Predador-Prey Interactions Fishes

7/27/2019 Predador-Prey Interactions Fishes

1/13

Transactions of the American Fisheries Society, 1982, vol. 111, iss. 3, pp. 255-266.

http://afs.allenpress.com

http://afs.allenpress.com/archive/1548-8659/111/3/pdf/i1548-8659-111-3-255.pdf

Online ISSN: 1548-8659

Print ISSN: 0002-8487DOI: 10.1577/1548-8659(1982)1112.0.CO;2

American Fisheries Society


2/13

TRANSACTIONS of theAMERICANFISHERIES SOCIETY

May 1982VOLUME 111

NUMBER 3

Transactionsf theAmericanisheriesociety11:255-266, 1982 Copyright y the American isheries ociety 982

Predator-Prey Interaction betweenLargemouthBassand Bluegills as Influenced by Simulated,SubmersedVegetationJACQUELINE. SAVINO

OhioCooperativeishery esearch nitRoY A. STEIN

Department f ZooloTheOhioStateUniversity, olumbus,hio43210Abstract

Data rom he iterature uggesthatpredatory uccesseclinesshabitat omplexityncreases.To explain his phenomenon, e studied he predator-prey nteraction etween argemouthbassMicropterusalmoidesnd bluegills epomisacrochirusn four laboratory ools 2.4-3.0mdiameter,0.7 m deep),eachwith a different density 0, 50, 250, 1,000 stems/m) of artificialplantstems. ehaviorwas uantifiedor bothpredator ndpreyduring argemouthasseedingbouts asting 0 minutes. redation uccessnumberof captures) y argemouth asswassimilarat 0 and 50 stems/m, thendeclinedo near zeroat 250 and 1,000stems/m. As stemdensityincreased, redatoractivitydeclined ue to a decreasen behaviors ssociatedithvisual ontactwith prey.Reduced redation uccessy largemouth assn habitats f increasedomplexityapparentlys related o increasesn visualbarriersprovided y plantstems swellas o adaptivechangesn bluegillbehavior.

Littoral zones,with their associated egeta-tion, are importantareasof fishproductionnnorth temperate quaticcommunities.n par-ticular, theseareasprovide habitat for manyfishes,ncludingmembers f the sunfishamily,Centrarchidae (DiCostanzo 1957; Hall andWerner 1977; Werner et al. 1977; Keast 1977,1978; Laughlinand Werner 1980).Juvenilesunfishes ight ive n vegetationorat least wo reasons: vailabilityof forage oravoidance f predators.Basedon relativehab-itat profitabilitiesmeasuredn a natural akeandoptimal-foragingconsiderations,Mittelbach(1981) predicted hat all sizesof bluegillsLe-

The Unit is ointlysponsoredy the UnitedStatesFish and Wildlife Service,Ohio Departmentof Nat-ural Resources,nd The Ohio StateUniversity.

pomismacrochirushouldmove out of the vege-tated littoral zone nto open waterand feed onDaphniaduring much of the summer.Becausebluegills maller han 100 mm total length donot move offshore as predicted, Mittelbach(1981) suggestedhat fish of this size emain inthe vegetation o avoid predation by large-mouth bassMicropterusalmoides.his is a rea-sonable ypothesisecause varietyof studiessuggesthat prey vulnerability ecreasessen-vironmental complexity ncreases Huffaker1958; Glass1971; Steinand Magnuson1976;Saiki and Tash 1979). However, no one hasdemonstrated xplicitly hat vegetationeducesvulnerabilityof bluegills.To test this assump-tion, we completed series f experimentshatquantified he relationshipbetweenpredationmortalityand vegetation ensity,using arge-mouthbass s predators nd bluegills s prey.

255


3/13

256 SAVINO AND STEIN

If vulnerabilityof bluegillsdeclineswith in-creasing tructural omplexity,election f thishabitatby bluegills, ven in the face of poorforaging eturn, couldbe explainedon the ba-sisof highersurvival n vegetationhan n openwater.A variety of mechanismsmight be involvedin reducingprey vulnerabilityn complexhab-itats. For example, vulnerability could be re-duced n vegetation implybecauseandomvi-sualencounters etweenpredatorand prey arereduced (Cooperand Crowder 1979). Alter-natively, ehavioralesponsesf theprey o thepredatorcouldalter the probability f prey de-tection and capture. And certainly, predatorbehaviorcould be modifiedby vegetation. odistinguish mong hesecompeting ypothesesin our experiments,we quantified he behav-ioral interactions etween argemouthbassandbluegillswith increasing tem density. f therandom modelexplainsour results, hen thesedata could be generalized o many predator-prey interactionsn the vegetation.However,if prey vulnerabilitys strongly nfluencedbybehavioral nteractions, hen our resultsmay bespecifico thisparticularpredator-prey ystem.

