellow perch, lake herring and troutperch but would
not greatly alter the niche of northern pike. Space and food in deep water
could substantially add to the space and predator-prey dimensions of the wall-
eye niche (Table 27) promoting increased production. Higher and stable hypo-
limnion oxygen concentrations should stimulate increased availability of
invertebrates to walleye, yellow perch, troutperch and herring and result in
offshore distribution of age 0 yellow perch, increasing their availability to
walleye. Higher prey density in offshore areas associated with the buildup
of troutperch populations could result in reduced cannibalism and increased
feeding success of walleye. Although walleye may eat more troutperch, the
percentage consumed would continue to be low. Swenson and Smith (1976) found
that predation on troutperch in Lake of the Woods, Minnesota, did not control
troutperch population size because of their large size and low availability.
SUMMATION OF EFFECTS
Application of available data and the niche concept in measuring the
influence of nutrient removal on the Shagawa Lake fishery resource indicates
72
-------
that impacts are generally limited to minor shifts in production of existing
populations stimulated through changes in the space and predator-prey dimen-
sions of the niche of each species. Changes in the space dimension of a niche
influenced distribution directly. Additional indirect effects of changes in
the space dimension were activated through the influence of distributional
change on predator-prey dimensions of the niche.
To date, changes in prey populations, transparency and macrophyte abun-
dance have resulted in minor reductions in the production and niche size of
walleye, yellow perch, lake herring and troutperch. The model and available
data suggest niche size and production of northern pike and the Centrarchidae
are increasing (Table 27). Changes in light intensity and macrophyte ;opula-
tions appear to play an important role in determining the availability of
energy to piscivorous predators and in controlling survival and abundance of
several species. The mechanisms identified require further study but would
appear to be responsible for the dominance of northern pike in clear water
systems and of walleye in more eutrophic dark water lakes.
Low abundance of fish in water exceeding 5 m, positive correlations
between abundance and oxygen concentrations and comparisons with abundance in
Lake of the Woods, Minnesota, show that the space- and feeding-related dimen-
sions of walleye, yellow perch, lake herring and troutperch would be expanded
if nutrient removal is continued beyond the point at which hypolimnetic oxy-
gen concentration is maintained above 5 pprn. Although information developed
within the limited time frame of this study is inconclusive, it supports the
hypothesis that higher production of the major species in the Shagawa Lake
community is promoted under mesotrophic conditions.
73
-------
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78
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APPENDIX TABLE 1. AVERAGE MONTHLY TEMPERATURES (°C) FOR THE UPPER 6 M
OF WATER NEAR BRISSONS POINT SHAGAWA LAKE (EPA
unpublished data)
Year
Month
January
February
March
April
May
June
July
August
September
October
November
December
1968 1969 1970
2.1
9.5 12.0 5.7
14.3 15.4 15.9
20.0 21.1
22.1 20.1
18.7 14.6
7.6 9.5
4.6
1.3 1.8
1971
1.
1.
1.
1.
8.
16.
20.
19.
17.
12.
4.
1.
6
5
5
9
3
9
2
5
5
2
4
8
1972
2.
2.
2.
2.
10.
15.
18.
19.
15.
7.
3.
2.
2
3
3
6
3
9
8
7
7
3
0
4
1973
3.0
2.5
3.1
5.4
10.2
18.0
21.3
21.8
17.4
10.4
3.1
1.5
1974
1.
1.
1.
1.
6.
14.
20.
19.
13.
6.
6.
1.
5
7
7
8
8
4
3
0
1
7
5
5
1975
2.
2.
2.
2.
10.
16.
21.
20.
15.
10.
6.
__
1
1
3
6
3
0
2
7
7
2
9
1976
3.2
3.3
3.1
6.5
11.0
19.5
22.2
21.4
16.9
9.9
3.0
_
79
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APPENDIX TABLE 2. FOOD HABITS OF YOUNG-OF-THE-YEAR WALLEYES EXPRESSED AS
PERCENTAGE FREQUENCY OF OCCURRENCE FOR STOMACHS CONTAIN-
ING FOOD WITH PERCENTAGE OF TOTAL INGESTED FOOD WEIGHT
IN PARENTHESIS
Collection Date
1974
Aug.
Sept.
Number of Stomachs
Percentage Empty
24
33.3
15
20
Invertebrates
Annelida
Fluke
Amphipoda
Hyalella azteca
Arachnida
Hydracarina sp.
Cladocera
Copepoda
Cyclops sp.
Diaptomus sp.
Diptera
Chironomidae
Chaoboridae
Emphemeroptera
Nematomorpha
Platyhelmenthes
Tricoptera
Unidentified
Fish
Catostomus commersoni
Etheostoma nigrum
Notropis hudsonius
Perca flavescens
Pomoxis nigromaculatus
Unidentified
56.3(15.3)
6.3(0.2)
12.5(4.0)
18.7(14.3)
12.5(33.0)
6.3(33.2)
16.1(2.6)
8.3(0.3)
16.6(5.9)
25.0(14.0)
33.3(54.3)
16.6(22.8)
(continued)
80
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APPENDIX TABLE 2 (Continued)
Collection Date
Number of Stomachs
Percentage Empty
July
82
46.4
Aug.
