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This is to certify that the

thesis entitled

EARLY LIFE HISTORY FACTORS INFLUENCING
LAKE NHITEFISH (COREGONUS CLUPEAFORMIS) YEAR-CLASS

STRENGTH IN GRAND TRAVERSE BAY, LAKE MICHIGAN
presented by

Mark Harlyn Freeberg

has been accepted towards fulfillment
of the requirements for

 

Major professor

Date /4/V27,//5/

0—7639 MS U is an Afiirrnatt’vc Action/Equal Opportunity Institution

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Ark" T w i :9 I ,t
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‘mLOBZMB
0 9 1 9 :1 4

 

 

 

 

 

EARLY LIFE HISTORY FACTORS INFLUENCING

LAKE WHITEFISH (COREGONUS CLUPEAFORMIS) YEAR-CLASS

 

STRENGTH IN GRAND TRAVERSE BAY, LAKE MICHIGAN

BY
Mark Harlyn Freeberg

A THESIS

Submitted To

Michigan State University

In Partial Fulfillment of Requirements for the Degree

MASTER OF SCIENCE

Department of Fisheries and Wildlife

1985

éo7f3/

5—54

ABSTRACT

EARLY LIFE HISTORY FACTORS INFLUENCING
LAKE WHITEFISH (COREGONUS CLUPEAFORMIS) YEAR-CLASS

STRENGTH IN GRAND TRAVERSE BAY, LAKE MICHIGAN

BY

Mark Harlyn Freeberg

The objectives of this lake whitefish (Coregonus

 

clupeaformis) early life history research were to:

1) Determine, in the laboratory, the effect of food ration
on larval lake whitefish growth and survival; 2) Measure
the magnitude of annual fluctuations in egg and larval
survival in Grand Traverse Bay, Lake Michigan: 3) Delineate
the factors responsible for these fluctuations and 4) Use
this information to estimate relative recruitment strengths
of whitefish year classes.

In. the laboratory, statistically significant
differences were found between larval growth and survival
at seven feeding levels. In the field, overwintering egg
survival equalled 1.7% through the winter of 1982/1983 when
ice did not cover whitefish spawning grounds. Percent
survival increased to 5.6% through the colder winter of
1983/84 when ice covered the spawning grounds soon after
egg deposition.

Following the warm winter of 1982/83, density of

larval whitefish declined slowly through the spring as the
number of zooplankton available to each whitefish larvae
(z/f) fluctuated little. Inl984 densities declined
catastrophically when z/f ratios fell to a two year low in
week five. Using final larval densities, the 1984 cohort
was estimated to be 2.1 times larger than the 1983 cohort,
approximating that determined from trawls for juvenile
whitefish. Estimates of relative year-class strength of
lake whitefish derived from a mathematical model using
these early life history data compare favorably (1984
cohort 2.4 times larger than 1983) with predictions made
using the aforementioned techniques.

These results indicate that the dynamics of early life
history stages of lake whitefish influence whitefish year-
class strength. Application of this information to
predictive models of Lake Michigan whitefish recruitment
should increase the reliability of whitefish yield

estimates.

For You, Dad

ii

ACKNOWLEDGEMENTS

This research was sponsored by the Michigan Sea Grant
College Program Grant No. NA-8OAA-D-OOO72 and NA-BSAA-D-
Sg045, Project No. R/GLF-16 from the National Sea Grant
College Program, National Oceanic and Atmospheric
Administration, U.S. Department of Commerce and funds from
the State of Michigan.

Myrl Keller of the Fisheries Division, Michigan
Department of Natural Resources provided invaluable support
to several aspects of this project through technical
assistance in the field and lodging near our field sites.

In addition to their teaching me a great deal about the
analytical approach to ecological research, members of my
thesis guidance committee, Dr. William Taylor, Dr. Darrell
King, Dr. Niles Kevern and Dr. Donald Hall, managed to
teach me even more about myself. Of special note are Dr.
William Taylor whose patience, instruction and friendship
can never be forgotten and Dr. Darrell King and the search
for science and excellence for which he stands.

Without the assistance of four interns, four friends,
Tim Feist, Doran Mason, Jim Lucas and Patmarie Maher, many
accomplishments and successes would not likely have been
possible. Similar sentiments are expressed to the many
graduate students who gave up weekends and warm beds to
head north and help me sample. Although too numerous to

iii

name them all, three deserve special mention: Dave Dowling,
Martin Smale and Andy Loftus. Thanks, guys.

It is hard to describe what the remaining people mean
to the achievement of the goal embodied in this manuscript.
I was blessed with a Mother and a Father who understood my
goals and contributed in every manner and with every
opportunity they could to help me attain them. These
words of gratitude are feeble attempts to express to them
what their lives have meant to mine.

Can you adequately thank those people who influence
your life and your direction through it? Valerie, thank
you for listening, for yourself. Con, thank-you for
letting me dream and for the thoughts that served.as the
foundation for many of those dreams. Thank you for your
laughter and your smiles; thank you for being there when no
one else was and for understanding when no one else could.

My deepest gratitude is extended towards Jesus Christ,
His sacrifice for me, the abilities He granted me and the
opportunities He provided to me to help me fulfill them.

Thank you, Lord.

"What lies behind us and what lies
before us are tiny matters compared to
what lies within us."

Emerson

iv

TABLE OF CONTENTS
Page
LIST OF TABLES..........................................vii
LIST OF FIGURES........................................viii
INTRODUCTION..............................................1
STUDY AREA................................................7
METHODS..................................................lO
Laboratory Feeding Study.............................10
Data Analysis.....................................11
Overwintering Egg Survival...........................12
Data Analysis.....................................13
Spring Zooplankton Research..........................14
Field Procedures..................................14
Laboratory Procedures.............................14
Data Analysis.....................................15
Spring Larval Whitefish Research.....................16
Field Procedures..................................17
Laboratory Procedures.............................18
Data Analysis.....................................l9
RESULTS..................................................22
Laboratory Feeding Research..........................22
Overwintering Egg Survival Research..................27
1982/1983.........................................27

1983/1984000OOOOOOOOOOOOOOOOOOOOOO0.0.0.000000000032

Spring Zooplankton Research..........................33
Densities.........................................33

Size Distribution.................................37

Spring Larval Whitefish Research.....................37
Density Changes...................................37

Growth and Survival Rates........ ..... ............39
Stomach Content Analyses..........................43
DISCUSSION...............................................52
Variability..........................................52
Field and Laboratory Research........................54
MANAGEMENT IMPLICATIONS..................................79
SUMMARY..................................................83

LIST OFREFERENCESOO..OOOOOOOOOOOOOOO00.00.000.000000000085

vi

LIST OF TABLES
Page

Table 1. Mean and standard error (i) of the mean
length, daily growth rate and cumulative
survival of laboratory reared larval
lake whitefish fed differing densities
of live plankton over a 25 day period.
Densities fed were 1.8, 3.2, 5.6, 10, 32
and 110 zooplankton/ fish /12 hour
period. Lengths are those measured on
day 21 of the experiment when all
feeding levels were represented................24

Table 2. Mean and standard error (i) of the mean
number of lake whitefish eggs sampled
from 1982 to 1984 in Grand Traverse Bay,
Lake Michigan..................................29

Table 3. Mean and standard error (i) of the mean
total zooplankton density, combined
density of adults and copepodites,
densities of copepods 0.7 to 1.10 mm
long and nauplii densities (number/l).
Data were collected in Grand Traverse
Bay, Lake Michigan during the spring of
1983 and 1984 and tabulated relative to
larval lake whitefish hatching and
development (Week -1 to Week 7)................35

Table 4. Mean and standard error (i) of the mean
larval lake whitefish density
(number/m3) and the growth (daily) and
survival (weekly) rates of these larvae.
Data were collected in Grand Traverse
Bay, Lake Michigan during the spring of
1983 and 1984 and tabulated relative to
larval lake whitefish hatching and
development (Week -1 to Week 7)........ ..... 41

Table 5. Actual mean weekly lengths of field
caught larval lake whitefish in 1984
(Weeks 3 through 7) and lengths
calculated from the predictive growth
rates and zooplankton/fish ratios..............65

vii

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

3.

LIST OF FIGURES
Page

Location map of Grand Traverse Bay,
Lake Michigan and lake whitefish early
life history study grids in East
Traverse.......................................8

Length of laboratory reared larval lake
whitefish fed differing densities of
live plankton over a 25 day period.
Densities fed were 1.8, 18, 32 and 110
zooplankton per fish per 12 hours.............23

Instantaneous daily growth in length of
laboratory reared larval lake whitefish
fed differing densities of live
plankton over a 25 day period.
Densities fed were ln8, 3.2, 5.6, 10,
18, 32 and 110 zooplankton per fish per
12 hours......................................25

Survival of laboratory reared larval
lake whitefish fed differing densities
of live plankton over a 25 day period.
Densities fed were 1.8, 3.2, 5.6, 10,
18, 32 and 110 zooplankton per fish per
12 hours......................................26

Depth distribution of lake whitefish
eggs spawned in Grand Traverse Bay,
Lake Michigan in December of 1982 and

1983...'...OIOOOOOOOOOOOOOOOO0.00.0.0....00.0.28

Departure (°C) from the mean winter air
temperature during the winter of 1982
and 1983 at Grand Traverse Bay, Lake
Michigan......................................30

Percent survival of lake whitefish eggs
overwintering in Grand Traverse Bay,
Lake Michigan in 1983 and 1984................31

Total zooplankton density (a), combined
densities of adults and copepodites (b)
densities of copepods 0.70 to 1.10 mm
long (c) and nauplii densities (d) in
Grand Traverse Bay, Lake Michigan
during the spring of 1983 and l984............34

viii

Figure

Figure

Figure

Figure

Figure

Figure

Figure

10.

11.

12.

13.

14.

15.

Page

Relative length frequency of
zooplankton in Grand Traverse Bay,
Lake Michigan during Spring 1983 and
1984 (a) and changes in the relative
length frequency of zooplankton 0.70
to 1.10 mm long during Spring, 1984..........38

Density of larval lake whitefish
caught in Grand Traverse Bay, Lake
Michigan during Spring, 1983 and 1984.
Catches pooled by week of capture....... ..... 40

Changes in larval lake whitefish
instantaneous daily growth (mm) (a) and
weekly survival (b) rates in Grand
Traverse Bay, Lake Michigan during
Spring, 1984. Larval whitefish pooled
by week of capture...........................42

Larval yolk volume (a), and the
percentage of larval stomachs
empty (b), as well as the mean number
of prey (c) and mean number of copepod
eggs (d) ingested by larval lake
whitefish in Grand Traverse Bay, Lake
Michigan during Spring, 1984. Catches
pooled by week of capture....................44

Relative length frequency of prey
organisms ingested by larval lake
whitefish in Grand Traverse Bay, Lake
Michigan during Spring, 1983 and 1984........45

Strauss' electivity indices for prey
organisms ingested by larval lake
whitefish in Grand Traverse Bay, Lake
Michigan in l984.............................47

Changes in the number of zooplankton

per fish larvae during Weeks 3 through
6 in Spring, 1983 and 1984.............. ..... 48

ix

Figure 16.

Figure 17.

Figure 18.

Figure 19.

Figure 20.

Page

Changes in larval lake whitefish
instantaneous daily growth (a) and
weekly survival (b) rates in relation
to changes in the number of
zooplankton available to individual
fish larvae in Grand Traverse Bay,
Lake Michigan during Spring, 1984.
Larval whitefish pooled by week of
capture......................................49

Instantaneous daily growth rate of
larval lake whitefish sampled in Grand
Traverse Bay, Lake Michigan in 1984.
Growth rate plotted as a function of
the number of zooplankton per fish
larvae. Larval whitefish pooled by
weekof capture..............................51

Catch per effort of l, 2 and 3 year-
old lake whitefish in Grand Traverse
Bay, Lake Michigan during June of 1984
and19850....00......OOOOOOOOOOOOOOOOOOOO0.0.68

Spring survival of lake whitefish
larvae as a function of changes in the
number of whitefish eggs hatching in
the spring and the amount of
zooplankton available to the
developing larvae. Zooplankton
densities are held constant at 1.92/1
(Spring, 1983) and 2.98/1 (Spring,
1984) while egg numbezrs are varied
from 0.001/m2 to 0.13/m .....................70

Range of year-class indices possible
during Spring, 1983 and 1984. Indices
determined by integrating
overwintering egg survival with spring
larval survival. Zooplankton density
held constant at 1.92/1 and 2.98/1
during 1983 and 1984, respectively...........72

Figure 21.