MethodsWe conducted1-hour observational xperi-ments in a shaded outdoor area during twosummers.Four circular,plasticpools 3 m di-ameter,0.7 m deep)with dark insidewallswereused he first year, whereas ircular,steelpools(2.4 m diameter,0.7 m deep)with white nsidewalls,which acilitated iewing f fish,wereusedduring the secondyear. Water temperaturesranged rom 16 to 24 C during the study,butfluctuated nly2 C duringany 24-hourperiod.

Between experiments, water was circulatedthrough a sand-flossilter to improve waterclarity and maintaindissolved xygenconcen-trationsat about7 mg/liter.Algal bloomswerecontrolledby an algicide containingmonuron,simazine, nd atrazine)applied wiceper sum-mer.

Lengthsof yellowpolypropyleneope (4 mmdiameter,0.5 m long) simulatednatural plantstems.Periphyton covered these rope strandsafter a few weeks,causing hem to resembleaquaticmacrophytes.opeendswere astened,in a uniform distribution, to wire mesh that wascoveredwith sandon the poolbottom; ree endsfloated to the water surface. Attached strands

remained flexible, allowing largemouth bassfreedomf movementhroughhepool.Eachpoolcontained different density f these opestrands 0, 50, 250, 1,000 stems/m2), ereafterreferredoas erolow,medium,ndhigh temdensity. nter-stemdistances ere about 14, 6,and 3 cm for low, medium,and high stemden-sity, respectively. hese densitieswere chosento reflect the range of macrophyte densitiesfound in natural communities (Ozimek et al.1976; Sheldonand Boylen 1977; CrowderandCooper 1979).To permit observerso estimatedistancesn the pools,we laid colored tones .3m apart on the sandbottom.Experimentswererun only in the morning to eliminate ime-of-day effects.

Largemouth bassused in our experimentswere collected rom RossLake, RossCounty,Ohio, arid ranged rom 33 to 37 cm total ength.In all, five largemouthbasswere used,one in-dividualper experiment.Bluegills sed angedfrom 35 to 44 mm total length and were ob-tained from localpondsand Hebron NationalFish Hatchery, Ohio. Bluegillsof this sizecom-monly are found within the littoral vegetationof natural lakes (Hall and Werner 1977; Mit-telbach 1981), and thus should be most affectedby changesn stemdensity.Largemouthbassand bluegillbehaviorcate-gorieswere determined rom preliminaryob-servations.Largemouth bassbehaviorswereseparated into six mutually exclusive cate-gories:Search:moving,but not orienting o the prey.Follow: moving, and orienting to particular

prey.Pursue: ollowingat burstspeed.Attack: strikingat prey.Capture: ngesting rey.Inactive: restingand motionless.Bluegillbehaviorand position n experimentalpoolswascategorized s follows:BehaviorSchooled:ndividualsaggregated nd movingabout as a unit.Dispersed: ndividualsnot associating tronglywith one another.PositionTop edge: upper 0.25 m of the water columnand within 0.3 m of the pool side.


4/13

PREDATOR--PREYNTERACTIONSALTEREDBY COVER 257

Bottomedge: ower0.25 m of thewatercolumnand within 0.3 m of the pool side.Top center:upper 0.25 m of the watercolumnand beyond0.3 m from the pool side.Bottom center: lower 0.25 m of the water col-umn and beyond0.3 m from the pool side.Experiments enerally onsistedf observingfeedingbehaviorof largemouthbass nd anti-predatorbehaviorof bluegillsn the four stemdensities.n eachexperiment,one largemouthbasswas combinedwith 35 prey. Before anyexperiment, argemouthbasswere acclimatedto the experimental ool for 2 to 7 days. ndi-vidualswere considered cclimatedwhen theyfed regularly n the presenceof an observer.

