Forage Fishes and Their Salmonid Predators in Lake Michigan

SEA GRANT COLLEGE PROGRAMUNIVERSITY OF WISCONSIN

WIS-SG-81-729

FORAGE FISHES AND THEIRSALMONID PREDATORS IN

LAKE MICHIGAN

Donald J. StewartJames F. KitchellLarry B. Crowder

Most Significant'per of the Year

Pa<

Reprinted from:Transactions of ihe American Fisheries Society

110:751-763, 1981

Transactions of the. American Fistwrifs Society 110:751-763, 1981© Copyright by the American Fisheries Society 1981

Forage Fishes and Their Salmonid Predators in Lake Michigan

DONALD J. STEWART/ JAMES F. KITCHELL, AND LARRY B. CROWDERLaboratory oj Limnology and Department of Zoology, University of Wisconsin—Madison

Madison, Wisconsin 53706

AbstractAlewife Alosa pseudoharengus and rainbow smelt Osmerus mordax dominate the planktivorous

fish fauna of Lake Michigan and are now the primary food of lake trout Salvelinus namaycuskand introduced salmonids. Their fluctuations in abundance have been a concern due to theireffects on native species and their present role as forage species. Each has been implicated as animportant factor in the local reduction or extinction of important native species. Mechanismsfor these interactions include competition for food and predation on eggs and larvae. Bioener-getic modeling simulations of alewife consumption by stocked salmonids suggest that as muchas 20 to 33% of the annual alewife production may be consumed in some years. Increasingstocking rates of salmonids in Lake Michigan yield a predator-prey system in which predatornumbers become relatively independent of prey dynamics. This suggests possible declines inalewife production, changes in major forage available to predators, and perhaps destabilizationof the current predator-prey system. Results of simulations indicate that a given number ofchinook salmon Oncorhynchus tshawytscha stocked in Lake Michigan will eat almost twice as muchforage fish as the same number of coho salmon O. kisutch; equal numbers of lake trout will eatabout 1.5 times as much as coho salmon.

The present fish populations of Lake Michi-gan form a management-dependent systemdominated by exotic fishes. The lake now hasa salmonid sport fishery of high social (recre-ational) and economic value. Without continu-ing human intervention to control the sea lam-prey (Table 1), and to propagate salmonids inhatcheries, the system rapidly would return toits low-value status of the mid-1960s.

The situation that led to salmonid introduc-tions in the mid-1960s stemmed from a seriesof events that began early in this century (Smith1968): (1) the rainbow smelt was introduced tothe Lake Michigan drainage in 1912 and sub-sequently spread throughout the upper GreatLakes; (2) the alewife and the sea lamprey in-vaded the upper Great Lakes via the WellandCanal; (3) the sea lamprey, together with a size-selective fishery, decimated the larger commer-cial fishes including lake trout, the primary pi-scivore, and this permitted the alewife (in the1960s) to attain high abundance; (4) rainbowsmelt and alewife provided strong competitionfor native planktivores and may also havepreyed on their eggs and larvae (Crowder

1 Present address: Fish Division, Department ofZoology, Field Museum of Natural History, Chicago,Illinois 60605.

1980), leading to extreme reduction or extinc-tion of native species; (5) alewife went througha population explosion and massive die-off in1967, which had high social and economic costs.Simply stated, in the mid-1960s Lake Michiganwas yielding very little of its natural productiv-ity to human benefit.

Through a monumental and successful effortto develop a selective toxicant for ammocetes(Lawrie 1970), sea lamprey populations weregreatly reduced, and this permitted reintro-duction of lake trout to Lake Michigan in 1965.To further rehabilitate the lake, increasingnumbers of salmonids, including coho salmon,chinook salmon, rainbow trout, and browntrout have been stocked. This has led to a valu-able salmonid sport fishery. Almost 14 millionsalmonids were stocked in 1979. Public enthu-siasm now creates pressure for further in-creases in stocking levels. Because very few laketrout have produced offspring for yet unknownreasons, and stream spawning habitat for salm-on is somewhat limited, salmonid densities highenough to support an important sport fisherymust be maintained by stocking. Sea lampreymay never be fully eradicated and must be con-trolled constantly to permit the salmonid fish-ery to exist. Without management, Lake Mich-igan's fishery resource would again dissipate(Walters! et al. 1980).

751

752 STEWART ET AL.

TABLE 1.—Fish species cited by common name in the text.