Year-class indices predicted by the
early life history recruitment model
for Springy 1983 and 1984.
Zooplankton density held constant at
1.92/1 and 2.98/1 during 1983 and
1984, respectively. Egg 2survival
equals 0.017/m and 0.056/m in 1983

Page

and1984' respectively00000.0.0.0000...0.0.0.74

xi

INTRODUCTION

Yield prediction in fisheries is an ever changing and
ever developing science. Advancement of this science has,
however, been slow since the inception of stock-recruitment
models in the 1950's (Ricker 1954, 1958; Beverton and Holt
1956, 1957). Although these models have provided limited
improvements in the description of fish population
dynamics, fisheries scientists have much to learn about the
relationship between system functioning and fishery yield.
Problems associated with the variability and unreliability
of existing yield.models will persist until mechanistic
solutions to these questions are forthcoming.

Many of the yield models now in use have arisen from
mathematical correlations between measurable
characteristics of a system (Pella and Tomlinson 1969;
Deriso 1980). These characteristics may offer little
explanation of the mechanisms responsible for observed
conditions. Historically, analytical models have almost
exclusively used characteristics of the adult stock (ie.
stock size, spawning biomass) to predict year-class
strength (Ricker 1958; Beverton and Holt 1957). This
practice has produced stock-recruitment relationships with
sufficient variability to raise questions as to their
usefulness to fisheries management. Inaccurate and

unreliable estimates of recruitment and yield will continue

to be made until the factors responsible for variations in
year-class strength are determined and incorporated into
predictive models of recruitment and yield. Admittedly,
delineating the mechanisms responsible for fish yield is a
much more difficult task than continuing current trends of
correlation and curve-fitting. It is, however, a necessary
task if advances are to be made in our understanding of the
fisheries resource.

These advances would be very applicable to the lake
whitefish (Coregonus clupeaformis) fishery in the Great
Lakes. In these lakes, the whitefish is the basis of one
of the largest commercial fisheries on the North American
continent (Baldwin, et. a1. 1979). However, due to dramatic
variations in whitefish year-class strength and subsequent
fluctuations in numbers recruited to the fishablezstock,
management of this fishery is difficult.(Jacobson 1983).
These difficulties are accentuated in regions of the lake
where an intensive commercial fishery is dependent solely
upon the abundance of a single cohort of whitefish
(Scheerer 1983). Thus, delineating the factors responsible
for year-class size and numbers of recruits would be useful
in the management and beneficial to the economic success of
the whitefish fishery.

The question as to what factors underlie and are
responsible for oscillations in lake whitefish population
size has not been answered (Bajkov 1930; Christie 1963;

Patriarche 1977). Recent attention, however, to the

inherent dynamics of the early life history stages of the
lake whitefish.(Hoagman 1973, 1974; Frederick 1982) is a
step towards an improved understanding of these population
fluctuations. The first months of life have proven to be
critical to fish survival and future recruitment.to several
other fisheries (Gulland 1965; Chenoweth.l970). During
these stages of development, total mortalities can reach as
high as 99.9% (Gulland 1965). As mortality experienced
during these stages is severe, strong year to year
variations in the future abundance of recruited fish may
arise from relatively small fluctuations in the mortality
rates. These rates are in turn established via weekly and
daily changes in growth or mortality as influenced by
interactions with biotic and abiotic mechanisms.

Although.previous studies of lake‘whitefish.early life
history have provided much needed information regarding the
biology of the egg and larval periods, they have failed to
describe the interaction between these biotic and abiotic
variables and early life history characteristics (Hart
1930: Faber 1970; Reckahn 1970: Hoagman 1973). The
importance of factors such as winter severity, prey
density, prey size selection and critical periods in larval
development largely’has been ignored. Furthermore, the
influence that these components have on larval growth and
survival rates and year-class size has not been
considered.

Survival of larval fish is often a function of the

amount of food available to these developing individuals
(Einsele 19633<rConnell and Raymond 1970L. Significant
mortality occurs during a critical period when larval fish
move from dependence upon endogenous to exogenous sources
of energy (Kurata 1959; Blaxter 1965, 1971). Low densities
of crustaceans (Lasker et.al. 1970; Beers and Stewart 1971)
or a preponderance of zooplankton sizes or species types
unusable by larval fish (Fluchter 1980: Teska and Behmer
1981) may increase mortality. Combined with poor search,
capture and feeding abilities of many larvae (Braum 1964),
these factors have the potential to substantially reduce
larval numbers during the first weeks of exogenous feeding.

In addition to larval mortality, the number of fish in
a year-class can be affected by losses during the egg stage
of development. If mortality attributed to the egg and
larval stages is severe, survival is bottlenecked during
the egg and larval periods of development and few fish pass
through to later stages. Year-class size is subsequently
reduced. If, however, these pressures are moderate, egg or
larval survival is increased and the resultant year-class
of fish is more abundant. Determining the extent and
variation of these pressures is a prerequisite to more
accurate estimates of year-class strength and yield.

It is with these thoughts regarding the perceived
importance of early life history dynamics to whitefish
population characteristics that the question of year-class

variability in the Great Lakes lake whitefish is

approached. In an effort to understand the source of this
variability, an extensive research program investigating
the early life history dynamics of lake whitefish in the
east arm of Grand Traverse Bay, Lake Michigan (East
Traverse) was initiated. This thesis serves as a synthesis
of the field research and additional work conducted in the
laboratory regarding the same question. In it I evaluate
aspects of the species biotic and abiotic environment,
including such variables as winter severity, overwintering
egg mortality and zooplankton/larval fish interactions.

The specific objectives of the research were to:

1. Determine, under laboratory conditions, the
influence of zooplankton abundance and
availability on larval lake whitefish growth
and mortality.

2. Document winter severity and corresponding
lake whitefish overwintering egg mortality on
the spawning grounds.

3. Describe spring larval lake whitefish.dynamics
in Grand Traverse Bay, Lake Michigan.

4. Describe spring zooplankton dynamics in Grand
Traverse Bay, Lake Michigan.

5. Integrate the information gathered in
objectives one, three and four and arrive at
conclusions regarding the importance of
zooplankton to larval lake whitefish

population characteristics.

6. Integrate these findings with data from
studies of overwintering egg mortality and
model the relationship between lake whitefish
early life history characteristics and year-

class strength.

The absence of this integrative approach in previous
studies of lake whitefish may be responsible for the
failure of these studies to successfully explain the
population oscillations typifying the whitefish fishery.
For decades fisheries research has paid little attention to
the importance of fish early life history in observed
population dynamics. Nor has it given much attention to
the ecosystem or community approach to fisheries problems.
As trophic level interactions during early life history
development are investigated, the research embodied in this
manuscript is a more holistic approach to the analysis of
whitefish populations. ‘Data analyzed in regard to each of
the above objectives will be considered separately as well
as integrated with that obtained from other objectives to
shed light on the contention that these factors are
responsible for much of the year-class dynamics of

whitefish populations in the Great Lakes.

STUDY AREA

Grand Traverse Bay, an inland extension of northern
Lake Michigan, consists of a large outer bay and two
smaller arms oriented in a general north-south direction.
Studies of overwintering egg survival were located on lake
whitefish spawning grounds in the east arm (East
Traverse) (Figure 1). Particular emphasis was placed on a
region at 44°53'7" N, 85°25'5" W identified through
historical fishery data and communication with Fisheries
Division, Michigan Department of Natural Resources (MDNR)
officials as a site where high numbers of spawning
whitefish had been recorded.

These spawning grounds cover an area of 69.7 hectares.
Water depths average 4.5 m and currents, although variable,
move in a general south to north direction over the
grounds. Clay overlainby sand, gravel, rock or cobble
covers much of the region. Larger particles prevail in the
shallower regions (5 6 m) while sand and clay are
predominantly found in depths of water greater than 6 m.

Spring zoo- and ichthyoplankton sampling were
conducted in shallow water regions (5 6 m) in East
Traverse (Figure 1). For sampling purposes, the eastern
shore of East Traverse was divided into twelve, 1000 m wide
grids. An additional 1000 m separated each grid. Grids
were further subdivided into three subgrids with each
subgrid extending from the shoreline to a depth of 10 111.
All grid and subgrid boundaries were identified using Loran

   

 

 

 

 

 

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Location map of Grand Traverse Bay, Lake Michigan and

lake whitefish early life history study grids in East
Traverse.

C latitude/longitude coordinates.

Substrate type characterizing these shallow water
areas also consists of clay overlain by sand, gravel, rock
or cobble. The lake bottom slopes gradually to depths of
roughly 10 111 before depths increase sharply to 40 to 60 m
and more. Rooted aquatic vegetation is lacking except for

small patches of Chara gpp.

MEmeS

Laboratory Feeding Study

Lake whitefish eggs from Lake Simcoe, Ontario that
were in late stages of development were obtained from the
Ontario Ministry of Natural Resources. These eggs were
further incubated and hatched at Michigan State University
aquaculture labs from 8 to 15 April, 1983. Immediately
prior to egg'hatchingy eggs.in.groups of 100 were counted
and placed in a series of 20 l aquaria cooled to 12°C by a
flow through water bath. Beginning the day after larval
hatch, seven densities of 2-day old brine shrimp, Artemia
gpp. were fed twice daily to fish larvae in each of four
replicates of each feed density. Any excess brine shrimp
remaining from the previous feeding were removed prior to
the next feeding by siphoning them off the aquarium bottom.
Feeding was conducted on a zooplankton per fish larvae
basis to keep the feeding level constant while larval
numbers changed over the course of the experiment.
Densities fed were 1.8, 3.2, 5.6, 10.0, 18.0, 32.0 and 110
zooplankters/whitefish larvae/12 hour period. The first
six densities were chosen.for statistical reasons, being
equally spaced on the logic scale, while the latter
provided an excess number'of zooplankton.to each larvae.
To prevent or reduce potential mineral buildup or toxicity
problems, 1 liter of water was removed, and clean water

added, twice weekly in each aquaria. An analysis of

11

conductivity at the close of the experiment showed this
practice to be effective in keeping mineral levels low.
Subsamples of five randomly selected larvae were
taken twice weekly from all tanks except where larvae were
receiving excess food (110 z/f). Lengths (mm) of these
larvae were determined by measuring their magnified image
(25x) on a microfiche reader. The presence or absence of a
yolk sac was determined microscopically. These procedures
were repeated on subsamples of three larvae per day taken
from the tank where larvae were fed to excess. Dead larvae

were removed daily and measured for length and weight.

--Data Analysis--

Since feeding regimes were equally spaced on the loglo
scale, statistically significant differences among
treatment means could be tested using orthogonal contrasts
(Steel and Torrie 1980). Specific growth rate was

determined using the equation:

u/day = (1n La - 1n Lb)/T

where u/day - specific or instantaneous growth rate per day
(mu/dam
L - larval length (mm)
a - length at end of experiment
b - length at beginning of experiment

T - time interval

12

Overwintering Egg Survival Research

 

Sampling for lake whitefish eggs began during and
continued past the completion of spawning activity in
December of 1982 and 1983. A 39 kg iron sled (Stauffer
1981) attached to a diaphragm pump at the surface via a
flexible hose 5 cm in diameter was used for egg
collections. The egg sled was towed for 5 minutes and
moved at an average speed of 0.5 m/s. The sled was towed
at depths of 1.5, 3.0, 4.5 and 6.0 m. Replicate trawls
were conducted at each depth except at the 3.0 m contour on
9 April, 1984 when wind and wave conditions prohibited
further sampling at this depth. The total number of trawls
run on each day of sampling varied from 12 to 20 in
accordance with weather conditions. .All transect locations
were identified using Loran C latitude/longitude
coordinates.