Largemouthbasswere starved or 24 hoursbe-fore eachexperiment. mmediatelybefore est-ing, bluegillspreviously ntested)were addedand isolated rom the predatorby a smallwire-meshcage 1 m diameter) or 5 minutes.Whenthe cagewas emoved,experiments eganandcontinued for 60 minutes. At the end of 60minutes, argemouthbasseither were satiatedor had quit trying to captureprey. Observationswere made rom 2-m-high addersplacednextto the pools.The first year, only predatorbe-havior was recorded; he next year, a secondobserver ocumented ntipredator ehaviorofbluegills. argemouth ass ehaviors erecod-ed directly nto a Datamyte900 (Electro/Gen-eral Corporation,Minnetonka, Minnesota).Anentry wasmadeeach ime the largemouth assexhibited changen behavior. he Datamyterecords ime of eachentry, therebyprovidinga record of time spent in each behaviorandnumber of occurrencesduring each experi-ment. Bluegillbehaviorwas ecordedon codeddata sheets t 5-minute ntervals,providing n-stantaneous observations of their behaviortwelve imesduring eachexperiment.At eachobservation,we recorded the percentageofbluegills either schooledor dispersedand ineach of the four locations. At the same time,distancesetweenhe largemouth ass nd thebluegill closest o it were recorded. Behavior,location, nd initial distance rom the predatorwere ecorded or bluegills ttacked uring ex-periments.From thesedata, we comparedbe-havior of bluegills ttackedwith thosenot at-tacked.After an experiment, argemouth asswere permittedcontinuedaccesso the remain-ing bluegillsor 24 hours;after 24 hours,blue-

gillswere emoved nd counted.Modificationsin bluegillbehavioracross tem densitycouldbe influenced either by the presenceof thelargemouth ass redatoror stemdensitytself.To distinguish etween hese wo effects,we alsoquantified he behaviorof bluegillswithout apredator at all stemdensities.Analysis f largemouthbass nd bluegillbe-haviorsdiffered because f the different typesof data collected for each fish: that is, contin-uous versus instantaneous observations. Weanalyzed redatordata on the basis f numberof occurrencesnd time spent n eachactivityand bluegilldata on the basis f the percentofindividualsarticipatingn each ehavior t eachobservation.Walshaverageswere used o cal-culate medians and 95% confidence intervals forbehaviors nd positionsHollander and Wolfe1973).To provide for replicationwe testedat leasttwo different argemouth ass total of seventimesat eachstemdensity.For bluegills, achtreatmentwasreplicatedat least ive timeswitha predator and two timeswithout a predator.About60% of the replicatesor the argemouthbasswere run in the first year; all replicates fthe low-density reatment were run in the sec-ond year. Results btained n the 2 yearswerenot significantly ifferent (P > 0.05) nor werethe three major behaviors mong ndividuallargemouthbass two-wayayout):

Behavior Years (P) Individuals (P)Captures number) 0.11 0.26Activity (minutes) 0.28 0.82Visual contact (minutes) 0.15 0.38Becausesome fish were used n experimentsmore often than others, we tested each individ-ual againsthe others combined sa group) odetermine f any one of them might bias ourresults. n thesecomparisons, o individualstestedweredifferent rom the group P > 0.05,two-way ayout). Furthermore,we added datafrom preliminaryexperimentsin whichwe re-cordedbehaviorsn a slightly ifferent ashion)to the group of final experimentsand againcompared ehaviorof single ish with that ofthe group.Asbefore, he behavior f single ishdid not differ from that of the group P > 0.05,two-wayayout).Thus, data rom bothyears orindividual largemouth basswere lumped forpresentation nd analysis.To relate our findings o a more natural sit-


5/13


z

C)Tmm

LLI

LLo

m

z

6O

4O20

06O

4O

200

200

4O2O

0

2O

L MEDIAN,95% ONFIDENCENTERVAL(8) (7)

ATTACK'0 50 250 IOO0

CAPTURES AFTER 60 MINUTES: 0 50 250 IOO0

o40

20o

40 - (4)20

oI

INACTIVE' 0 50 I000 250

(8) (5)

CAPTURES AFTER 24 HOURSo 50 25O iooo

I0 50 250 I000

6O

- 20z-- 0L 60LLd o

m_ 20coP- 0_z 2oFF 0__0 0I 20-r 0

2O

0

2O

0

MEDIAN,95% ONFIDENCENTERVAL(7) (7)

(8) )EARCH= O 50 250 IOO0FOLLOW. 0 50 20 I000

PURSUE 0 5O 250 iooo

AFFACK. 500 250 Iooo

T I , ,,[ , , ,[ , , ICAPTURE. 0 50 250 IOO0

ACTIVE. 0 50 250 I OO0

I I0 50 250 I000

STEM DENSITY NUMBER ER M+ 1)FIGUr.(A) Numberf imesach fseveralargemouthassehaviorsccurredithncreasingtem ensityuring

an experiment.B) Time pentn each ehaviory argemouthass ithncreasingtem ensityuringn experiment.Visual ontactas he um f timehe redatorpent otionlesshile bservingluegills,ndollowing,ursuing,attacking,nd apturingluegills.ediansot ignificantlyifferentrom achtherreunderlinedKruskal-Wallistest, > 0.05).Sampleizesinparentheses)or eachreatmentor all behaviorsregivenn the op anel, xceptwherendicatedifferently.