Alewife Alosa pseudoharengusAtlantic menhaden Brevoortia tyrannusBlackfin cisco Coregonus nigripinnisBloater Coregonus hoyiBrown trout Salmo truttaBurbot Lota lotaChinook salmon Oncorhynchus tshawytschaCoho salmon Oncorh^nchus kisutchDeepwater cisco Coregonus johannaeDeepwater sculpin Myoxocephalus thompsoniEmerald shiner Notropis atherinoidesKiyi Coregonus kiyiLake herring Coregonus artediiLake trout Salvelinus namaycushLongjaw cisco Coregonus alpenaePacific sardine Sardinops sagax.Rainbow smelt Osmerus mordaxRainbow trout Salmo gairdneriSea lamprey Petromyzon marinmShortjaw cisco Coregonus zenitkicusShortnose cisco Coregonus reighardiSlimy sculpin Cottus cognatusSpottail shiner Notropis hudsoniusTrout-perch Percopsis omiscomaycusYellow perch Percaflavescens

Forage for salmonids in Lake Michigan is fi-nite. It is possible, with available hatchery tech-nology, to produce more salmonids for stockingthan the system can sustain. Excessive stockingcould result in high mortality or poor growthof stocked salmonids, or it could cause local ex-tinction of a major forage organism. Chinooksalmon have been successfully used to com-pletely eradicate rainbow smelt from certainsmall lakes in New Hampshire (Hoover 1936).The question is "How many predators are toomany?" Lake Michigan may be a suitable systemfor exploring such a question. Modeling simu-lations of Lake Michigan fishes lead to infer-ences about management possibilities. Experi-mental management manipulations may beused to test model inferences. Inferences fromanalyses presented herein may apply to the oth-er Laurentian Great Lakes, and the general en-ergetics approach used to evaluate predationmay be viable for any system if appropriate dataare available.

The number of salmonids stocked in LakeMichigan may increase in coming years. An es-timate of the capacity of the system to supportfurther increases is greatly needed. Our specificobjectives are: (1) to review historic and currentinteractions among the forage fishes of LakeMichigan and discuss the implications of man-aging their predators; (2) to review past, pres-

ent, and projected stocking levels for the fivemain salmonids being stocked into Lake Mich-igan; (3) by bioenergetic modeling simulations,to estimate past, present, and future alewifeconsumption by the three most abundant sal-monids—coho salmon, chinook salmon, laketrout—and extrapolate to rainbow and browntrout; (4) to compare relative predation impactof coho salmon, chinook salmon, and lake trouton various types of forage; (5) to contrast rel-ative distribution of predation over time (pre-dation inertia) by coho salmon, chinook salmon,and lake trout on their forage; and (6) to eval-uate some possible alternatives for managingthe Lake Michigan forage-fish assemblage withsalmonid predators through manipulationssuch as changing species mix or total numbersstocked. All the above should provide infer-ences about consequences of increased stock-ing. Relative stability of the existing predator-prey system can be inferred from proportionof the total annual forage production beingconsumed by salmonids.

Interactions Among Forage SpeciesAlewives have dominated the planktivore

community in Lake Michigan for 20 years. Theincrease of this species is well documented(Brown 1968; Wells and McLain 1973), but themechanisms of the species' interactions withother planktivores and its effects on forage-fishcommunity structure are much less clear (Smith1968, 1970; Brown 1972; Christie 1974). A sec-ond exotic, rainbow smelt, has been commonsince the early 1930s.

During the period of alewife or rainbowsmelt increase (1930s-1960s), several species ofnative fishes declined and are extremely rare orlocally extinct in Lake Michigan. Emerald shin-er, lake herring, and six species of ciscoes(blackfin cisco, deepwater cisco, longjaw cisco,shortnose cisco, shortjaw cisco, kiyi) are no lon-ger common in the lake. Populations of othernative species (for example, yellow perch, bloat-er, and deepwater sculpin) have varied in sizeinversely with that of alewives (Wells andMcLain 1973). Numerous hypotheses havebeen suggested to account for these dynamics.Size-selective fishing, size-selective predation bysea lamprey, competition for food with rainbowsmelt and alewife, and predation by alewife orrainbow smelt are the most commonly pre-sented (Smith 1970; Wells and McLain 1973).

FORAGE-SALMONID DYNAMICS IN LAKE MICHIGAN 753

Competition requires the joint use of a lim-iting resource. Adults of planktivorous speciesin Lake Michigan share many common prey in-cluding Mysis relicta, Pontoporeia hoyi, and largerzooplankton species (Wells and Beeton 1963;Morsell and Norden 1968; Janssen and Brandt1980; Wells 1980; Crowder et al. 1981). Smallfishes in this community may have even largerdiet overlaps, because most species seem to con-centrate on small copepods, cladocerans, androtifers (Norden 1968; Siefert 1972).

Adult planktivores often consume the largestavailable prey and thus may have profound ef-fects on the size structure of zooplankton com-munities (Brooks and Dodson 1965; Hall et al.1976). The size and species compositions ofLake Michigan zooplankton have undergonelarge shifts (Wells 1960, 1969). Between 1954and 1966, during the period of alewife in-crease, mean zooplankton size declined sharply.Large zooplankters (1.1-5 mm average length)became extremely rare. Medium-sized andsmall species (0.43-0.93 mm average length)became more abundant. After a massive alewifedie-off in 1967, the zooplankton communitybegan to recover to its 1954 composition. Thissuggests intense planktivory and perhaps a lim-itation on food resources for planktivores dur-ing the alewife peak.