After passing through the sled, hose and pump, eggs
were deposited on a fine mesh screen at the surface. Live
eggs were separated from dead eggs and detritus and
preserved in 10% formalin for later identification and
enumeration in the laboratory. Eggs that appeared damaged
or having an opaque shell were classified as dead. The
finding of few dead or damaged eggs indicates, in the very
least, that our methods were not destroying the eggs as
they passed through the apparatus.

Spring egg sampling commenced during March of both
years before the lake whitefish eggs began to hatch. Two

13

methods were used to detect hatching. If empty egg cases
or motile larvae were collected by the egg sled, sampling
was terminated and the preceding set of egg density data
were used to estimate overwintering mortality. Hatching
also could be detected by trawls for larval lake whitefish
conducted before and after each day of egg collection.
Using these procedures, hatching time could be reliably
determined.

Temperature data were obtained from the National
weather Service in Traverse City, Michigan. Mean monthly
temperatures were determined from average daily
temperatures in Traverse City and compared to the 25-year
mean for that month in that region. Degrees of departure
(°C) from the long term mean were determined for each month
from December through March. These values were averaged
over these months to arrive at the index of winter

severity.

--Data Analysis--

Between year differences in the number of eggs
deposited on the grounds in the fall and the number
remaining on these grounds in the spring were tested for
statistical significance using the Mann-Whitney U
nonparametric procedure (Siegel 1956). The mean number of
eggs at each depth and date and the standard error of the

mean also were calculated.

14

Spring Zooplankton Research

 

--Field Procedures--

Zooplankton sampling in Grand Traverse Bay began
during March of 1983 and 1984 and continued twice monthly
through May of each year. One subgrid on each of three
grids was selected at random on each sampling day; 'Using a
63 u mesh Wisconsin net, three replicate samples were taken
at 1, 3 and 5 m (filtering approximately 10.1, 30.3 and
50.5 1” respectively) on each subgrid. Sampling was
conducted between 1000 and 1600 hours. To reduce
ballooning of crustacean carapaces, all samples were
preserved in Koeche's solution (2.3 kg sugar dissolved in 2
l of formaldehyde and 8 l of water) for laboratory

analysis.

--Laboratory Procedures--

Plankton in subsamples of two ml were counted and
measured using a Sedgewick-Rafter counting cell in
conjunction with a calibrated Whipple grid. A minimum of
two subsamples were analyzed from each sample.
Measurements of total length and maximum width were
recorded for all organisms counted in 1983. Total length
was measured to the distal end of the caudal rami in all
copepods. To improve the reliability of size comparisons
between copepods available in the field and those found in
larval lake whitefish stomachs, these measurements were
augmented by measurements of metasomal length and maximum

width observed from dorsal and lateral views in 1984. Sex

15

and number of eggs also were recorded when possible.

During all counts, adult and copepodite copepods were
classified to suborder. Nauplii and cladocerans were not
subdivided further. In order to obtain some knowledge of
the types of zooplankton being analyzed, individuals from
March and early April samples were keyed to species. In
addition, cladocerans and other organisms which differed
noticeably from zooplankton already classified were
identified as they became apparent in samples taken later
in the spring. Zooplankton were identified using Ward and

Whipple (1959) and Pennak (1978).

--Data Analysis--

In addition to within year comparisons, zooplankton
densities and relative size distributions on each sampling
date were compared between years on the basis of length of
time before (Week -1), during (Week 1 and 2) and after
(Week 3 to 7) the hatching of larval lake whitefish.
Because more than 86% of the total number of larvae
captured during the two springs were located in one meter
of water, analyses of plankton densities and size
distributions were focused on this depth. All
statistical tests were made using nonparametric methods.
Mean density of zooplankton and standard error of the mean

density are presented.

16

Spring Larval Whitefish Research

 

 

--Field Procedures--

Ichthyoplankton trawls began during the third week of
March in 1983 and.during the first week of April in 1984.
Trawls continued through May 21 and May 30 of each
respective year. Using the network of grids described
earlier, surface trawls lasting five minutes were conducted
daily when wind and wave conditions permitted. Special
attention was directed towards Grid 5 where high numbers of
spawned lake whitefish eggs had.been found. Sampling on
Grid 5 consisted of three surface trawls at 1 m and two
surface trawls at three and five meters. One, ten minute
bottom trawl also was conducted at three and five meters.
To examine larval dispersal, surface transects at 1 m were
made onsuccessive grids to the north and south of Grid 5
after sampling there had been completed. Trawls were
conducted between 0600 and 1400 in 1983 but, because the
ability of larval fish to escape capture declines at night
(Lenarz 1981), were run between 1800 and 0900 in 1984.
Night to day capture ratios were determined via catch
records from twenty-four hour sampling periods and were
used to adjust night to day caught larval fish densities.

A 363 u mesh net with a mouth diameter of one-half
meter was used in all trawls in 1983, while one-half meter,
363 and 560 u mesh nets were used in 1984. Nets were three
meters in length, cone-shaped and towed at speeds of 0:7 to

1.1 m/s. Larval whitefish remain susceptible to such gear

17

until they attain lengths of 33 mm. Capture efficiency
does decline after larvae reach lengths greater than 20 mm
(Hoagman 1974). Towing speed, distance travelled and
'volume of water filtered were calculated.from.equations
incorporating flowmeter readings taken with a General
Oceanics flowmeter mounted in the mouth of each net.

Before each trawl, time of day, depth, grid number and
Loran C latitude/longitude coordinates were recorded and
correlated to trawl number. After each trawl, captured
larval fish were removed from the net and placed into
chilled 90% alcohol to prevent stomach content egestion.
Larvae were separated from algae and detrital matter'and
transferred to Koeche's solution within 24-48 hours of
capture.

The reliability of estimates of relative year-class
strength based on the final larval density in 1983 and 1984
was investigated by comparing relative densities of l to 3
year-old whitefish sampled in East Traverse. Through
cooperation with the Fisheries Division, MDNR. 10 minute
transects using a 12.2 m x 2.4 m otter trawl with 1.9 cm
cod end mesh were made at depths ranging from 6 to 122
meters. Captured lake whitefish were measured for length
and weight and aged using scale analysis. The catch.per
effort statistic for each age group was determined in order
to compare the relative strengths of these classes to the
predictions based on the early life history dynamics of
this species.

18

--Laboratory Procedures--

.All preserved larval whitefish were measured for
length and weight. Weights were recorded to 0.0001 g while
lengths (mm) were taken by measuring their magnified image
(25x) on a microfiche screen.

Morphological measurements of mouth size (maximum
width) and yolk sac volume (length x width x height) were
made on all larvae captured in 1983. Owing to the larger
sample size in 1984, larvae were pooled by week of capture
and these measurements taken on a random subsample of
larvae representing 50% of the total number of individuals
in each pool.

Stomach content analyses were conducted on those
larvae for which morphological measurements were made.
Stomach contents were removed via dissection and number and
size of all individuals recorded using a compound
microscope (10x) in conjunction with a calibrated Whipple
grid. Total lengths, as well as metasomal lengths for
copepods, were recorded and width measured from both the
dorsal and lateral aspects. Linear regression equations of
total length on metasomal length.and.dorsal and lateral
widths were constructed. Life stage, number of eggs
carried and sex of each individual were determined when

possible.

19

--Data Analysis--

Total numbers of larvae and volume of water sampled
were used to estimate larval densities during each spring.
Although circumstances prevented sampling during weeks two
and three of 1983, densities during these periods were
estimated using regression analysis. Mean densities and
the standard error of the mean density are presented.

Mean lengths were also determined for larval lake
whitefish pooled by week. Larval instantaneous growth

rates were determined using the equation:
u/day = (1n La - 1n Lb)/T

where u - specific or instantaneous growth rate (mm/day)

L - larval length (mm)

a - mean length in week t

b - mean length in week t-l

T - time interval.
Growth rate also was plotted as a function of the number of
zooplankton per fish larvae -(z/f) using a threshold
corrected hyperbolic equation for substrate limited growth:

u/day 3 [umax((Z/f)'(Z/f)q)l/[(K5'(Z/f)q)
+ ((Z/f)'(z/f)q)]
where u - specific or instantaneous growth rate (mm/day)
“max - maximum growth rate
K3 - one-half saturation value

z/f - substrate level

z/fq - maintanence substrate level.

20

Constants of “max and K8 were calculated using the

regression equation:

((z/f)-(z/f)q)/u- Ks/umax + (l/umax) ((Z/f)-(Z/f)q).

Instantaneous weekly survival (s) rates were

calculated with the equation:
s/week a Nt/Nt-l
where Ni 8 larval abundance (#/m3).

Survival and growth rates were plotted in conjunction with
zooplankton/fish ratios at 1 m.

Mean yolk volume, gut width and number of eggs and
number of prey ingested were determined for larvae pooled
by week. The identity (Ward and Whipple 1959: Pennak
1978), life stage and sex of captured prey and the
percentage of larval stomachs empty during each week were
also recorded. Frequency distributions of total lengths of
organisms ingested were compared between weeks. The linear
index of prey selection proposed by Strauss (1979) was used
to characterize the electivity of prey organisms by larval

whitefish. The index:
L=r1-p1

is the difference between the relative abundance of a prey
item in a fish gut (r1) and the relative abundance of that
prey item in the field.(pi). ‘Values of the index range

21

from +1 to -1 with positive values indicating selection for
a size, 0 no selection and negative values selection

against a size class.

RESULTS

Laboratory Feeding Research

Significant differences (Orthogonal contrasts; p <
0.05) in length gain (Figure 2; Table 1) were observed
among feeding regimes with greater food rations producing
greater fish growth. After 21 days, larvae receiving 110
z/f (the number of zooplankton/fish larvae) were 1.37 times
as long as those fed 1.8 z/f. For the sake of clarity,
only four of the seven feeding levels are depicted in
Figure 2 as length gains for feeding levels of 3.2, 5.6 and
10 z/f were intermediate to feeding levels of 1.8 and 18
z/f. No statistically significant length differences were
found between tanks within a feeding regime (Orthogonal
contrasts: p > 0.05).

Instantaneous daily growth rates, as reflected in
length gain through the course of the experiment, differed
among feeding levels (Friedman two-way analysis of

variance: x2

- 54.6: p < 0.001) and again were positively
correlated to food ration (Figure 3: Table 1). Larvae fed
1.8 z/f grew at a rate 50% of those receiving excess (110
z/f) rations.

The survival of larval lake whitefish depended on the
amount of food provided to each fish (Figure 4; Table l)
and was linearly related to growth rate by the equation:

% Survival - -1oa.2 + 8275(Daily Growth Rate) (r2 - 0.97)

There was no difference in survival between feeding regimes

until day 15, when yolk sac resorption was complete and the

Length (mm)

 

23

 

 

 

 

22 -
- llO
l8 "
32 __
- Io ’—
L8
I4 -
IO "'
0 IO IS 20 25
Time (Days)
Figure 2. Length of laboratory reared larval lake whitefish

fed differing densities of live plankton over a
25 day period. Densities fed were 1.8, 18, 32
and 110 zooplankton per fish per 12 hours.

24

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1.8 3.3 5.8 10 18 32 110
Feeding Level (z/f)
Figure 3. Instantaneous daily growth in length of laboratory

reared larval lake whitefish fed differing densities

of live plankton over a 25 day period.

Densities

fed were 1.8, 3.2, 5.6, 10,.18, 32 and 110 zooplank-
ton per fish per 12 hours.

lOO

26

Volt Sac
LOCO

 

001-

‘I. Survival

20"

 

Figure 4.