uation,we comparedhe amountof coverpro-videdby polypropyleneine with hatprovidedby a natural aquaticmacrophytePotamogetonnatans, y quantifying he percentcover pro-videdby each t the same temdensity. ercentcover was defined as the percentvertical areaof the water column occupiedby vegetation(artificialor natural). If refuge for prey is re-

lated to the amount of visual isolation betweenpredhtor nd prey,percent over hould ro-vide somemeasureof protectionavailable oprey at any given stem density.To measurepercent over,we photographedrom the sidea 0.5-m strip (extending12.5cm nto an aquar-ium) of eachstem ype (artificialor natural)atthe three experimental ensities. hotographs


6/13

PREDATOR--PREY INTERACTIONS ALTERED BY COVER 259

a

100

8o

6o

4o

2o

oIoo

8o

6o

4o

I

ATTACKS PER FOLLOW:0 50 250 I000

I i i

(7) MEDIAN,95% CONFIDENCEINTERVAL

I

CAPTURESEr ATTACK:T-o 250 0 II I I i II I I III I0 50 250 Iooo

STEM DENSITY (NUMBERPER M2 + 1)FIGURE.-Percentfattacksnbluegillser ollowandpercentfcaptureserattackn an experimentith argemouthbaas nder onditionsf ncreasingtem ensity.umber f experimentser reatments ndicatednparentheses.ediansnotsignificantlyifferentKruskaI-Wallisest,P > O.05) rom each ther re underlined.o attacks ereobseroedthighstem ensity.then were digitized Hewlett-PackardDigitizer,Model 9874) and percent coverwascalculatedfor each stemdensity.

ResultsChanges n stem densitymodified the pred-atory tacticsof largemouthbass.Predator be-havior,asmeasuredby numberof occurrencesor time spent,did not change rom zero to lowdensity, or did it change rom medium o high(Fig. 1). However,nearly all predatorybehav-iors declined significantlyas stem density in-creased from zero to medium or from low to

high, suggestinghat thesebehavioralpatternswere modified between densities of 50 and 250stems/m 2.

Participationby largemouthbass n preda-tory behaviors, s measured y numberof oc-

currences of the behaviors, declined as stemdensity ncreasedFig. 1A). As thesebehaviorsdecreased, o did the number of bluegills ap-tured by largemouthbass,during both 1-hourand 24-hour eedingbouts.Consistent ith theresults f Glass 1971), ncreasing temdensityreduced the predatory ability of largemouthbass.

Time spent n eachpredatorbehavior id notreflect the number of its occurrences. Searchtime remained constant whereas follow timedecreased s stemdensity ncreasedFig. lB).When active, argemouthbassspentmost oftheir time searching nd following.Pursuing,attacking, and capturing contributed little tototal eeding imes t zeroand owstemdensity.As these behaviorsdeclined and approachedzero at mediumand high stemdensity, ignif-


7/13


- 100oo 80

_ 0

_ 40o z0zu 0

MEDIAN,5% ONFIDENCENTERVAL

I I0 50 250 I000STEM DENSITY NUMBER ER M2 + 1)

FCURE .--Percent choolingf bluegills ithand without largemouthass resent,n relation o stem ensity. ig-nificant ifferencesKruskal-Wallisest,P < 0.05) betweenhepercentagesf schoolingithandwithoutargemouthbass ithin treatmentredesignatedyan asterisk*).

icant differences occurred between zero-low andmedium-high stem densities. Becauseswim-ming speedand metabolic ate are exponen-tially related Glass1971), the contribution fpursuit,attack,and capture behaviorshat in-volve astswimming)o costs f predationmaybe substantial,even though these behaviorsmake up a small portion of the time budget.Time spent active by largemouth bassde-creasedwith increasing temdensities; reda-tots did not appear o compensateor reducedcapturesby increased earching r following.Largemouthbasscould not capturebluegillsat high stemdensitybecauseheycouldnot findor follow hem hrough he artificial egetation.Visual contactbetweenpredatorand prey de-clined precipitouslywith increasing temden-sity(Fig. lB). Both the total numberof follows(Fig. 1A) and the percentof follows eading oan attack Fig. 2) decreasedwith increased temdbnsity, herebydecreasinghe possible um-ber of attacks.However, the percent of cap-tures resulting rom an attack did not changewith stemdensity Fig. 2), a resultsupported ythe work of Glass 1971).Thus, the abilityof alargemouth bass o follow and attack was in-strumentaln determiningcapturesuccess.nce