Competition for food may be minimized bysegregation on other niche axes (Moermond1979). Brandt et al. (1980) have evidence forsegregation of major planktivore species bythermal habitat in Lake Michigan. Alewives,rainbow smelt, spottail shiners, trout-perch,deepwater sculpins, and yellow perch differ sig-nificantly in the temperatures they occupy onthe bottom where the thermocline intersects thelake substrate. Temperature-related differ-ences in spatial distribution also occur withinspecies. Native species that overlap the thermalhabitats of alewife and rainbow smelt use dif-ferent foods (Crowder et al. 1981). This patternof resource subdivision often has been attrib-uted to competition (Schoener 1974), thoughother interactions certainly will influence re-source use. Partitioning of food and habitat re-sources reduces the potential for interspecificcompetition (Larkin 1956).

Predation by alewife or rainbow smelt alsomay have affected native species (Crowder1980). Of 21 fish species common in LakeMichigan prior to the invasions of alewife and

smelt, 10 species have pelagic or semipelagiceggs or larvae (Balon 1975). After the increaseof alewife and smelt, only one of these species,bloater, remained abundant. Pelagic eggs andlarvae are more available than demersal onesto pelagic predators such as alewife and rain-bow smelt. The eggs of species that are nowrare (emerald shiner, lake herring, the six cis-coes, burbot) are certainly large enough to beenergetically profitable to pelagic planktivores,especially if other food is somewhat limited.Other native species tend to have demersal eggsand larvae.

Of the rare species, emerald shiner probablyhas been most affected by alewife predation.Abundant in southern Lake Michigan untilabout 1960, emerald shiners showed a rapiddecline as alewives increased. Although bothalewives and rainbow smelt are potential pred-ators on the pelagic eggs of emerald shiners,rainbow smelt had been present since the 1930sand alewives were much more abundant duringthe cyprinid's decline. Emerald shiner appar-ently declined first in northern Lake Michiganand somewhat later in the southern portion ofthe lake (Wells 1977). This directly correspondswith the pattern of alewife increase. A conge-ner, spottail shiner, with similar feeding habits,size, predators, and distribution (Scott andGrossman 1973) has not disappeared fromLake Michigan, but it spawns over sand and hassmall, adhesive, demersal eggs (Scott andGrossman 1973; Balon 1975).

Undoubtedly, both alewife and rainbowsmelt have had significant competition and pre-dation interactions with native forage species.Competition between alewife and rainbowsmelt can be severe (Smith 1970); in Canan-daigua Lake, New York, introduced alewifecompletely replaced rainbow smelt as the prin-cipal forage species (Eaton and Kardos 1972).In Lake Michigan, alewife and rainbow smeltcoexist, perhaps due to habitat partitioning(Crowder et al. 1981).

Salmonid Stocking and Predatory Impact

Interactions among forage fishes might alsobe influenced by differential predation fromthe various native and introduced salmonidsthat now are maintained at relatively high den-sities by stocking. Evaluation of predatory im-pact of the salmonids and of how variousspecies of predators differentially influence

754 STEWART ET AL.

TABLE 2.—Proportional composition by weight of four foodcategories in the diet of coho salmon and chinook salmon

from southern Lake Michigan at various times of the year.Day 1 of a modeling simulation is April 1 for coho salmonand May I for chinook salmon. Based on data from Judeand Miller (personal communication, Great Lakes Re-search Division, University of Michigan, Ann Arbor).Alewives over 8 e* were considered adults.

Food type

Simulationdays

Cohort

1

2

Start

1121181

161

121151

End

Coho

120180360

60120150240

Inverte-brates

salmon (N

0.00.00.00010.00010.00.00.0

Otherfish

= 187)

0.890.400.420.060.110.0030.05

Young-of-yearalewife

0.110.600.210.290.040.020.02

Adultalewife

0.00.00.370.650.850.980.93

Chinook salmon (N ~ 44)a

I

2

3

4

161

121

191

121181

1

1

60120360

90120180360

360

210

0.970.160.0020.00010.00010.00.0

0.0

0.0

0.030.00.390.420.060.110.05

0.05

0.05

0.00.840.610.210.290.040.02

0.02

0.02

0.00.00.0

0.370.650.850.93

0.93

0.93

a Diet composition for large chinook salmon was assumedto be similar to that for large coho salmon.

their forage base is the focus of the followingsections. For simulation results presented be-low, we assumed no functional links betweenthe various predatory salmonid species such ascompetition or other density-dependent inter-actions. Possible predator-prey interactions be-tween salmonids were also omitted. These sim-plifying assumptions may not be valid whenpredation pressure approaches its upper limit,but may be reasonable for these exploratorysimulations intended to determine how closepresent densities are to their upper limit.

Energetics Model

Energetics-based population models for laketrout, coho salmon, and chinook salmon wereused to estimate total annual food consumptionby each species. These models are fully docu-mented elsewhere (Stewart 1980; Stewart et al.,in press); they are similar to other models(Kitchell et al. 1977) and will only be outlined

here. The growth model for an average indi-vidual is a mass-balance equation of processrates as follows:

A£ =C - (R +F +E);AB = growth;

C = consumption;R = metabolism, including standard, ac-

tive, and specific dynamic action;F = egestion;E = excretion, primarily ammonia lost

via the gills.