 

 

Time (Days)

Survival of laboratory reared larval lake whitefish
fed differing densities of live plankton over a 25
day period. Densities fed were 1.8, 3.2, 5.6, 10,
18, 32 and 110 zooplankton per fish per 12 hours.

27

larvae became dependent.upon.exogenous energy resources.
Survival of larvae receiving low rations decreased
dramatically after this date. Larvae fed low rations (5 10
z/f) exhibited 100% mortality within 7-10 days while those
fed excess rations (110 z/f) experienced no mortality.
Fish fed medium rations of 18 and 32 z/f exhibited
significantly different mortality rates of 90 and 10%,
respectively (Kruskal-Wallis one-way analysis of variance;
H - 16.9; p < 0.01). Additionally, time to 50% survival

increased with increasing ration.

Overwintering Egg Survival Research

 

--l982/l983--

In 1982, densities of spawned lake whitefish eggs
peaked on 4 December. Although eggs were found to a depth
of 6 m, most were at depths of 4.5 m and shallower where
large (> 2.5 cm diameter) spawning substrates were most
predominant (Figure 5: Table 2).

Unseasonably warm weather during the winter of
1982/1983 prevented ice formation on East Traverse (Figure
6). Consequently, whitefish spawning grounds were exposed
to wind and wave action throughout the winter.

Estimates of overwintering egg survival during
1982/83 were calculated from egg densities sampled on 12
March, 1983. Although a total of 15 trawls were conducted
from depths of 1.5 to 6.0 111, eggs were found only at the
1.5 m contour in the spring of 1983 (Figure 7: Table 2).

Percent survival at this depth equalled 2.4% while egg

28

[:1 = 1982: N= 261
E = 1983: N= 449

3 S 8

Percent of Total Eggs Deposited
N
O

 

1.5 3.0 4.5 6.0
Depth (m)

Figure 5. Depth distribution of lake whitefish eggs spawned in Grand
Traverse Bay, Lake Michigan in December of 1982 and 1983.

29

Table 2: Mean and standard error (+) of the mean number of lake
whitefish eggs sanpled From 1982 to 1984 in Grand
Traverse Bay, Lake Michigan.

our DEPTH naxarn or NUMBER 0! new NUMBER STANDARD
rnaxsrcrs recs origaas gggon

 

 

 

 

 

12/4/32 1.5 3 171 57.0 13.2
3.0 2 55 27.5 1.5
4.5 2 20 10.0 1.0
5.0 2 5 2.5 2.5
3/12/03 1.5 5 7 1.4 0.2
3.0 4 0 - -
4.5 3 0 - -
5.0 3 0 - -
12/11/53 1.5 3 103 51.0 13.5
3.0 3 151 50.3 32.7
4.5 3 102 34.0 19.0
5.0 3 '13 4.3 2.4
4/9/54 1.5 4 15 3.0 0.5
3.0 1 1 1.0 -
4.5 4 0 2.0 1.1
5.0 3 1 0.3 0.3

30

6.0

4.0

5‘
0

Degrees of Departure (‘ C)
E" N
O O

 

Figure 6. Departure (’ C) from the mean winter air temperature
during the winter of 1982 and 1983 at Grand Traverse
Bay, Lake Michigan.

31

10D

:l= 1983; N= 7
0.0 g = 1984; N= 25

Percent Egg Survival

 

1.5 3.0 4.5 0.0
Depth (In)

Figure 7. Percent survival of lake whitefish eggs overwintering in
Grand Traverse Bay, Lake Michigan in 1983 and 1984.

32

survival in the entire shallow water region of the spawning

grounds (5 6.0 m) equalled 1.7% (Figure 7).

--1933/1934—-

Densities of spawned lake whitefish eggs were highest
on 11 December, 1983. The depth distribution of spawned
eggs followed the same pattern as in 1982 with much higher
densities occurring in shallower water (Figure 5: Table 2).
No significant difference was observed between the two
years in terms of the number of whitefish eggs deposited at
depths 5 6.0 m (U -= 47.5: p > 0.05) or at the 1.5 m contour
only (U - 4: p > 0.05).

During the colder winter of 1983/84 (Figure 6), ice
covered the spawning grounds soon after egg deposition and
persisted until 4 April. The number of live eggs remaining
on the spawning ground in 1984 was significantly higher
than the number recorded in 1983 (Mann-Whitney U: U - 36: p
< 0.01). Unlike 1983, live eggs were found in depths of
water through 6.0 m (Figure 7; Table 2). Percent survival
of eggs at depths 5 6.0 m equalled 5.6%, 3.4 times greater
than the percent surviving in 1983. Using the least
variable estimates (1.5 10), percent survival equalled 6.2,

2.6 times greater than the value recorded at.1.5:m in 1983.

33

Spring Zooplankton Research
Calanoid and cyclopoid copepods dominate the spring
zooplankton assemblage in Grand Traverse Bay. Of the

species present in the bay, Diaptomus sicilis and gyclops

 

bicuspidatus thomasi were most frequently observed.

 

Limnocalanus macrurus was noted only occasionally.

 

Cladocerans, predominately 9525553 £22. and ggggigg

longirostris, were scarce until the final week of May.

 

--Densities--

With the exception of densities recorded five weeks
after larval lake whitefish had hatched (Week 5), 1984
post-larval hatch zooplankton densities (: SE) at a depth
of 1 m exceeded those present in East Traverse in 1983
(Figure 8: Table 3). In 1983, total densities of
zooplankton (all species and life stages) increased from
5.1 (i 2.1) individuals/liter (ind/l) in Week -1 to 10.2 (_-I_-_
2.8) ind/l in Week 1. Densities dropped slightly to 9.8 (i
2.5) ind/l in Week 5 and then increased sharply to 26.5 (i
4.4) ind/l in Week 7. Naupliar densities followed a
similar pattern. Adult/copepodite densities exhibited the
largest fluctuations. Densities changed little from Week
-1 to Week 1. Abundance of these individuals declined from
3.2 (i 1.6) ind/l in Week 1 to 1.3 (i 1.1) ind/l in Week 3.
The rate of change in density slowed between Week 3 and

Week 5, with Week 5 densities equalling 1.2 (i 0.5) ind/l.

34

8.0

 

 

4M

rot-I1 Density (m)
.3

16 i

 

 

—- '1933 —— -1003

Adnn.CopqwmiruulChpepodfle Dmmdhr(§/D

 

 

 

 

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b

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p
O

 

 

 

 

 

 

 

-1 1 5 5 7
C
1904
-1 i 3 8 7
InuGISenuNed Ibeklfiunphul

Figure 8. Total zooplankton density (a), combined densities of

adults and copepodites (b), densities of copepods
0.70 to 1.10 mm long (c) and nauplii densities (d)
in Grand Traverse Bay, Lake Michigan during the
spring of 1983 and 1984.

35

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36

Densities increased to 1.4 (_-_l-_ 0.4) ind/l by the final week
sampling in 1983 (Figure 8: Table 3).

Patterns of change in total numbers of plankton and
numbers of nauplii in 1984 were similar to patterns
described for 1983. Adult/copepodite density fluctuations
in 1984 were, however, unlike those observed the previous
year. Densities increased sharply from 0.2 (1 0.07) ind/1
in Week -1 to 5.0 (1; 2.3) ind/l in Week 1 before declining
to 3.0 (_-I_- 0.8) ind/l by Week 3 (Figure 8: Table 3). Unlike
1983 when the rate of decline in adult/copepodite numbers
slowed between Week 3 and Week 5, the high rate of decline
persisted through this time span in 1984 and densities
decreased to 0.9 (i 0.4) ind/l by Week 5. Adult/copepodite
densities increased after Week 5, reaching a value of 3.1
(1- 1.4) ind/l in Week 7 (Figure 8; Table 3).

The nonparametric randomization test was used to
compare Week 1, 3 and 5 zooplankton densities between the
years of 1983 and 1984. Total zooplankton densities
differed significantly at a probability of 0.15. Adult
densities differed at the 0.20 level of significance while
naupliar densities differed significantly at the 0.10
level.

Fluctuations in the density of adult and copepodite
zooplankton 0.70 to 1.10 mm long were further scrutinized
in 1984. Densities, which reached a peak of 5.3 (i 1.4)
ind/l during Week 1, fell to a density of 0.7 (i 0.04)
ind/l by the fifth week of larval growth (Friedman two-way

37

analysis of variance: x2 = 5.7: p < 0.10%. Abundance
increased by the final week of the study, attaining a
density of 2.7 (i 0.5) ind/l at this time (Figure 8; Table

3).

--Size Distribution--

The 1983 and 1984 frequency distributions of numbers
of zooplankton in size categories ranging from 0.1 to 1.2
mm in total length were similar. In both years, a large
peak at 0.10 mm in length and a much smaller one at 0.90 mm
was apparent (Figure 9). Analyzing the 0.70 to 1.10 mm
lengths in 1984 reveals significant (Friedman two-way
analysis of variance: pI<IL01) changes in the frequency
distribution of these sizes within a year (Figure 9).
Except for the 0.70 mm length group, for which the
abundance increased between Week 1 and 3, the relative
abundance of these sizes decreased between Week 1 and 5.
Between Week 3 and 5 this decline was also apparent for
sizes in the 0.70 mm group. Relative abundance of all

sizes increased in Week 7.

Spring Larval Whitefish Research
--Density Changes--

A total of 848 larval lake whitefish were captured
during the two years of the study, 119 in 1983 when trawls
were conducted during the day and 729 in 1984 when sampling
occurred throughout the 24 hour period. Densities in 1984

remained significantly higher than those recorded in 1983

38

1.00

0.75 .

0.50 <

Relative Frequency

0.25 ‘

 

 

 

0e15 ‘

0e10 ‘

Relative Frequency

0.05 a

 

 

 

0.70 0.00 0.90 1.00 1.10
Zooplankton Total Length (mm)

Figure 9. Relative length frequency of zooplankton in
Grand Traverse Bay, Lake Michigan during Spring,
1983 and 1984 (a) and changes in the relative
length frequency of zooplankton 0.70 to 1.10 mm
long during Spring, 1984.

39

even after the 1983 data were adjusted using night to day
capture ratios based on 1984 trawl records (Randomization
test: p < 0.01). Densities of larval lake whitefish
equalled 0.004 (f: 0.002) and 0.014 (1 0.003) ind/m3 during
the first week of life in 1983 and 1984, respectively
(Figure 10; Table 4). Hatching continued into Week 2 in
1984 with densities reaching a spring maximum of 0.071 (1
0.0073) ind/m3 at this time. The Week 2 density was
estimated to be 0.019 ind/m3 in 1983.

Densities declined slowly from Week 2 to 6 during
1983. A similar pattern was observed in 1984 until three
to four weeks after feeding was initiated, at which time
densities dropped sharply from 0.065 (1; 0.005) to 0.039 (5
0.021) ind/m3. This loss rate slowed between Week 5 and 6
of 1984 but remained greater than recorded between Week 5
and 6 of 1983. Densities declined sharply during Week 7 of
each year when larval whitefish moved into deeper water

(Figure 10: Table 4).

--Growth and Survival Rates--

Weekly estimates of larval lake whitefish
instantaneous growth rates are presented in Figure 11 and
Table 4. In 1984, daily growth rates were high during the
first two weeks of growth. Growth rates declined to a
value of -0.0033 during Week 5, then increased to 0.0091 in
Week 7 (Figure 11; Table 4).

Survival rates remained high throughout the spring of

1983 and cumulative survival equalled 0.724 (Figure 11;

40

 

0.072 A
I \
I \
I \\ 5 —— =1983
I \ ——- =1984
I \
0.054« 1’ . \
I \
I \
I \
I \
I \
0.036 - l \

0.018 ~

Larval Lake Whitefish Density (#/m’)

 

 

 

 

0 T T 1 f l

1 2 3 4 5 6 7
Week Sampled

Figure 10. Density of larval lake whitefish caught in
Grand Traverse Bay, Lake Michigan during
Spring, 1983 and 1984. Catches pooled by
week of capture.