begun,attacksed to captures bout70% of thetime in a total of 476 attacks.Even given thisresult,more attackswere requiredper captureat low than at zerodensity.Numberof captureswere similar between these two densities; how-ever,energy equiredper capturewasprobablyhigher at low than at zero stemdensity.Bluegillbehaviorwasmodifiedby both stemdensityand the presence f largemouthbass.Generally, bluegill behavior was lessvariablewhen largemouth basswere present. At eachstemdensityexcept he highest, he percentoffish schoolingwas similar, whether a predatorwaspresentor not (Table 1; Fig. 3). At highdensity, predator presencesignificantly e-duced he percentof bluegills chooling. f thebluegillsattackedby largemouthbassat zero,low, and medium stem densities, few wereschoolingmean = 6.0%; Clopper-Pearson5%confidence interval = +_2.7%; N = 302; Hol-lander and Wolfe 1973). The ratio of attackson schooled o dispersedbluegillswas similaracross these three treatments (binomial test:P > 0.14; N = 302; Hollander and Wolfe 1973).Thus, because so few schools were attacked,schooling asadvantageoust thesedensities.Yet, at high stemdensity,no bluegillswere at-


8/13

PREDATOR--PREYNTERACTIONS ALTERED BY COVER 261

T BI 1 --Null probabilitiesof changesn behaviornddistributionf bluegillscrosstem ensitiesithandwithoutco-occurringargemouthass. lusandminus ignsn parenthesesndicate hether behaviorfbluegill as nhanced(+) or depressed-) asstem ensityncreased.robabilitiesre rom distribution-freeultiple omparisonsased nKruskal-WallisanksumsHollander nd Wolfe1973). Within-treatmentomparisonsanbe ound on the iguresreferenced.Plantdensitiesstems er m2)

0 0 0 50 50 250Bluegillbehavior Figure versus versus versus versus versus versusor distribution reference 50 250 1,000 250 1,000 1,000

Schooling%) 3With bass __b __ *** (_)Without bass -- -- --Top edge %) 4With bass *** (-) *** (+) 0.06 (-)Without bass 0.22 (+) -- --Bottom edge %) 4With bass *** (+) *** (+) 0.10 (-)

Without bass -- -- 0.36 (+)Top center %) 4With bass -- -- *** (+)Without bass -- -- --Bottom center (%) 4With bass -- ** (+) *** (+)Without bass -- -- 0.22 (-)Distance o bass m) 5Closestschooled * (+) 0.39 (+) ** (+)School attacked * (+) -- Closest ispersed -- -- 0.07 (-)Dispersed ttacked 0.49 (+) 0.39 (+) c

*** (-) *** (-)

-- 0.34 (+) ** (+)0.15 (-) ** (-) --*** (-) *** (-)

*(+) --*** (+) *** (+)

*** (+) *** (+) *** (+)

-- 0.48 (+)c c

*** (-) *** (-)e e

ap 0.5.No attacks n bluegills y largemouth ass ccurred t high stemdensity.

tacked and only schoolingbluegillswere fol-lowed.Apparently,bluegills educed heir sus-ceptibility o the predator by dispersing mongthe stemsat high densityand schooling t lowdensities.Bluegilldistribution asalsomodifiedby stem

densityand predators. n the absence f pred-ators,bluegillsmoved throughout he pool, al-though heyshowed omeaffinity or edges Fig.4). When a largemouthbasswaspresent,blue-gills stayednear pool edges especially ottomedges),except at high densitywhen they dis-persed hroughout he pool. Largemouthbassresponded to this distribution by exclusivelyattacking ndividualsat the edge in the threelowest temdensities;nly 1%of all attacksN =302) were on bluegills n the center.Center at-tacksexcluded, argemouthbassdid not differ-entially ttackbluegills t the top or bottomedge(binomial est:P > 0.12; N = 129)exceptat lowdensity, n which largemouthbassshoweda

slight preference or attackingbluegillsat thetop edge binomial est:P = 0.05; mean= 57%;N = 169).Given hat so ew bluegillswere oundat the top edge (Fig. 4) and about 50% of thepredator attacksoccurred here, the probabilitythat an individual at the surface would be eatenwashigh. Thus, bluegills educed heir vulner-ability o a predatorby congregating ear bot-tom edgesn all pools.Typically, luegills tayedin areas of discontinuity,either air-water orsediment-waternterfaces,at the pool edge. Inthis way,bluegillswere protected rom two at-tack directions:dorsally and laterally or ven-trally and aterally.Bluegillsmovedonly o cen-ter pool when stem densitywas high enough(1,000 stems/m) to provide protection rompredation.