53
39.4
1975
Sept.
59
28.4
Oct.
11
10
Nov.
17
21.4
Invertebrates
Annelida
Fluke 3.3(0.7)
Amphipoda
Hyalella azteca 11.1(0.1)
Arachnida
Hydracarina sp. 3.3(0.2)
Cladocera 3.5(0.5) 21.6(1.8) 28.4(0.8) 61.6(3.3) 78.7(7.0)
Copepoda
Cyclops sp. 4.3(0.7) 3.0(<.l)
Diaptomus sp.
Diptera
Chironomidae 6.7(0.4) 8.7(0.8) 9.1(0.2) 11.1(2.8) 27.2(5.2)
Chaoboridae 12.1(0.6) 22.2(0.1) 45.4(1.8)
Ephemeroptera 6.7(3.6) 3.0(0.4)
Nematomorpha
Platyhelmenthes
Tricoptera
Unidentified
Fish
Catostomus commersoni 3.3(3.7) 3.0(2.3)
Etheostoma nigrum 6.7(2.2) 9.1(10.5)
Notropis hudsonius 3.0(2.3)
Perca flavescens 80.0(88.7) 73.9(81.8) 69.7(82.8) 44.4(93.5) 27.2(85.9)
Pornoxis nigromaculatus 13.0(15.2)
Unidentified
(continued)
81
-------
APPENDIX TABLE 2 (Continued)
Number of Stomachs
Percentage Empty
July
66
41
1976
Aug.
57
34.4
Sept.
42
25
Invertebrates
Annelida
Fluke
Amphipoda
Hyalella azteca
Arachnida
Hydracarina sp.
Cladocera
Copepoda
Cyclops sp.
Diaptomus sp.
Diptera
Chironomidae
Chaoboridae
Ephemeroptera
Nematomorpha
Platyhelmenthes
Tricoptera
Unidentified
Fish
Catostomus commersoni
Etheostoma nigrum
Notropis hudsonius
Perca flavescens
Pornoxis nigromaculatus
Unidentified
46.7(14.1)
6.!(<.!)
3.0(0.2)
7.6(0.9)
7.6(4.5)
4.5(7.9)
3.0(1.3)
12.1(9.9)
43.9(60.9)
1.5(0.2)
14.0(0.2)
55.6(6.6)
19.3(1.9)
8.8(3.3)
5,2(0.9)
1.8(0.2)
21.0(21.8)
29.0(64.9)
7.1(1,6)
23.5(2.7)
33.3(12.5)
11.9(3.9)
14.2(1.4)
21.4(19.3)
26.1(58.1)
82
-------
APPENDIX TABLE 3.
FOOD HABITS OF SHAGAWA LAKE WALLEYE BEYOND THE
FIRST YEAR OF LIFE EXPRESSED AS PERCENTAGE FREQUENCY OF
OCCURRENCE WITH PERCENTAGE BY WEIGHT IN PARENTHESIS
1974
\_.U-L-Lei_l_-L
-------
APPENDIX TABLE 3 (Continued)
1975
Collection Date
May June July Aug. Sept,
Number of Stomachs
Percentage Empty
74
21.6
104
20.2
139
34.5
83
15.7
176
17.6
Invertebrates
Amphipoda
Hyalella azteca 27.6(0.8) 18.1(5.2) -- 2,!(<.!)
Cladocera 8.1(0.1) 25.9(13.9) 2.1(2.1) 5.1(0.1)
Coleoptera
Copepoda 1.7(0.1) 6.0(0.1) 1.!(<.!)