41

Table 4: Mean and standard error (3;) of the sen larval
lake whitefish density (number/m ) and the
growth (daily) and survival (weekly) rat. of
these larvae. Data were collected in Grand
Traverse Bay, Lake Michigan during the spring
of 1983 and 1984 and tabulated relative to
larval lake whitefish hatching and development
(Week -1 to Week 7).

 

 

 

1&1
Week Larval Standard Growth Survival
Density Error Rate Rate
1 0.0045 0.0019
2 0.0193 -
- 0.93
3 0.0179 -
- 0.92
4 0.0164 0.0042
- 0.98
5 0.0161 0.0149
- 0 e 87
6 0.0140 0.0144
7 0.0022 0.0013
1.9.51
1 0 . 0136 0 . 0031
0.0143 -
2 0.0714 0.0073
0.0083 0.96
3 0.0683 0.1120
0.0066 0.96
4 0.0653 0.0054
-0.0033 0.60
5 0.0393 0.0214
0.0091 0.74
6 0.0290 0.0017
0.0191 -
7 0.0069 0.0034

42

 

0020

0.015 j

0.010 4

 

0005

 

lndhuflaneouslbafly GrowflhIRah3(nun)

 

 

 

 

0.70 i

0.60 1

‘Ieekhrlervalshuvhnd Rate

 

 

 

050

Figure 11.

V T—

4 5 3
Week Sampled

Changes in larval lake whitefish instantaneous
daily growth (mm)(a) and weekly survival (b)
rates in Grand Traverse Bay, Lake Michigan during
Spring, 1984. Larval whitefish pooled by week of

capture.

43

Table 4). In 1984, survival was initially high but
declined dramatically in Week 5 and increased only slightly
by Week 6. Cumulative survival through the spring of 1984

is 0.0406, 56% of the 1983 value.

--Stomach Content Analyses--

Larval yolk volume declined from 4.7 to 0.5 mm3 during
the first two weeks of life (Figure 12). By Week 7, all of
the yolk had been resorbed. Although 83% of all larval
stomachs were empty during Week 1, this percentage declined
throughout the spring until all stomachs contained food of
some type in Week 7 (Figure 12). The mean number of prey
in larval stomachs increased dramatically between Week 1
and Week 4, changed little in Week 5 and more than doubled
by Week 7 when it reached a maximum of 13.3
individuals/stomach (Figure 12). The mean number of
copepod eggs in larval guts fluctuated even more severely.
Copepod egg numbers increased dramatically through Week 4
before dropping sharply in Week 5 to less than one-half the
number recorded the previous week. Egg number again
increased in Week 6 and 7 (Figure 12).

The distribution of total length of organisms in
larval whitefish gut contents pooled over all weeks
demonstrates the importance of copepod adult and late
copepodite stages to the‘larvae (Figure 13). In 1983, two
modes were observed in the data, one centered at 0.60 mm
and another at 0.90 mm. Only one mode centered at 0.80 mm

characterizes the 1984 frequency distribution.

44

 

 

 

 

 

 

 

 

 

 

 

 

 

 

as 100
i a b
5’
E
"ae 750<
E a
d
’ B
.5. a
O
" 2.4~ 50.1»
.5
O
>-
‘6
E .
1.21 525.0.
i
a.
o 1 Y r 1' o I 1 1 i v
II a a It 6 e I! 1 s s «1 6 o 7
1&9 as:
c
a a d
c 2
8 8
3 3
.9 .8
r i
‘6 '.‘a
5 E
g 8.84 a ..o.
2 z
0 v . 1 v 1 T o t v v v 1
ll 8 I 4 6 I 'I 1 8 8 4 8 6 ii
Veek Sampled 'eek Sampled
Larval yolk volume (a) and the percentage of larval

Figure 12.

stomachs empty (b), as well as the mean number of
prey (c) and mean number of copepod eggs (d) ingested
by larval lake whitefish in Grand Traverse Bay, Lake
Michigan during Spring, 1984. Catches pooled by week
of capture.

45

1.001

0.75 3

0.50 -I

Relative Frequency

0.25 -

 

 

 

Y I I

0.10 0.30 0.50 0.70 0.00 1.10

Zooplankton Total Length (mm)

Figure 13. Relative length frequency of prey organisms ingested by
larval lake whitefish in Grand Traverse Bay, Lake Michigan

during Spring, 1983 and 1984.

46

Strauss' electivity indices calculated using these
data increase from a low of -0.85 for prey 0.10 mm long to
a maximum of 0.99 for prey 0.80 mm long (Figure 14).
Indices are slightly negative for prey 0.20 to 0.40 mm in
length, near zero for lengths of 0.50 and 1.20 mm and
slightly positive for zooplankton 0.60, 1.00 and 1.10 mm
long. Zooplankton 0.70 to 0.90 mm in length occurred in
larval whitefish diets at much higher rates than their
abundance in the environment would predict (Figure 14).

Integrating this prey selection information with prey
abundance and larval fish densities provides a more
accurate estimate of the amount of food available to the
larvae each year. This estimate, labelled the z/f ratio,
is the amount of adult and copepodite zooplankton available
to an individual larval whitefish in East Traverse.
Dramatic differences in the value of this ratio exist
between the spring of 1983 and 1984 (Mann-Whitney U: U - 0;
p < 0.05) (Figure 15). Ratios were high and fairly constant
through the spring of 1983 when larval densities were low.
With higher numbers of larvae in 1984, values of the z/f
ratio were markedly lower and decrease through much of the
spring. These characteristics may help to explain the
changes in growth and survival documented in 1984.

Zooplankton/fish ratios are plotted in conjunction
with 1984 daily growth and weekly survival rates in Figure
16. As the ratios declined from Week 3 to 5, growth and

survival rates decreased. These rates increased when the

47

 

 

 

1.00

0.754

0.50-4
'7
Q
.9. 0.25«
'U
a
H
5“
E
'2; 0 4L— _________
0
d
m
'0:
m
3 0 25
b .
(O

-0.75 1
f1.00 7 1 T r v r 1 v 1 1 1
0. 10 0.30 0.50 0.70 0.90 1. 1 0

Zooplankton Total Length (mm)

Figure 14. Strauss’electivity indices for prey organisms
ingested by larval lake whitefish in Grand
Traverse Bay, Lake Michigan in 1984.

48

 

 

 

 

 

100
C) ’g””’
3:: x
a 00 " ____ ’.’/l
"3 ----- 1003
a 50 1 —- 1904
\
C3
C)
33
04
C)
(3
N

0 r . .

3 4 5 6

Week Sampled

Figure 15. Changes in the number of zooplankton per fish larvae during
Weeks 3 through 6 in Spring, 1983 and 1984.

49

 

0030

0016 +

 

 

 

 

Instantaneous Daily Growth Rate (mm)

 

80
{I
-——-- Gnuflh.RauI
--- - Z/F Ratio //1L 80
.40
I- 20
0
8

 

 

0.00 i

0.80 3

Weekly Larval Survival Rate

 

 

 

 

080 . . 0
3 4 6 6
Week Sampled
Figure 16. Changes in larval lake whitefish instantaneous daily

growth (a) and weekly survival (b) rates in relation
to changes in the number of zooplankton available to
individual fish larvae in Grand Traverse Bay, Lake
Michigan during Spring, 1984. Larval whitefish
pooled by week of capture.

Zooplankton/Fish Ratio

Zooplankton/ Fish Ratio

50

z/f indices increased after Week 5.

A plot of growth rate as a function of
the number of zooplankton per fish larvae revealed a
hyperbolic relationship with increasing z/f ratios
increasing the instantaneous daily growth rate of larval
lake whitefish (Figure 17). The threshold corrected
hyperbolic equation for substrate limited growth rate
fitted to this data was:

u/day - [0.0199((z/f)-25.0)]/[24.3 + ((z/f)-25.0)]

with the appropriate constants being calculated from the

slope and y-intercept values of the regression of ((z/f)-

(Z/flql/u on (Z/fl-(Z/f)q=

((z/f)-(z/f)q)/u = 1221.0 +
50.3 ((z/f)-(z/f)q). (r2 - 0.99)

51

 

 

 

 

A 0.000.
S n
V
g 0.0104
.0!
E 0.010.
a n I w
:3: '4
g 0.0054
3
O
0
g 0
g ..
.0.” 1 r 1 t I j
0 100 sec 000 400 000 000

Number of Zooplankton/Fish Larvae (z/f)

Figure 17. Instantaneous daily growth rate of larVal lake whitefish
sampled in Grand Traverse Bay, Lake Michigan in 1984.
Growth rate plotted as a function of the number of 200-.
plankton per fish larvae. Larval whitefish pooled by
week of capture.

DISCUSSION

--Variability--

Quantitative analyses of ecological systems are made
more difficult by the complexity of these systems. This
complexity can be accompanied by variability within and
between component parts of the ecosystem. Representative
samples removed from the system should reflect, and their
analysis consider, this variability. This was accomplished
with egg, zooplankton and larval fish samples collected in
East Traverse throughout the course of this study.

Although lake whitefish prefer a gravel or cobble-
sized substrate for spawning, these broadcast spawners will
deposit eggs over a wide range of bottom types. Eggs
deposited on fine substrates are moved by wind or wave
action to other poor substrates where damage or death may
occur or to coarse substrates where they may collect. Eggs
spawned on coarse substrates will be disturbed to a lesser
degree. The non-random distribution of these eggs can be
documented by sampling a lake bottom of mixed substrates
such as those in East Traverse.

This non-random pattern will be accentuated in egg
samples taken after a winter of heavy egg mortality, when,
relative to egg densities in the fall, very few eggs will
remain on the spawning grounds by the time hatching begins
in the spring. Surviving eggs will be those deposited on
substrates which provided the best incubation sites.

53

Estimates of variability in numbers of eggs alive and
sampled reflect this clumped distribution pattern.

This clumped, non-random distribution is also observed
in zooplankton and larval fish samples collected in East
Traverse. Patterns of zooplankton and larval fish
patchiness are phenomena frequently recorded in the marine
literature.0Cassie 1963: Smith 1973; Steele 1978: Hewitt
1981: Omori and Hamner 1982). Individuals aggregate or
swarm in response to behavioral or environmental conditions
that are not completely understood (Laurence 1974: Omori
and Manner 1982). The occurrence of these aggregations in
East Traverse is reflected in plankton samples collected
there. As this pattern appears to be a characteristic
inherent in plankton and larval fish populations,
intensified sampling efforts may reduce but likely“will not
eliminate such variability. Considering the non-random,
patchy distribution of individuals in the population,
commonly used statistical significance levels of 0.05 or
0.01 are often difficult to attain. Given the amount of
time and effort expended on sampling such patchy
distributions, significance levels as low as 0.10 or 0.20,
obtained in this study, appear to represent good precision.
Had time allowed acquisition of a much greater number of
samples, higher significance levels would have been

expected.

54

--Field and Laboratory Research--

Lake whitefish egg survival appears to be dependent
upon the severity of the winter through which the eggs
incubate. The exact mechanism responsible for this
difference in survival is unknown. However, physical
factors, primarily wind and wave action whose impact is
regulated by ice cover, are likely candidates (Miller 1952;
Clady and Hutchinson 1975). When ice forms over whitefish
spawning grounds soon after the eggs are deposited, spawned
eggs are protected from damage and mortality caused by wave
and wind action. Eggs are less likely to be moved to
substrates where damage or death may occur due to burial or
abrasion. Higher rates of egg survival result.