Occurrence of an attack depended on dis-tancesbetween argemouthbassand bluegills.Largemouthbassgenerallyattackedbluegillswithin about0.5 m from the predator;bluegills


9/13


ILl 80

if' 4oo

m o

4o

.J o

TOP DGE + MEDIAN,95% CONFIDENCEINTERVAL

,,- WITHBASS I>58 OBSERVATIONS).WiH::TsAs...S'''"... ;,..N=24 OBSERVATIONS)TOPENTERI0 50 250 I000 BOTTOM CENTERt I0 50 250 I000STEM DENSITY (NUMBER PER M2+1)

FOUR[, .-Distribution f blueglls ithina poolwith ncreasingtem ensities, ithand without largemouthasspresent.ggncant ifferencesKtuakaI-Wallisest,P 0.05) betweenluegrillistributionsithandwithout predatarin eachocation ithina treatmentre designatedyan asterisk*).

were mostcommonly arther than 0.5 m fromthe predator Fig. 5). Generally, chooled lue-gills remained arther from largemouthbassthan dispersed nes not attacked; ig. 5). Asstem density ncreased, chools tayed artherfrom argemouthass,whereasndividuals erecloser.Largemouthbassattacked rom aboutthe samedistance egardless f stemdensityordispersion atternof bluegillsFig. 5).Our comparison f artificialwith natural cov-er showed hat Potamogetonatansprovidedmore cover han artificialvegetation t similardensities Fig. 6). From this relationship,wewouldpredict hat naturaldensities fP. natansof 130 stems/m (about 250 artificial stems/m)or greaterwouldmodifypredatory ehavior flargemouth ass nd antipredator ehavior fbluegills.

DiscussionRecent work suggests hat intermediatestructural omplexitywithin a habitatproducesoptimum conditions or predator growth, be-cause t ensuresa long-term supplyof prey(Glass 971;Cooperand Crowder1979).Thispredictiondependson a positive elationship

betweenstructuralcomplexity nd prey num-bers (DiConstanzo1957; Saiki and Tash 1979)and an inverse elationshipbetweenpredationrate and structural complexity--assumptionssupported y our results nd by thoseof otherinvestigators Huffaker 1958; Glass 1971;Crowder and Cooper 1979; Saiki and Tash1979). However, rather than a linear relation-ship, we found a break in capture rate occur-ring at a moderately igh structural omplexity(greater han 15% cover),beyondwhich arge-mouth basspredatorsof greater han 300 mmwere severelyimited. This limitationwaspro-ducedby a combination f factors,ncluding heantipredator esponse f prey and a reductionin visualcontact f prey by predators aused yan increasing umberof barriers.Largemouthbass lsochanged heir tacticswith changesnstructural omplexity; t low densitiesheywereactive earchers hereas t high densities,heybecame "sit and wait" or ambush predators.These behavioralshifts may operate to mini-mize energycosts equired for prey capture.Vulnerable prey often seek cover to avoidpredation (Stein and Magnuson1976; Stein1977; Werner et al. 1977; Saikiand Tash 1979).


10/13


0.5

01.0

0.50

MEDIAN,5% ONFIDENCENTERVAL (28)() ICLOSEST SCHOOL /

SAMPLE IZE .- ...

SCHOOL TTACKED

-(4) CLOSESTNDIVIDUALc, (49)