Consumption was modeled as a function oftemperature and predator size. Metabolism wasconsidered a function of temperature, predatorsize, swimming speed, and, for specific dynamicaction, food assimilated. Waste losses, F and E,were considered functions of temperature, ra-tion, and food type (fish versus invertebrates).Energy content of salmonids was varied as afunction of size. Energy content of the primaryforage species, alewife, was varied seasonally;energy values for other, less important foodswere input as constants. All growth computa-tions were done in terms of joules.

Specific data required from Lake Michiganincluded water temperatures, diet compositionfor various seasons and age-classes of each sal-monid species, annual end points of growth foreach cohort of each species, and mortality rates.It was assumed that a particular species of sal-monid would stay at the warmest available tem-perature up to, but not exceeding, its preferredtemperature as measured in the laboratory.Diet composition expressed as proportion byweight was estimated for lake trout by Stewartet al. (in press) from preliminary data providedby G. Eck (Great Lakes Fishery Laboratory,United States Fish and Wildlife Service, AnnArbor, Michigan), and was closely similar todata for lake trout diet reported by Rybicki andKeller (1978). D. Jude and T. Miller (GreatLakes Research Division, University of Michi-gan, Ann Arbor, Michigan) provided data onthe diet of coho salmon and chinook salmon(Table 2). For each cohort, the year was dividedinto seasons defined by their first and last daysand within each season, diet was partitionedinto as many as four forage types (for example,adult and young-of-the-year alewife, other fish-es, invertebrates).

Growth and mortality rates for lake troutwere based on data from Rybicki and Keller


(1978). Nine years of growth were simulatedwith a starting size of 20 g (stocked as 18-month-old yearlings on 30 June); weight ingrams at the end of each successive year was asfollows: 1 = 260; 2 = 659; 3 = 1,216; 4 =1,828; 5 = 3,044; 6 = 3,842; 7 = 4,281; 8 =4,520; 9 = 4,760. Survival was assumed to be63% for each of the first 3 years in the lake,56% in the fourth year (fishing mortality startsnear the end of that year), and 52.7% for eachof the remaining years of life (Rybicki and Kel-ler 1978).

Growth and survival estimates for the twosalmon species are summarized in Table 3. Thesalmon were divided into growth classes ac-cording to when they return to spawn, with thefaster growing fishes returning relatively earlier(Ricker 1976). For example, chinook salmonjacks return to spawn after about 17 months inthe lake and presumably grew faster than con-specifics that waited until their third or fourthyear to spawn. Partitioning of numbers stockedamong the various growth classes was based onrelative numbers surviving to spawn and back-extrapolation with approximate estimates ofsurvival rates from Rybicki (1973). Full deri-vation of the estimated growth and survivor-ship values is beyond the scope of this paper.Both growth classes of coho salmon were start-ed at 30 g (18-month-old yearlings, stocked 1April), and the three chinook salmon growthclasses were started at 4.54 g on 1 May (Borge-son 1977). A negative exponential mortalitymodel was assumed for both salmon species aswell as for lake trout.

Daily consumption rate is difficult to measurein the field, but growth provides an index ofconsumption integrated over time and is moreeasily measured. Consumption was estimatedfor each cohort by iteratively adjusting con-sumption to a constant proportion of maximumpossible consumption (based on laboratorystudies) until a good fit of the observed annualend point of growth was obtained (Kitchell etal. 1977). With the growth curve fit, a final it-eration was done to sum daily consumption es-timates over the year. The simulation modelused difference equations with a 1-day timestep over a 360-day year, except for the finalcohort of each growth form for the two salmonspecies, each of which was terminated on 30November.

For the sake of simplicity, we have not mod-

TABLE 3.—Total survival for various time periods andweights at various end points for the different growthforms of coho salmon and chinook salmon in Lake Mich-igan; derived by Stewart (1980) from information inRybicki (1973), Borgeson (1977), Ricker (1976), andJude (unpublished data, Great Lakes Research Division,University of Michigan, Ann Arbor).

Simu-lationcohort

Age-class

Time periodMt

Start Endday day p<

unthsin:riod

Sur-vivalfor

period8

Weight(g) at

end ofperiod

2-year coho salmon

11

1222

11

1-2222

1 180181 240

3-year coho salt

1 360361 450451 540541 600

62

-non

12332

0.540.02"

0.340.760.64C

0.06"

1,214

1,043

3.918

2-year chinook salmon

122

0-111

1 360361 510511 570

1252

0.460.720.03"

493

2,644


12333

0-11-2222

1 360361 720721 780781 870871 930

1212232

0.480.480.880.69C

0.07"

3622,149

7,179


1233444

0-11-22

2-3333

1 360361 720721 870871 1,080

1,081 1,1401,141 1,2301,231 1,290

121257232

0.480.480.740.670.890.70=0.14"

2311,552

5,914

10,362

* Proportion of individuals starting a period that live toend of period.

b Reflects natural mortality plus heavy fishing mortalityduring spawning run. Those surviving to end of period diein tributary streams.

c Reflects natural mortality plus light (offshore) fishingmortality.

eled geographic and year-to-year variation indiet composition, growth, or mortality rates.We thus implicitly assume that values used rep-resent lake-wide averages, while recognizingthat further refinements of our estimates ulti-mately will be possible for those who have (orget) the needed data.