Eggs are exposed to these mortality factors throughout
a warm, iceless winter or for extended periods of time
during mild winters when ice does not form until late
January or February. Eggs deposited on or moved to sand,
silt or clay bottoms would suffer the highest rates of
mortality as these substrates provide little protection
from wind and wave action (Johnson 1961: Rupp 1965). Egg
mortality on large substrates prevalent in shallow water
regions in northern Lake Michigan also would be high during
mild winters. However, eggs settling into the deepest
crevices in these substrates would not be destroyed and a
small percentage of eggs would survive through the winter.
Overlying anchor ice which extends short distances out from

the shoreline would also improve shallow water egg survival

55

during mild winters.

In protective embayments or on lee shores away from
prevailing winds, mild winters may influence egg survival
in a manner different than observed in this study. With
little fetch, wind driven waves that destroy deposited eggs
cannot develop. In addition, some ice may develop in these
calm, protected areas during mild winters even though
temperatures are warm enough to prevent its formation in
regions of the lake where wind and waves are omnipresent.
If suitable spawning substrates are available and dissolved
oxygen is not limiting, spawning grounds located in these
areas should be ideal incubation sites and consequent egg
survival should be high. '

In more exposed regions like East Traverse, annual
fluctuations in overwintering egg survival likely would be
more severe, and their influence on lake whitefish year-
class strength, more extensive. Because of the large
number of eggs deposited by whitefish, small amounts of
variation in egg mortality from year to year have the
potential to produce substantial differences in the number
of fish recruited to the fishery. With an average
fecundity in East Traverse of 30,000 eggs/female,
approximately 1100 more eggs/female reached the larval
stage in 1984, when egg survival equalled 5.6%, than in
1983 when it equalled 1.7%. Over 500,000 more larvae were
present in 1984 than in 1983 if this change in egg survival

is extrapolated to a hypothetical spawning population of

56

500 females in East Traverse. Application of these
survival percentages to later stages of development, the
juvenile period for example, will remove far fewer
individuals.from the latter cohort and influence year-class
size to a lesser degree. Thus, the importance of winter
severity to year-class strength lies as much in its
existence as in the timing of its occurrence during the
life history of the whitefish. Moving its severity from
the egg stage where the potential for'massive changes in
numbers is great to a later period would reduce its impact
on whitefish dynamics. In its present position, however,
it may serve as one of the driving forces behind these
changes.

The effect of winter severity and overwintering egg
mortality on year-class strength goes beyond the impact of
the direct mortality imposed by it on developing embryos.
It also may play a controlling role in the growth and
survival dynamics of the larval stage of development.
Fundamental to growth and development is the availability
of energy to the individual organism. This energy becomes
inadequate for further growth and survival when the demand
for it exceeds the rate of its supply. By influencing the
density of food organisms serving as sources of energy for
larval whitefish, as well as determining the number of fish
entering the larval period, winter severity influences both
the supply and demand sides of the energy equation.

Energy is supplied to developing larvae in the early

57

spring in the form of copepod zooplankton that have
overwintered in East Traverse. These copepods begin
producing eggs soon after breeding in March and early
April (Selgeby 1975). The largest peak in the overall size
distribution of zooplankton in the bay is comprised of
early instar naupl ii that have hatched from these eggs. A
second and much smaller peak in this size distribution
consists of the late copepodite as well as the early adult
stages and arises from an increase in the length of time
spent by copepods as adults relative to the time spent in
other development stages (Gehrs and Robertson 1984).

Changes in zooplankton densities and length frequency
distributions through the spring result from factors
inherent in the mature population of overwintered adults
typifying early spring zooplankton assemblages (Selgeby
1975: Makerewicz and Likens 1982). Densities decline
through April and much of May when recruitment from younger
copepodite stages fails to balance the losses due to
mortality in the adult size classes (Selgeby 1975). In
addition, high loss rates of reproductive individuals
precludes large increases in naupl ii and early copepodite
densities at this time.

Initial densities of these zooplankton may, however,
be influenced by the severity of the winter through which
individual plankton live. Periphyton present on the lake
bottom during dives through the ice in 1984 was not present

during dives in the warm, iceless winter of 1983. Energy

58

is expended by zooplankton searching for the phytoplankton
food resource kept dispersed throughout the water column by
wave action in 1982/83. Zooplankton overwintering during
1983/1984 utilized an energy resource that, relative to
1982/83, may have.been more readily available and abundant.
Energy expended on search costs the previous winter may
have been available to meet growth and maintenance costs in
1983/84 when the resource appeared to be aggregated at the
lake bottom. If there was more energy to meet maintanence
and other requirements in 1984 than in 1983, higher
densities of zooplankton should have survived through the
winter of 1983/84.

The upward trend in standing crops of zooplankton
following the loss of ice cover in 1984 is driven by
additional amounts of sunlight penetrating into the water
and warming water temperatures at this time. Coupled with
these factors is the increased agitation of the water
column due to wave action after ice out. Thus, zooplankton
samples collected in Week 1 accurately depict between year
differences in densities. Samples collected at earlier
dates may not have reliably estimated plankton densities.
Distributional differences of these plankton at these
earlier dates may have made them inaccessible to the
sampling gear.

Improvements in the density of the zooplankton
resource are not the only consequence of the cold, ice-

covered winter of 1983/1984. High egg survival during this

59

winter produced high numbers of larvae hatching in early
April. .Although zooplankton were more abundant in 1984,
fewer plankton were actually available to individual
whitefish larvae because densities of these larvae in 1984
were 3.7 times higher than in 1983. These larvae may have
found it difficult to obtain enough energy for growth and
maintenance.

Laboratory studies illustrated the impact of
inadequate food rations or energy supplies on larval
dynamics. In the laboratory, larval lake whitefish growth
and survival rate were affected by altering available
energy. .A reduction in growth rate at low z/f values could
influence larval lake whitefish dynamics. The larval stage
of fish is potentially very susceptible to predation
(Lindstrom 1962). Larval fish can minimize this predation
pressure by growing rapidly into a less vulnerable size
class where fewer predators can ingest them. If larvae
encounter low z/f ratios in the spring, larvae would
experience a reduced rate of growth into a stage less
vulnerable to predation. Consequent increases in predation
mortality might prove costly to year-class strength.

Physical or chemical stresses such as temperature
changes and wave or current action also may influence
larval survival and year-class strength. Even if such
factors were relatively constant from year to year, their
influence on larval dynamics could depend to a large extent

on the zooplankton/fish ratio. If the ratio is low, not

60

only will larvae be unable to acquire enough food to
adequately deal with the stresses, but growth and survival
also may be hindered. Larval survival would be lower than
during years when the z/f ratio is high and growth is more
rapid. Laboratory observations of larval behavior revealed
that larvae receiving higher rations were robust and active
while those being starved or fed reduced rations spent much
of their time lying quietly at the bottom of the tank. It
appears these larvae could ill afford to expend energy on
stresses additional to maintanence and growth.

Predation and the influence of physical or chemical
stresses are indirect means by which larval lake whitefish
might be affected by low zooplankton/fish ratios.
laboratory experiments further demonstrated survival to be
directly influenced by food rations. Three findings in
particular merit discussion: the presence of a critical
period, the ability of even poorly fed larval lake
whitefish to survive at 12°C for 2 to 3 weeks and the
eventual mortality these larvae experience relative to
larvae held under conditions of high plankton densities.

The presence or absence of a critical period in fish
has long been debated (Marr 1956). My laboratory study
delineates its presence in larval lake whitefish.
Regardless of feeding level, mortality was essentially'zero
until day 15 of the experiment. After this date, which
coincided with yolk sac absorption and the switch by the

larvae to complete reliance on exogenous food sources for

61

energy, mortality increased in tanks receiving rations of
18 z/f and less. Prey densities in these tanks did not
provide enough energy to compensate for the loss of energy
resources previously available in larval yolk reserves. In
contrast, no large increase in mortality was experienced in
tanks where higher prey densities were available (32 and
110 z/f). Critical period expression thus appears to be a
function of available food densities rather than something
intrinsic to larval fish.development.

The ability of larval lake whitefish to live on poor
rations for roughly two weeks while experiencing very
little mortality could be considered adaptive for these
larvae. These fish hatch in late winter to early spring
when zooplankton densities are customarily low. Being able
to survive for an extended period of time with low
zooplankton densities may allow larvae to survive until the
spring zooplankton bloom occurs. They could then take
advantage of these higher densities and grow more rapidly,
thereby circumventing high mortality which would otherwise
occur if they required zooplankton soon after hatching.

It is important to note that if no improvement in
zooplankton abundance occurs in 2-3 weeks, post-hatch
larval mortality would be severe. Larvae experiencing low
zooplankton rations went from 100 to 0% survival within a
week to 10 days of yolk sac resorption. As major changes
in survival are noted with minor changes in food density, a

slight change in food abundance may thus have a significant

62

effect on the number of fish in a cohort.

Several characteristics of larval lake whitefish
growth and survival dynamics apparent in or hypothesized
from laboratory results also were expressed by field
populations of larvae in East Traverse. Changes in the
number of zooplankton available to individual fish larvae
were followed by corresponding fluctuations in the rate of
daily growth in length of these larvae. Rates decline from
:L43%/day when larvae were drawing upon yolk reserves and
available zooplankton for energy to -0.33%/day when yolk
reserves were exhausted and zooplankton populations
depleted. .At this later date, growth declines as energy is
diverted from growth and allocated to meet maintenence
requirements. Survival declines when available energy and
energy intake are not sufficient to meet maintenence costs.
Thus, variation in growth appears to be a precursor to
other changes in cohort characteristics that occur during
the larval stage of development.

Their exist a plethora of equations or models that
attempt to reliably predict individual growth in size (von
Bertanlaffy 1938: Silliman 1967: Knight 1969). In these
equations, size is modelled as some function of past size
and growth rate, with 1ittle,if any, attention given to
factors responsible for these values. The Monod or, more
correctly, threshold corrected hyperbolic equation for

substrate limited growth addresses these factors by

63

incorporating into its formulation values characterizing
individual growth kinetics and values representing energy
units from which these indices are derived. When applied
to field data from East Traverse and calculated growth
rates plotted as a function of available energy (z/f), the
resulting threshold corrected equation describes a curve
that falls from high to zero growth with little change in
the z/f ratio. Over this range, larval growth rates are
severely altered by relatively small changes in zooplankton
density or the abundance of larval whitefish. As such,
larval growth may be a sensitive indicator of the amount of
energy available to larval fish in their environment.
Because growth is sensitive to environmental
conditions and because changes in it often precede
fluctuations in other characteristics of the cohort,
accurate estimates of growth would be useful in the
analysis of larval dynamics. Accurate growth estimates
also would be a prerequisite to the successful use of these
rates as predictive tools. Thus, it would be wise to
investigate the reliability of the predictive threshold
corrected growth equation discussed previously and from
which these rates are calculated. Internal reliability of
this equation was checked by using it to calculate growth
rate based upon densities of zooplankton and larval fish
measured in the field. Larval length predicted from these
growth rates could then be compared to means determined for

field caught larvae from East Traverse. Actual and

64

predicted length values differ very little through the
course of the spring (Mann-Whitney U: U - 11: p - 0.42).
Actual final larval lengths are only 0.15 mm less than
predicted (Table 5). This result lends credence to the
usefulness of this equation as a tool in further modelling
of larval dynamics.

It is apparent that larval survival parallels larval
growth quite closely. Reductions in survival are
coincidental to decreases in growth which are a consequence
of lower amounts of energy available to individual larvae.
This becomes particularly apparent in 1984 when higher
larval densities reduced the amount of zooplankton
available to individual fish larvae.

Available energy is further limited by larval
whitefish prey size selection. These larvae do not ingest
the small naupl ii and early instar copepodites which are
very abundant in the water column. Instead, they utilize
pools of energy available in the large copepodite and adult
stages of copepods, particularly those individuals 0.70 to
1.10 mm long. The resorption of the yolk sac, coupled with
the near exhaustion of densities of these sizes of
organisms in Week 5 of 1984, reduced larval growth and
survival rate. Zooplankton/fish ratios fell below levels
necessary to meet maintenence costs and larval survival
declined sharply. The decline in density over this time

span accounted for 59.9% of the total reduction in spring

Table 5:

65

Actual mean weekly lengths of field caught larval
lake whitefish in 1984 (Weeks 3 through 7) and
lengths calculated from the predictive growth
equation derived using field growth rates and

zooplankton/fish ratios.