' -- NDIVIDUALTTACKEDI I0 50 250 I000

STEM DENSITY (NUMBERPER M2 + 1)FIGU.E .--Distancesetweenargemouthass ndbluegillsboth choolednddispersed)ith ncrearingtem ensity.Bluegillshatwere ttackedrecomparedith hosehatwere losesto he redatorutnotattackedigncant ifferences(Kruakal-Wallisest,P < 0.05) betweenluegillsttackedndnotattackedithin treatmentredesignatedyanasterisk*). Theclosestchooledluegillnotattacked)as ignificantlyartherrom he argemouthassP < 0.05)than heclosestispersedluegillnot ttacked)n all treatmentsxceptor 0 stems/m.Sampleizesndicatehenumberof times ach ehavior asobserved.Immobilizationor "freezing" (Smythe 1970;Curio 1976) combineswith cryptic coloration(Endler 1980) to permit prey to blend in withtheir background and avoid detection. Al-though percent cover increased inearly withstem density, t did not afford protection ordispersed luegillsn our experiments ntil highstem densitieswere reached (about 40% cover).At high coverdensities, luegillsmay well bedifficult to detectowing to their barred colorpatterns.Thesecolorpatterns ombinewith theantipredator acticof becoming ompletelymo-tionlessn the presence f a predator o reducesubstantiallyhe vulnerabilityof bluegills,evenwithin striking angeof largemouthbass.At stem densities less than 250 stems/m2,bluegillsexhibiteda different behavioral ep-

ertoire to reduce their susceptibilitychoolingcan reduce he probability f captureof a givenindividual, for as the number of individuals ina group ncreases, redationsuccessnd prob-ability of encounter with a predator decline(Brock and Riffenburgh 1960; Hamilton 1971;Neill and Cullen 1974; Taylor 1976; Major1978; Shaw 1978). Furthermore, imitation ofneighbors in defense movements enablesschooling rey to respond o an attack morerapidly than solitaryprey (Radakov1973). Asgroup size increases, t becomes ncreasinglydifficult for a predator to singleout and attackan individual Neill and Cullen 1974; Seghers1974;Major 1978). n fact, ew schooled lue-gills n our studywere attacked,perhapsowingto the increasen difficultyof capturing chooled


11/13


n- 60O(D 40z(D 2_0EL 0

(4)+MEDiAN,AN6EY=4.93 + O.06Xr2=0.97. P. natans / (4)

SZE (3) t / .. -. -- -'" 1/(6,) .... -- -- -'

-'" (T Y=2.680.04r 2 = 0.960 200 400 600 800 I000

STEM DENSITY (NUMBER ERM2 +1)FIGURE .--Percent over rovided ydifferent ensitiesf Potamogeton atansandsingle trands f polypropyleneline (4 mmdiameter, .5 m long).over dispersed rey or to the abilityof schooledprey to respondand stay farther from large-mouth bass than dispersed prey. Increasedstructural complexityalso increased he dis-tanceschools ouldmaintain rom largemouthbass.However, bluegills eventuallyswitchedtheir strategy rom primarily schooling t lowstemdensitieso dispersing t the highestden-sity, n responseo advantages ssociated ithdispersal. t high densitydispersed rey couldhide effectively;hus ewer advantagesccruedto schoolingndividuals. ndeed, schoolingmaybe disadvantageoust thisdensity s t providedgroupsargeenough or largemouth asso findand follow.

Optimal foraging heory predicts hat strat-egieswill be selectedor maximizing et energyintake (providing owestcostsper benefit to apredator:Pyke et al. 1977). Costoften is mea-suredas ime spent n active oragingbehaviors(Werner 1974). Predator activitymay be influ-encedby the relativecostsn searchingor, pur-suing,and capturingprey (Griffiths 1980). Ifprey attack and capture s energeticallynex-pensive, hen predatorscanafford to adopt anactive oraging strategy--increasingime spentin searchand pursuit. Given that prey attackand capture s energetically ostly, hen pred-ators shouldminimizeenergyexpendituresnother activities suchas pursuit) and become

ambushpredators. Our resultssuggest hatpredators witch trategiesspreyvulnerabilitychanges; argemouth basswere far-rangingpredators t low stemdensitiesn whichpreywere highlyvulnerable, ut ambush redatorsat high stemdensities, henprey vulnerabilitywas ow. Thus, we believe hat predatory acticsof largemouth assmaybe a function f habitatas well as a species-specificharacteristicsuchasbodyshape). learly,piscivoreshattypicallyassociatehemselveswith inshoremacrophytes(largemouthbass;northern pike Esox ucius)would be ambush predators, whereasopen-water specieswhite crappiePomoxisnnularis;white bassMoronechrysops)ould be active,searching redators. rom our perspective,helargemouth ass s probably lexible n its pre-dationstrategies. hether hisspeciessan am-bushor an actively earching redatordependson the complexity f the habitatn which t (orits prey) happens o live.In addition to using different tactics n dif-ferent stemdensities,argemouthbassalsore-duced costsby attackingonly prey with thegreatest hanceof being captured.Almostal-ways, hesepreyweredispersed ithina rathershortstrikingdistance.Oncethe predatorat-tacked, apturewasnearlyassured;uccessateswerehigh--70-80%---correspondingo the 90%rate measured y Nyberg (1971).The mosten-