History of Salmonid Stocking

Numbers of lake trout, coho salmon, and chi-nook salmon stocked in Lake Michigan havebeen summarized in annual reports and variousother documents of the Great Lakes Fishery

756 STEWART ET AL.

12

5

Q

OO

a:UJQQ

0

A C T U A L IPROJECTEDl

iBROWN TROUTl

RAINBOW TROUTl

[CHINOOK SALMON!

cof jo SALMON;:.

L A K E TROUTl

1965 1970 1985 19901975 1980

Y E A R

FIGURE 1.—Actual and projected numbers of five dominant salmonid species stocked in Lake Michigan, 1965—1990.

Commission. A summary of stocking rates forrainbow and brown trout did not exist for theyears 1965-1974; these data were tabulated byStewart (1980) from numbers provided by thevarious state agencies. Projected stocking ratesfor all species represent a mix of actual plansthat have been approved or funded, informedspeculation, and reserved judgements of fish-ery managers in the four states surrounding thelake. It is now known that naturally spawnedsalmonids are contributing to the stocked pop-ulations, at least on the Michigan side of thelake (Carl 1980; Patriarche 1980), but data arenot available for all species or areas in the lake.We suspect that, on a lake-wide basis, annualnatural reproduction of predatory salmonidsother than lake trout (which are not reproduc-ing) could contribute an additional 10-15% tothe total salmonid population. Predation attrib-utable to naturally spawned salmonids was notconsidered in the analyses that follow, but ourconservative estimates can be adjusted upwardreadily when accurate lake-wide data becomeavailable.

Salmonid stocking since 1965 generally hasincreased strongly (Fig. 1). During the 6-yearperiod 1973-1978, total numbers stocked an-nually stayed close to 12 million, but increasesto near 15 million per year are projected forthe near future. The system thus has been giv-en very little chance to establish an equilibrium.Whether or not the total numbers stocked ac-tually will level off after 1985 depends on anumber of biological, political, and economicvariables. Projections indicate that a reduction

in coho salmon will be balanced or more thanbalanced by an increase in lake trout and chi-nook salmon. Increases also are planned forrainbow and brown trout.

Model Results and Inferences

Alewife Biomass versus SalmonidConsumption

Total consumption of alewife by salmonidswas compared to annual estimates of alewifeavailable to trawls for the period 1968-1980(Hatch et al. 1981). These alewife biomass es-timates (Fig. 2) are not corrected for vulnera-bility or availability to trawls, and do not includeGreen Bay. They are, therefore, minimal esti-mates.

Consumption estimates from energetics sim-ulations of lake trout, coho salmon, and chi-nook salmon were extended to include rainbowand brown trout by increasing total consump-tion in proportion to numbers stocked. Thisassumes that annual consumption by an indi-vidual rainbow or brown trout is equal to theaverage value for individuals of the other threespecies. This may be a reasonable assumptionbecause the life-history characteristics of rain-bow trout and brown trout are variously brack-eted by the three species that were modeled.For the three species simulated, number in eachcohort during each year is based on actualnumbers stocked that year and in previousyears combined with survival schedules for eachspecies.

A notable feature of the alewife biomass isthat it tends to vary with as much as a twofold


150

x 100-

<nzo

t-IUS

COHO SALMON,CHINOOK SALMON8 LAKE TROUT

1967 1969 1971 1973 1975 1977 1979 1981 1983 1985

FIGURE 2.—Biomass (90% confidence limits, CL) of adult alewife available to trawls in Lake Michigan, 1968-1980(Hatch et al. 1981; data for 1979—1980 are from Hatch, personal communication, Great Lakes Fishery Laboratory,Ann Arbor, Michigan), compared to modeling simulation estimates of total alewife consumption by coho salmon, chinooksalmon, and lake trout. "All salmonids" extrapolates consumption to include rainbow and brown trout.

change in apparent abundance from one yearto the next (Fig. 2). In marked contrast, pre-dation may be increasing steadily. With mostsalmonid recruitment coming from hatcheries,the functional feedback link from prey abun-dance to predator abundance is essentially lack-ing. Alewife apparently increased in abundancebetween 1977 and 1978 even though predationremoved a quantity greater than the October1977 estimate of biomass available to trawls. Aconversion factor is needed to translate biomassavailable to trawls to a total biomass estimate.This conversion factor is presently the subjectof intensive research; recent estimates based inpart on acoustic surveys (Brandt 1978) rangefrom 3 to 5 but the final multiplier must awaitconclusion of that study (S. Brandt and J. Mag-nuson, personal communication, Laboratory ofLimnology, University of Wisconsin, Madison).