 

Week Actual Predicted
Length Length

3 13.83 13.91

4 14.48 14.30

5 14.15 14.20

6 15.08 15.43

7 17.91 18.06

66

larval density. This period appears to be critical to
larval survival and year-class strength in 1984. Thus,
critical period expression does not appear to be a
phenomena confined to laboratory observations.

Much of the debate over the existence of’aicritical
period in field populations of developing larvae might be
explained if its expression is in large part dependent upon
prey abundance factors. The presence of a critical period
would not be recorded in the field if high zooplankton
densities are available when endogenous sources of energy
are exhausted. In this case, low mortality would occur
during the transition to exogenous feeding. Such reasoning
may explain why higher mortality rates did not coincide
with severe reductions in yolk volumes between hatching and
Week 3 of 1984. However, as in Week 5 of 1984, a critical
period would be apparent if extant zooplankton densities do
not provide enough energy to maintain larval growth and
survival rates when the larvae are forced to rely upon
these exogenous food sources for much of their energy.
Slowed growth rates result and are followed by high rates
of mortality and low larval fish densities.

Larval density declined by 60% through the spring of
1984 when initial larval abundance was high and z/f ratios
reduced. Larval abundance declined by only 27.5% in 1983
when z/f ratios were higher. Thus, 1984 density initially
3.7times greater than recorded in 1983 falls to level 2.1
times higher than the 1983 value by the end of the spring

67

of 1984. If, as hypothesized, the egg and larval stages of
lake whitefish are critical to establishing lake whitefish
year-class strength, the 1984 cohort should be slightly
more than double the size of the 1983 year-class. Trawls
for juvenile lake whitefish in East Traverse demonstrated
that the 1984 year-class was 2.5 to 3.0 times larger than
the 1983 cohort (Figure 18).

It is possible to develop a model predicting year-
class strength in East Traverse by integrating zooplankton
abundance and larval fish growth and survival datawwith
records of overwintering egg survival in the bay. Again,
it is assumed that year-class size and recruitment are a
function of these periods of development. Reliability of
the model could be checked by comparing its year-class size
predictions to those documented during the larval and
juvenile stages of development.

Cumulative larval survival from Weeks 2 through 6 of
1983 equalled 0.724 and corresponded to a calculated daily
growth rate of 0.0164 over this time span. Growth rate
dropped to 0.0081 in 1984 and survival declined to 0.0406.
Assuming linear increases in survival with growth rate,
the following equation is produced when cumulative survival

rate is regressed on growth rate:
Survival/Spring a 0.0957 + 38.3 (u/day)

It has already been demonstrated that larval growth is
reliabily predicted by the threshold corrected hyperbolic

68

 

 

 

 

 

 

 

 

 

 

 

 

25-
9
k 30- ——-- r—
2‘.
[-1
\
3 I:
16d 0
E 2
a —
hi
8 10‘ E
Ch be ll
{3' o 3 r--—
..3 2 *
3 ~ -
5d
0

June, 1984 June, 1985

Figure 18. Catch per effort of l, 2 and 3 year old lake whitefish
in-Grand Traverse Bay, Lake Michigan during June of
1984 and 1985.

69

equation for substrate limited growth and
the zooplankton/larval fish ratio embodied in it. Thus,

substituting this equation for u produces:

Survival/Spring - 0.957 +

38.3[(0.0l99 (z/f - 25))/(z/f + 49.3 - 50)]

Using this equation, changes in survival/spring as a
function of changes in the z/f ratio are depicted in Figure
19. This equation describes the survival of larval lake
whitefish from the time of larval hatch to deepwater
movement of the individuals. To estimate larval survival
over this time span, the weekly estimate of zooplankton
density in the numerator of the z/f ratio in the equation
is replaced by the mean density of adult and copepodite
zooplankton recorded in the spring during weeks preceding
the deepwater movement of the whitefish. This mean value
provides an index of spring zooplankton abundance each
year. The fish denominator of the ratio is replaced by the
density of eggs surviving through the preceding winter,
hatching in the spring and encountering the extant
zooplankton densities.

Assuming zooplankton density does not increase in
parallel with the number of eggs surviving through the
winter, any increase in egg survival will lower survival
through the larval stage as the amount of energy available
to each individual is reduced. Energy available to
individual larvae is lowest following winters of high egg

70

 

 

0.00 n
E!"
D.
Q
A
% ...
E? Zun»-800
0300
:3;
E 0.80 1
08 loop. I- 1.”
.2 03%”
E
0 v . . r . . .
0 0.03 0.0‘ 0.00 0.00 0.10 0.13 0.14

Egg Survhnfl.nndex

Figure 19. Spring survival of lake whitefish larvae as a function of
changes in the number of whitefish eggs hatching in the
spring and the amount of zooplankton available to the
developing larvae. Zooplankton densities are held constant
at 1.92/1 (Spring, 1983)and 2.98/1 (Spring, 1984) while egg
numbers are varied from 0.001/m2 to 0.13/m2.

71

survival and low zooplankton production. Thus, larval
survival dynamics will influence cohort size and whitefish
year-class strength to the greatest extent following
cold, ice-covered winters. The rate of supply of food is
adequate to meet the requirements of the low number of
larvae hatching after open winters. In these years,
survival during the egg, not the larval, stage of
development is critical to year-class strength.

Regardless as to what stage of whitefish early life
history is most responsible for determining year-class
size, the model demonstrates that interrelated changes in
the number of eggs spawned, the survival of these eggs
through the winter and the density of zooplankton available
in the spring are the driving forces behind these dynamics.
Stock size and winter severity determine the number of
larvae hatching each spring. Varying these two factors
while holding zooplankton density constant will produce a
wide range of year-class sizes. For my purpose,
zooplankton density was held constant at the mean for the
spring of 1983 and 1984 and the number of eggs, and
consequently larvae, entering this period varied from 0.001
to 0.13/m2. This procedure produced two hyperbolic curves
reminiscent of the Ricker stock-recruitment function under
resource limited conditions (Figure 20). As egg number,
which incorporates into it the number of eggs spawned and
overwintering survival of these eggs, increases, larval

survival declines and year-class strength increases to a

72

 

 

0.031
loop. - 2.98
(1064)
N 0.02
0
'U
:l
H
m
m
d
H
0
m
>‘ 0.01 ‘
flnpu-lsz
(1003)
o v Y i v v v i
0 0.02 0.04 0.00 0.00 0.10 0.12 0.14

Egg Survival Index

Figure 20. Range of year-class indices possible during Spring, 1983
and 1984. Indices determined by integrating overwintering
egg survival with spring larval survival. Zooplankton

density held constant at 1.92/1 and 2.98/1 during 1983
, and 1984, respectively.

73

maximum of 0.187 in 1983 and 0.291 in 1984. Further
increases in egg number reduce year-class strength. Over
the descending portion of the curve, low z/f ratios
eliminate gains made in density due to high egg survival.
This result once again points to the degree of interaction
between overwintering egg survival and the extent of
mortality during the larval stage of development.

Using this equation in collaboration with data
pertaining to overwinter egg survival in East Traverse will
provide estimates of spring larval survival and relative
year-class strength to which values actually recorded in
the bay can be compared. Predicted values of larval
survival of 0.693 and 0.505 in 1983 and 1984 approximate
values recorded in the field during each respective year.
The prediction of relative year-class strength made by the
model (1984 cohort:2.4 times the size of the 1983 cohort;
Figure 21) compares favorably to other estimates of
relative size. As mentioned previously, cohort abundance
was 2.1 times higher at the close of the larval period in
1984 than it was in 1983. Trawls for juvenile whitefish in
East Traverse demonstrated that the 1984 cohort was 2.5 to
3.0 times greater than the 1983 year-class (Figure 18). It
appears relative year-class size of whitefish in East
Traverse can be fairly accurately predicted with data
pertaining to overwintering egg and.spring larval survivala
Incorporation of these data into a mathematical description

of the system produces a reliable estimator of the relative

74

 

 

0.03 .
---------- I
I
I
I
I
I
: loop. - 2.00
I one»
m J .
” I
0
‘6 l
.5 I
u I
g I
—o I
O l
I
a ...... I
o I I
>‘ 0.01 . I 2
I 2001:. - 1.02
I I
I ' ,
. I
1 l
| I
I I
' i
l
o 1. . i . e . . .
0 0.“ 0.04 0.00 0.00 0.10 0.12 0.14

Egg Survhnfl.nndex

Figure 21. Year-class indices predicted by the early life history
recruitment model for Spring, 1983 and 1984. Zooplankton
density held constant at 1.92/1 and 2.98/1 during 1983
and 1984, respectively. Egg survival equals 0.017/m2 and
0.056/m2 in 1983 and 1984, respectively.

75

size of year classes of whitefish in the bay.

Analysis of model behavior also can provide clues as
to the stage of development controlling these annual
fluctuations in cohort abundance. Given the zooplankton
conditions in 1983, a year-class curve is produced that
peaks at a cohort size index of 0.0187 (Figure 21). If the
egg survival (ie. fish number) recorded during that year is
plotted on this curve, the index corresponding to the point
of intersection of the egg survival value and the year-
class parabola is well below the maximum value possible
given available spring zooplankton densities (Figure 21).
Severe overwintering egg mortality during the iceless
winter of 1983, not the ensuing spring larval dynamics, was
responsible for the reduced size of the 1983 year-class.
Increases in cohort abundance to a value as much as 1.6
times higher than observed would have been possible if egg
survival had been higher. Higher densities of spring
zooplankton only would have made this discrepancy between
realized and potential year-class size more severe.

The scenario is different in 1984. Egg survival
during the preceding winter corresponds to a year-class
near the peak of the curve of cohort indices (Figure 21).
Given existing zooplankton densities, further increases in
the number of eggs surviving through the winter would do
little to increase and may actually decrease eventual year-
class strength. Thus, plankton densities limit the size of

the 1984 cohort. Higher densities of zooplankton would

76

have produced a curve of greater magnitude and severe
mortalities would have been avoided as more zooplankton
would have been available to individual fish larvae.
Cumulative larval survival would have been increased and
more fish would have passed through the critical early life
history stages of development. Under prevailing
conditions, however, the amount of zooplankton produced in
the bay was not adequate to meet the energy requirements of
the developing larvae. Although zooplankton abundance was
greater following the ice-covered winter of 1983/84, this
increase could not keep pace with changes in the number of
surviving eggs or larvae. Excess fish unsupportable by the
system were eliminated and densities at the close of the
larval stage were near the maximum sustainable by spring
zooplankton conditions.

It is evident that the dynamics of the larval stage of
whitefish development are tied intimately to the severity
of the previous winter and its corresponding egg mortality.
The larval stage of whitefish development appears to act as
a bottleneck retarding the movement of these fish to older
stages and larger sizes. This bottleneck operates most
severely when winter conditions have been favorable for egg
survival. Because of its position before other stages of
development, winter severity and egg mortality can function
as a driving force behind early life history dynamics.
The larval period responds to the annual fluctuations in

density in a manner analogous to an engine governor.

77

Control over the rate of flow through the system is
tightened when too many particles, ie. larvae, enter it and
loosened when their rate of entry is low. Thus, larval
dynamics are dependent upon and operate within limits
described by overwintering egg mortality to further
delineate year-class size.

In considering these changes, it is critical to
realize that larval dynamics cannot be interpreted without
regard to the importance of fluctuations in the ratio of
zooplankton numbers to fish numbers. Without considering
larval densities, cumulative larval whitefish survival in
1984 would be expected to increase over 1983 values with
higher zooplankton densities in 1984. Owing to improved
overwintering survival of lake whitefish eggs, however,
high densities of larval whitefish in the spring of 1984
lower the ratio relative to what was recorded in 1983.
Thus, fewer zooplankton are available to individual larvae
in 1984, energy became limiting and survival declined.