Decapoda
Astasidae 2.3(0.3)
Diptera
Chaoboridae 17.2(0.1) 16.9(0.3) 2.3(<.l) 0.7(<.l)
Chironomidae 37.9(0.3) 47.0(2.2) 14.1(0.4) 10.0(1,5) 13.1(1.5)
Ephemeroptera 22.9(3.6) 20.2(9.5) 9.3(1.0) 2.4(<.l) 9.1(0.4)
Hirudinea 3.4(0.5) 3.6(3.8) 2.3(<.l) 0.7(0.2)
Megaloptera (Sialis)
Notostraca -- 1.2(<.l)
Odonata 1.2(<.l)
Pelecypoda
Plecoptera
Tricoptera 3.4(<.l) 1.2(<.l) 2.3(<.l)
Fish
Ambloplites rupestris 1.4(0.9)
Catostomus commersoni 4.5(1.5) 4.8(5.9)
Coregonus artedii
Etheostoma nigrum 5.2(1.3) 2.4(2.2) 1.4(0.2) 2.1(0.3)
Ictalurus melas 1.2(0.4)
Lota lota 1.1(1.4)
Micropterus dolomieu 0,7(0.8)
Notropis hudsonius 1.7(0.2) -- 2.2(2.1) 2.9(5.8) 8.3(5.1)
Noturus gyrinus
Perca flavescens 53.4(88.6) 38.6(43.0) 87.6(91.4) 87.1(85.8) 75.9(71.0)
Percina^ caprodes 0.7(0.1)
Percopsis omiscomaycus 10.3(4.4) 2.4(5.0) 3.4(<.l) 2.9(1.0) 1.4(1.5)
Pomoxis nigromaculatus 1.2(0.2) 8.6(5.2) 9.7(9.6)
Stizostedlon vitreum 2.4(14.1) 0.7(2.3)
Unidentified 1.4(<.l)
(continued)
84
-------
APPENDIX TABLE 3 (Continued)
Collection Date
Number of Stomachs
Percentage Empty
Invertebrates
Amphipoda
Hyalella azteca
Cladocera
Coleoptera
Copepoda
Decapoda
Astasidae
Diptera
Chaoboridae
Chironomidae
Ephemeroptera
Hirudinea
Megaloptera (Sialis)
Notostraca
Odonata
Pelecypoda
Plecoptera
Tricoptera
Fish
Ambloplites rupestris
Catostomus commersoni
Coregonus artedii
Etheostoma nigrum
Ictalurus melas
Lota lota
Micropterus dolomieu
Notropis hudsonius
Noturus gyrinus
Perca flavescens
Percina caprodes
Percopsis omiscomaycus
Pomoxis nigromaculatus
Stizostedion vitreum
Unidentified
June
97
35.1
20.6(0.6)
11.1(1.2)
38.1(0.8)
38.1(28.2)
6.4(0.2)
9.5(0.2)
1.6(<.l)
1.6(0.1)
1.6(0.1)
3.2(1.8)
1.6(0.3)
1.6(8.5)
1.6(1.0)
47.6(56.6)
1.6(0.6)
July
73
41.1
18.6(5.9)
7.0(0.4)
2.3(1.6)
2.3(0.7)
4.7(3.2)
88.4(88.2)
1976
Aug.
115
37.4
4.2(<.l)
1.4(0.2)
1.4(0.7)
2.8(0.3)
6.9(5,1)
88.9(88.6)
2.8(2.1)
1.4(2.7)
2.8(0.4)
Sept.
116
30.2
30.9(5.0)
4.3(0.4)
2.5(0.2)
1.2(0.5)
3.7(4.6)
77.8(81.9)
1.2(7.1)
2.5(0.3)
Oct.
69
27.5
8.0(<.l)
2.0(<.l)
2.0(2.9)
4.0(33.3)
2.0(0.4)
4.0(1.4)
76.0(56.3)
2.0(5.1)
6.0(0.6)
85
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/3-80-036
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Influence of Advanced Wastewater Treatment on the
Fishery Resource of Shagawa Lake, Minnesota
5. REPORT DATE
March 1980 issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
William A.
8. PERFORMING ORGANIZATION REPORT NO.
Swenson
9. PERFORMING ORGANIZATION NAME AND ADDRESS
University of Wisconsin
1800 Grand Avenue
Superior, Wisconsin 54880
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Research LaboratoryCorvallis, OP
Office of Research and Development
Environmental Protection Agency
Corvallis, Oregon 97330
13. TYPE OF REPORT AND PERIOD COVERED
final 1974-1977
14. SPONSORING AGENCY CODE
EPA/600/02
15. SUPPLEMENTARV NOTES
16. ABSTRACT
This project was conducted to measure the response of the warmwater fish populations
in Shagawa Lake, Minnesota, to altered trophic conditions brought about by phosphate
removal from sewage discharges entering the lake. The project also served to provide
basic information on the mechanisms through which a lake's trophic status controls
its fish species complex.
Abundance, distribution, growth, and feeding interrelationships of walleye
(Stizostedion vitreum vitreum), northern pike (Esox lucius), yellow perch (Perca
flavenscens), and lake herring (Coregonus artedii) were described in relation to
changes in prey density and other environmental factors. A conceptual model based on
the "niche concept" was developed to identify the influence of changes in various
parts of the system on individual fish populations and on the total fish species
complex.
The field data and model suggest that production of walleye, yellow perch and lake
herring is being reduced while the "niche" and production of the northern pike popula-
tion is expanding in response to changes in several feeding-related "niche dimensions
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
water pollution
freshwater fishes
sewage treatment
models
phosphorus
lakes
Shagawa Lake
Minnesota
Stizostedion vitreum
vitreum
eutrophication
13B
8H
6F
18. DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (ThisReport)
unclassified
20"SEZCTjfffrY CLASS (This page)
unclassified
21. NO. OF PAGES
92_
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
86
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