To actually evaluate the impact of consump-tion, however, it is necessary to know the ratioof production to biomass for alewife. The trawlsurveys mostly were done in October. Based onpreliminary modeling simulations for the ale-wife (Stewart 1980), we estimated that peak ofthe annual population biomass cycle occurs inOctober and the ratio of total annual produc-tion to alewife biomass in October may be justover 1.0. The October biomass estimates of ale-wife may be a good index of alewife productionavailable to salmonids in a given year if the bio-mass estimates are corrected for vulnerabilityand availability to trawls. From the foregoing

it can be inferred that salmonids could be con-suming as much as 20 to 33% of the annualalewife production in some years.

Edsall et al. (1974) used a somewhat similarenergetics modeling simulation to estimate totalconsumption of alewife by the coho salmonstocked in Lake Michigan in 1971. They esti-mated that either 13,600 or 36,000 metric tonsof alewife were eaten, depending on model as-sumptions. Both of their simulations assumedthat most of the forage eaten by coho salmonwas alewife. Their lower estimate is comparableto our estimate for consumption of all foragetypes by coho salmon in 1971, but about 50%higher than our estimate of 8,300 t for alewifeeaten. However, rainbow smelt can be an im-portant component in the diet of young cohosalmon (Jude and Miller, unpublished data,Great Lakes Research Division, University ofMichigan, Ann Arbor). This could account formost of the difference between our estimateand the lower of two estimates made by Edsallet al. (1974).

With the present or a steadily increasing levelof salmonid recruitment that is independent ofrapid alewife population fluctuations, it mayonly be a matter of time before alewife pro-duction cannot fulfill predator forage needs. Asthis happens, other forage fishes, such as rain-bow smelt, bloater, deepwater sculpin, or slimysculpin should contribute a greater proportionof salmonid diets. It is unknown if all salmonidswill switch to other forage, or what reduction

758 STEWART ET AL.

Q _, 700UJ a-

600co

x aco— co"- z

- o 400

300UJ u_

°- o

§2£ 5- co

o Co

ro O O

— 0 O

INVERTEBRATES

RAINBOW SMELT,SLIMY ANDDEEP WATERSCULPINS

ADULTALEWIFE

jLiy.YOUNG-OF-YEARALEWIFE

COHOSALMON

CHINOOKSALMON

LAKETROUT

FIGURE 3.—Estimated total consumption by various salmonid predators in Lake Michigan (per million fish stocked), witha breakdown of diets into their major components.

in alewife abundance must occur before aswitch takes place. Before invasion of the ale-wife, lake trout fed heavily on bloaters and scul-pins. Pacific salmon are likely to switch readilyto rainbow smelt, and perhaps also to bloaters.

Such a shift in forage composition may beunderway at present. A bottom-trawling studynear the thermocline off Grand Haven, Mich-igan indicated that bloaters have increased inrelative abundance from less than 1% of thecatch in 1977 to nearly 60% of the catch in 1980(Crowder and Magnuson, unpublished data,Laboratory of Limnology, University of Wis-consin, Madison). Resurgence of the bloaterlikely is due to the almost total ban on com-mercial harvest of the species that was imposedin 1976 and to the reduction in competitivepressure from alewives. Lake wide trawl surveysindicate increases in bloater stocks, but no ob-vious reductions in alewife biomass have oc-curred (Great Lakes Fishery Laboratory, AnnArbor, Michigan). We do not know if other for-age species could sustain present levels of sal-monid consumption in the event of an alewifecollapse, but recent recovery of the bloater sug-gests that a reasonably rapid increase of pro-duction by at least that forage species may bepossible. Provided the production of otherspecies increases rapidly enough to compensate

for an alewife decline and provided the preda-tors switch readily, a decline of the alewife maybe beneficial to the system as a whole as it couldpermit the development of a more stable sal-monid-forage interaction. Time lags involvedin such shifts may be the critical determinantsof the final scenario.

Salmonid Impact on Forage Species

To compare relative predatory impact ofcoho salmon, chinook salmon, and lake troutupon their different forage organisms, a sim-ulation was run starting with a standard onemillion fish for each species and accumulatingconsumption until all individuals have died.This represents the relative impact of a singlestocking event, not the result of continuousstocking at a fixed level. From the results ofthese simulations (Fig. 3), it appears that totalconsumption per million fish stocked is highestfor chinook salmon; both chinook salmon andlake trout eat noticeably more than coho salm-on. Management decisions involving shiftsfrom coho salmon to either of the other twospecies perhaps should take these differencesinto account. It is not a direct tradeoff.