Hoagman (1974) states, ”It is exceedingly doubtful
that wild (whitefish) larvae would be unable to find and
capture sufficient food for growth and maintanence. Thus,
natural starvation seems remote...". In focusing only on
characteristics of the larvae, however, Hoagman failed to
consider trophic level interactions, interactions critical
in controlling fish population dynamics. Interpreting the
dynamics of larval lake whitefish in East Traverse, in

light of their integration with zooplankton dynamics and

78

overwintering egg mortality, reveals that larval lake
whitefish are at times unable to find and capture
sufficient food for growth and survival at critical stages
of development. The integration of these dynamics into a
comprehensive model describing whitefish year-class
characteristics provides a critical prerequisite to an
accurate management perspective of mechanisms controlling

changes in the whitefish resource.

MANAGEMENT IMPLICATIONS

The stock-recruitment model has generally failed as a
predictive tool in fisheries management. Due to the scope
of the variability inherent in the stock-recruitment
relationships, yield estimates can be inadequate and are
often incorrect. The source of much of this variability
and, consequently the low reliability of these
realtionships, lies in the failure of these models to
consider factors other than the fish stocks themselves as
agents responsible for changes in year-class size.
Attention is not directed to mechanisms operating in other
life stages or originating from dependencies upon other
trophic levels and abiotic factors. .As these factors alter
numbers throughout the.pre-recruitment.period, it is not
surprising to find actual recruitment is often times much
different than what is predicted by stock values.

By delineating some of the mechanisms which interact
to control lake whitefish dynamics, this study may provide
insights into improved forecasts of whitefish recruitment
and yield, ‘Within Grand Traverse Bay these estimates
should be particularly reliable. Variations in winter
severity influence both.the number of eggs surviving to the
larval stage and the initial abundance of zooplankton in
the spring. During mild winters, control originating at
the egg stage is responsible for eventual recruitment
strength. The larval stage exerts its influence when

numbers of larvae overwhelm the production capacity of food

80

in the bay. During these years, survival through the
larval stage is very important in determining year-class
strength. The integration of these factors into a
deterministic model provides a relationship between egg
survival, a surrogate for stock size, and year-class
strength similar to that described by Ricker for resource
limited conditions. Basic differences, however, separate
the two models. Instead of being highly correlative in
nature, the equation described herein is mechanistic in
scope. Recruitment is a function of factors clearly
demonstrated as influencing year-class size rather than
being related simply to the given stock size from which it
is spawned. The early life history equation is also
dynamic rather than fixed in nature. The magnitude of the
curve produced.by the equation responds to variations in
the amount of energy available to individual organisms. .As
such, the year-class strength predicted from a given egg
survival varies in accordance with prevailing zooplankton
conditions each spring. Thus, a given egg survival may
result in an array of recruitment levels.as zooplankton
density varies from year to year. Traditional models fail
to incorporate these changes and predict one level of
recruitment for each stock size.

Variations in spawning location and lake productivity
may alter between variable relationships delineated in.East
Traverse. With different relations to prevailing wind

conditions, ice movements and upwellings at other spawning

81

locations in the lake, the interaction between these
controlling mechanisms is likely site specific. This is
most clearly demonstrated when latitudinal differences are
considered. In higher latitude lakes, the role winter
severity plays in year-class variability is reduced. Lakes
are covered with ice each year and the catastrophic effects
of open winters are eliminated. The larval stage is left
as the period when year-class size is established.
Differences in lake productivity between lakes within a
year, or between years within a lake, largely determine
recruitment. Increases in available energy with higher
levels of productivity‘will allow a greater percentage of
emergent larvae to live through the spring. Year classes
will be higher than in lakes where or years when
productivity, available energy and survival is poor. As
the whitefish is well within its range in these northern
lakes, biotic factors appear to be controlling its dynamics
(Lack 1954). '

In regions farther south, the importance of winter
severity and overwintering egg mortality increases. Some
years will be ice covered and others will not. Recruitment
will be a function of egg survival during years without ice
as losses during this stage cannot be compensated for with
higher survival at later stages. During years of ice cover
the year-class control sequence becomes much like that
already described for more northern latitudes.

Unlike these northern regions, ice may seldom, if

82

ever, cover spawning grounds at still more southern
latitudes. As the whitefish is now on the edge or boundary
of its range, abiotic factors replace biotic mechanisms as
those controlling lake whitefish dynamics (Andrewartha and
Birch 1954). Year-class strength becomes highly dependent
upon the severity of the winter through which eggs
incubate. Lake productivity, although generally higher at
lower latitudes, (Brylinsky and Mann 1973) will influence
year-class strength little as numbers of individuals have
very likely been severely reduced previously. A few strong
year classes will be apparent and resulting from years
with cold, ice-covered winters. Survival of these cohorts
at other stages of development is high, owing to the higher
level of productivity and available energy at these
southern latitudes.

Consideration of these early life history factors and
their inter-relationships may prove enlightening in the
analysis of the dynamics of this cosmopolitan species of
fish. Further studies investigating the question of
interactions between environmental factors, the dynamics of
fish life history and their relationship to recruitment are
indeed necessary. Without them, improvements in our
understanding of a species dynamics, and the biotic or
abiotic mechanisms responsible for much of this variation,
will be slowed. With them, advances in the ability of
fisheries biologists to accurately predict yield will be
forthcoming.

3.

SUMMARY

Using data collected from laboratory experiments as
well as field research in Grand Traverse Bay, Lake
Michigan, overwintering lake'whitefish.egg survival was
integrated with larval lake whitefish growth and
survival rates and concurrent levels of prey abundance.

Annual variations in overwintering egg survival appear
to influence lake whitefish year-class strength.
Abundance of individuals in cohorts incubated in mild
winters on spawning grounds lacking ice cover is
significantly reduced relative to numbers hatching
after severe winters when lakes are covered by ice.
The number of eggs remaining on the spawning grounds
following the cold winter of 1983/84 was 3.4 times
greater than the number remaining after the warm winter
of 1982/83 when ice did not form over the spawning
ground.

A linear increase in larval survival with increasing
specific growth rate was recorded in the laboratory.
Larval growth rate and survival increased as the amount
of zooplankton per fish larvae per 12 hour period
became greater. Dramatic changes in survival occur
between prey levels of 18 and 32 zooplankton per fish
larvae per 12 hour period (z/f). Total mortality of
larvae receiving low food rations (5 10 z/f) rises
sharply after larval yolk reserves are exhausted.
Larvae being fed 32 and 110 z/f, however, exhibit
little fluctuation in mortality subsequent to yolk sac
resorption. Thus, prey density factors appear to be
largely responsible for the expression of critical
periods in larval whitefish development.

Trends similar to those recorded in laboratory
research are apparent in field studies.of larval
growth and survival rates in 1984. Growth and.survival
rates again vary in proportion to the number of
zooplankton available to each fish larvae (z/f). As
z/f ratios decline to 23.5, growth rates fall below
zero, survival rates decrease sharply, larval densities
decline and a critical period in larval whitefish
development is realized. Larval densities do not
decline and a critical period in development is not
apparent in 1983 when z/f ratios are high and constant.

84

Characteristics of larval whitefish prey selection are
in large part responsible for the critical nature of
Week 5 in 1984. Higher densities of smaller particles
are ignored through much of the spring. Diets instead
consist almost exclusively of adult/copepodite stages
of copepods, with larvae ingesting zooplankton 0.70 to
1.10 mm long at higher rates than the abundance of
these lengths in the environment would predict. Thus,
near exhaustion of pools of energy available in larger
prey organisms in Week 5 of 1984 contributes to the low
survival and reduction in density observed at this
time.

Incorporating prey selection data with information
regarding overwintering egg survival, spring larval
survival and spring zooplankton density results in a
predictive model of whitefish year-class size in East
Traverse. Using this model, the 1984 year-class of
whitefish appears to be 2.4 times as large as the 1983
cohort. This prediction corresponds well with year-
class estimates derived from spring larval whitefish
samples (1984 2.1 times larger than 1983) and trawls
for juvenile whitefish in East Traverse (1984 2.5 to
3.0 times larger than 1983). It is apparent that lake
whitefish year-class strength can be reliably predicted
given an accurate description of the early life history
factors influencing the recruitment process.

LIST OF REFERENCES

LIST OF REFERENCES

Andrewartha, 11.6. and L.C. Birch. 1954. The Distribution
and Abundance of Animals. Univ. of Chicago Press:
Chicago. pp. 648-665.

Bajkov, A. 1930. A study of whitefish in Manitoban lakes.
Contrib. Can. Biol. Fish. 5:443-455.

Beers, SLR. and G.L. Stewart. 1971. ‘ Microzooplankters in
the plankton communities of the upper waters of the
eastern tropical Pacific. Deep-Sea Research. 18:861-
883.

von Bertanlaffy, L. 1938. A quantitative theory of
organic growth. Hum. Biol. 10:181-213.

Beverton, R.J.H. and S.J. Holt. 1956. A review of methods
for estimating mortality rates in fish populations
with special reference to sources of bias in catch
sampling. Rapp. P.-v. Reun. Cons. Perm. int. Explor.
Mer. 140:67-83.

Beverton, R.J.H. and SJ. Holt. 1957. On the dynamics of
exploited fish populations. U.K. Min. Agric. Fish.,
Fish. Invest. (Ser. 2) 19:533 pp.

Blaxter, J.I-I.S. 1965. The feeding of herring larvae and
their ecology in relation to feeding. Calif. Coop.
Oceanic Fish. Invest. Rep. 10:79-88.

Blaxter, J.H.S. 1971. Feeding and condition of Clyde
herring. Rapp. P.-v. reun. Cons. Perm. int. Explor.
Her. 160:128-136.

Braum, E. 1967. The survival of fish larvae in reference
to their feeding behaviour and the food supply. In:
The Biological Basis of Fish Production (S.D. Gerking,
ed.) pp. 113-131. Oxford, Edinburgh: John Wiley and
Sons.

Brylinsky, u. and K.H. Mann. 1973. An analysis of factors
governing productivity in lakes and reservoirs.
Limno. and Ocean. 18:1-14.

Cassie, R.M. 1963. Microdistribution of plankton.
Oceanogr. Mar. Biol. Annu. Rev. 1:223-252.

86

Chenoweth, 8.3. 1970. Seasonal variation in condition of
larval herring in the Boothbay area of the Maine
coast. J. Fish. Res. Bd. Can. 27:1875-1879.

Christie,‘W¢L 1963. Effects of artificial propagation
and weather on recruitment in the Lake Ontario
whitefish fishery. J. Fish. Res. Bd. Can. 20:597-646.

Clady, M. and B. Hutchinson. 1975. Effect of high winds
on eggs of yellow perch (Perca flavescens) in Oneida
Lake, New York. Trans. Am. Fish. Soc. 104:524-525.

 

Deriso, R.B. 1980. Harvesting strategies and parameter
estimation for an age-structured model. Can. J. Fish.
Aq. Sci. 37:268-282.

Einsele, W.G. 1963. Problems of fish larvae survival in
nature and in the rearing of economically important
middle European freshwater fishes. Calif. Coop.
Oceanic Fish. Invest. Rep. 10:24-30.

Faber, DJ. 1970. Ecological observations on newly
hatched lake whitefish in South Bay, Ontario. In: The
Biology of Coregonid Fishes (C.C. Lindsey and C.S.
Woods, eds.) pp. 481-500. Univ. Of Manitoba Press.

Fluchter, J. 1980. Review of the present knowledge of
rearing whitefish (Coregonidae) larvae. Aquaculture
19:191-208.

,Frederick, IuL. 1982. Ecology of juvenile whitefish
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