Predatory impact on young-of-the-year ale-wife is almost equal for all these species. Thus,changing salmonid species composition might


have very little effect on abundance of age-0alewives. In contrast, adult alewives may be di-rectly affected by a shift in salmonid speciescomposition. Consumption of adult alewives bychinook salmon is almost three times greaterthan that by coho salmon (Fig. 3). If alewifestocks are depleted, a stocking shift from chi-nook salmon to coho salmon could be benefi-cial. For forage fishes other than alewife, cohoand chinook salmon currently may be eatingabout the same amount of rainbow smelt, andlarge chinook salmon may also eat sculpins (D.Jester, personal communication, Michigan De-partment of Natural Resources, Lansing), butat present we do not have sufficient data forlarge chinook salmon to make a detailed com-parison. Lake trout eat substantially more ofother fishes than the salmon; these are a mix-ture of rainbow smelt and the two sculpinspecies. Coho salmon larger than 20 g fromsouthern Lake Michigan (Table 2) were eatingfishes almost entirely, but a few smaller individ-uals (naturally spawned?) were eating terrestri-al insects. Peck (1974) found that coho salmonmay eat invertebrates, mostly insects, at leastduring the first couple of months after releasein northern Lake Michigan. Chinook salmonare stocked at a smaller size than coho salmon(4.5 g versus 30 g) and during their first sum-mer in the lake feed largely on terrestrial in-sects, probably taken at the surface near shore.Of the three species studied, lake trout appearto eat the most invertebrates and may be theonly one that preys on Mysis relicta and Ponto-poreia hoyi to any extent. To get a better appre-ciation of what these dietary differences meanin a management context, it is useful to exam-ine the distribution of consumption over time.

Relative Predation Inertia of SalmonidsIt is important to recall that one of the major

criteria used in choosing the salmon species forstocking Lake Michigan was their potential rolein controlling the alewife "nuisance." When es-timates of alewife consumption by salmonidsfrom simulations discussed in the previous sec-tion are plotted over time (Fig. 4), some strikingdifferences among the salmonid species emergethat have important management implications.For the purpose of this discussion, we use theconcept of predation inertia which is defined asthe time from stocking until most of the pre-dation impact has occurred. Excessive preda-

300-

0 2 4 6 8 1 0(JAN.)

YEARS (+2 MO.) FROM HATCHERY EGG COLLECTION

FIGURE 4.—Comparison of estimated predation on LakeMichigan alewife by three salmonid fishes (per million

fish stocked). Origin of the time axis corresponds to wheneggs are normally collected, for the hatchery; "S" indicateswhen smolts are normally stocked into Lake Michigan."YOY" is young oj year.

tion inertia can preclude the possibility forshort-term management manipulations in re-sponse to forage-fish fluctuations.

Coho salmon have the least inertia in thatthey only spend about 18 months in the lake.Their overall impact is about half that of chi-nook salmon and lake trout (Fig. 4). In thissense, they have the greatest potential for short-term management manipulations of stockinglevels, but the need to rear them in the hatcheryfor almost 18 months detracts from their man-agement utility. Time from making the decisionto collect eggs for hatchery rearing until peakpredatory impact in the lake is close to 3 years,the same as for chinook salmon. Coho salmon isthe only species for which stock reductions areplanned (Fig. 1).

Chinook salmon may be favored over cohosalmon in part because they smolt after only 6months and hence can be produced at a costper individual that is less than one-third thecost of coho salmon ($0.02-0.03 versus $0.09-0.10 per fish, Borgeson 1977). Chinook salmonhave more inertia than coho salmon in that they

760 STEWART ET AL.

spend 3 to 4 years in the lake. Chinook salmonalso grow to a larger size, and consume consid-erably more food. Time from egg collection topeak impact, 3 years, is comparable to that forcoho salmon and much shorter than for laketrout. This makes chinook salmon ideal for anyattempt to curtail an alewife population in-crease. Reduction in chinook salmon stockingmight be seriously considered in the event ofan alewife collapse and if other forage fishesfailed to show compensatory increases. As chi-nook salmon constitute less than a third of thetotal predator assemblage, it may be necessaryto reduce other salmonid species as well to ef-fect a forage-fish recovery.

Lake trout are the slow component in the sys-tem, with 7 to 8 years of inertia (Fig. 4). Timefrom egg collection to peak impact is 5 to 6years, and it takes 9 years to establish a fullcomplement of cohorts in the lake. This specieshas relatively little potential for responsivemanagement manipulations, other than thoseeffected through fisheries. Lake trout havelower consumption rates (compared with maxi-mum rates) than other salmonids, however, andthus may be more responsive to prey dynamics.They may, then, help provide a stabilizing in-fluence on forage-fish abundance with almostinstantaneous response time compared to the 3-year lag needed to manipulate salmon numbers.

Rainbow and brown trout live from 5 to 7years and, if included in Fig. 4, would be po-sitioned between chinook salmon and lake troutbased on relative inertia. Further evaluation ofpossibilities for these two species will have toawait development of population models.

Implications for Management

Preliminary population simulations for thealewife suggest that a given year class will com-plete 89% of its lifetime consumption within 24months of hatching (Stewart 1980). This is be-cause young alewife have relatively high con-sumption capacity and growth rates in combi-nation with relatively high mortality rates. If astrong alewife year class is recognized in thefall, additional salmonid eggs taken at that timewill yield salmon that will prey on that year classof alewives when the latter are age 2 + and mayhave spawned for the first time. The implicationis that short-term, year-to-year variations in ale-wife abundance cannot be moderated bychanging salmonid stocking levels. Such manip-

ulations might be reasonable if year-classstrength of forage fish could be predicted 3years before that cohort reaches sexual matu-rity.

Salmonid predators may be seen as a depen-satory mortality agent (Walters2 et al. 1980);they consume alewives (and other prey) at ratesmore proportional to their own than to alewifedensities. Thus, as weak year classes of alewivesappear, they are and will be subject to veryintense predation pressure. The resultant re-duction in reproductive contribution by thosecohorts, coupled with stochastic and density-independent increases in mortality (such asweather-related phenomena), could amplifypopulation cycles and result in a catastrophicdecline of alewife stocks similar to those ob-served in other clupeoid fishes such as the Pe-ruvian anchoveta Engraulis ringens, Atlanticmenhaden, various herring stocks, and the Pa-cific sardine (Murphy 1977). Due to the pre-dation inertia of salmonid stocks, a rapid switchto other forage species could similarly depressthe other forage populations before any man-agement action based on stocking rates couldbe effective. If this scenario were quickly per-ceived, one effective management responsewould be to encourage rapid exploitation of sal-monids. Another response, perhaps more ben-eficial in the long term, would be to view theentire Lake Michigan salmonid stocking pro-gram as the massive experiment that it is andto carefully monitor the results of this experi-ment as a guide to future management. Thiscontrasts with the foregoing alternative in thatno attempt would be made to manipulate num-bers after stocking. Similar management-relat-ed experiments have been advocated for GreatLakes sea lamprey (Walters, et al. 1980.)

Given existing technology, management ma-nipulations should be done as experiments tolearn more about functional interactions amongspecies, or about predator responses to appar-ent long-term (3-year or more) trends in abun-dance of a forage species. It might be antici-pated that a system dominated by salmon andalewife will fluctuate more than one based onlake trout and a diversified forage complex(Smith 1968). This is not necessarily bad. A co-ordinated management plan is necessary fordealing with the possible depression of alewifestocks. For example, if monitoring of foragestocks reveals that population cycles in the var-


ious Great Lakes are asynchronous, then shift-ing of salmonid stocking emphasis from lake tolake to match forage cycles may be an appro-priate management option.

Our central thesis of predator-inducedchanges in Lake Michigan's forage fish com-munity yields three hypotheses:

(1) Salmonid predation should reduce ale-wife dominance, increasing diversity amongzooplanktivorous fishes and their zooplanktonprey. Species that are rare or absent duringperiods of alewife dominance should increasewhen alewives decline.

(2) As alewife populations decrease due toincreased predation by salmonids, growth ratesand condition factor indices for alewives andtheir competitors should increase (at least ini-tially); both should decrease for salmonids ifother forage fish are slow (or unable) to com-pensate for alewife declines.

(3) Changes in competitive interactionsshould result in habitat shifts and increased di-versity of diet for various species. The lattershould be first apparent in salmonids.

Until feeding habits and preferences are bet-ter defined for Lake Michigan fishes, we cannotmake quantitative predictions. Lake Michigan'sfisheries have undergone dramatic changesduring this century—perhaps the most dra-matic of any among the Great Lakes. It is ap-parent that another change is currently under-way. If carefully monitored, the results couldyield important insights to Great Lakes re-source managers and to the scientific commu-nity.

AcknowledgmentsWe thank John Magnuson for his support

and constructive comments in the developmentof this project. We also wish to thank severalother colleagues for their valuable review com-ments. This research was funded by the Na-tional Oceanic and Atmospheric Administra-tion, Office of Sea Grant, Department ofCommerce, through an institutional grant tothe University of Wisconsin. The University ofWisconsin Graduate School provided supple-mental computer funds.

Numerous people generously provided dataor information without which this synthesiswould not have been possible. These includedD. Rottiers, T. Edsall, G. Eck, E. Brown, and R.Hatch (Great Lakes Fishery Laboratory, United

States Fish and Wildlife Service, Ann Arbor);C. Fetterolf (Great Lakes Fishery Commission,Ann Arbor); D. Jude and T. Miller (GreatLakes Research Division, University of Michi-gan, Ann Arbor); D. Jester, R. Rybicki, and M.Keller (Michigan Department of Natural Re-sources); R. Koch (Indiana Department of Nat-ural Resources); B. Meunch (Illinois Depart-ment of Natural Resources); F. Binkowski andS. Yeo (University of Wisconsin, Milwaukee); S.Brandt, J. Magnuson, R. Horrall, and A. Scholz(University of Wisconsin, Madison). R. Poff andJ. Pfender (Wisconsin Department of NaturalResources) provided data from studies sup-ported by the Anadromous Fish ConservationAct as part of Wisconsin Project AFS-9.

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Forage Fishes and Their Salmonid Predators in Lake Michigan