12/13


ergy-costly ehaviors re probablypursuit, at-tack, and capture, and these behaviorsweredirected at prey within a short distance romlargemouthbass.With this strategy, ostsperunit of benefit were minimized.This studydemonstrateshat vegetation seffectiveas cover n reducingpredationmor-tality of juvenilebluegills. herefore,we haveat leastpartly explained he distribution f ju-venile centrarchids in natural communities on

the basisof predation pressures,which causeprey to move nto relatively afeareasof vege-tation or areasprovidingsimilaramountsofcover.The resultant eduction n vulnerabilitycan be explainedby increasesn the number ofvisualbarriers or percent over), swellasbymodificationsn prey behavior.The random-encountermodel is supported n part by thecontinual decline in encounter rates as stemdensityncreases.n addition, reymodify heirbehavior decreasedchooling)n the presenceof predators, urther reducing heir visibilityand susceptibility.

AcknowledgmentsWe thank Vanessa Murchake for her untir-

ing assistancend enthusiasmn the faceof te-dious behavioralobservations, nd B. L. John-son, R. F. Carline, J. F. Downhower, L. B.Crowder,D. L. Johnson, nd K. Laub for pro-vidingcritical,highlyconstructiveeviews f thismanuscript. his researchwassupported n partby funds from the Federal Aid in Fish Resto-ration Act under Dingell-JohnsonrojectF-57-R, the National Science Foundation (DEB77-16167),and the Department f Zoology.

ReferencesBROCK, V. E., AND R. n. RIFFENBURGH.1960. Fishschooling: possibleactor n reducing reda-tion. Extrait du Journal du Conseil,Conseil n-ternationalpour l'Explorationde la Mer 25:307-317.COOPER,W. E., AND L. B. CROWDER. 1979. Patternsof predation n simpleand complexenviron-ments. Pages257-267 in R. H. Stroud and H.Clepper,editors.Predator-prey ystemsn fish-eries management.Sport Fishing nstitute,Washington, istrictof Columbia,USA.CROWDER, . B., AND W. E. COOPER. 979. Structural

complexity nd fish-prey nteractionsn ponds:a pointof view.Pages -10 in D. L. Johnson ndR. A. Stein, editors.Response f fish to habitatstructure in standing water. North Central Di-

vision,AmericanFisheries ociety, pecialPub-lication6, Bethesda,Maryland,USA.Cwmo,E. 1976. The ethology f predation.Spring-er-Verlag, New York, New York, USA.DCosTANZO,.J. 1957. Growthof bluegill,Lepomismacrochirua,ndpumpkinseed,. gibbosus,f ClearLake, Iowa. Iowa State CollegeJournal of Sci-ence 32:19-34.ENDLER,. A. 1980. Natural selection n color pat-terns in Poecilia reticulata. Evolution 34:75-91.GLASS, . R. 1971. Computeranalysis f predationenergeticsn the argemouth ass. ages 25-363in B. C. Patten,editor.Systemsnalysis nd sim-ulationecology, olume1. Academic ress,NewYork, New York, USA.GRIFFITHS,. 1980. Foraging osts ndrelativepreysize. American Naturalist 116:743-752.HALL,D. J., ANDE. E. WERNER. 977. Seasonal is-tribution and abundance of fishes in the littoral

zone of a Michigan Lake. Transactions f theAmericanFisheries ociety 06:545-555.HAMILTON,. D. 1971.Geometryor theselfish erd.Journalof TheoreticalBiology31:295-311.HOLLANDER,., ANDD. A. WOLFE. 973. Nonpara-metricstatistical ethods. ohnWileyand Sons,New York, New York, USA.HUFFAKER,. B. 1958. Experimental tudies n pre-dation:dispersionactors nd predator-prey s-cillations.Hilgardia 27:343-383.KEAST,. 1977.Mechanismsxpanding ichewidthand minimizing ntraspecificompetitionn twocentrarchid fishes. Evolutionary Biology 10:333-395.KEAST,. 1978. Feedingnterrelationsetween ge-groupsof pumpkinseed Lepomisibbosus)ndcomparisonsith bluegill L. macrochirua).our-nal of the Fisheries Research Board of Canada35: 12-27.

LAUGHLIN, D. R., AND E. E. WERNER. 1980. Resourcepartitioningn two coexisting unfish: umpkin-seed Lepomiibbosus)nd northern ongearsun-fish Lepomi egalotieltates).anadian ournalof Fisheries nd AquaticSciences7:1411-1420.MAJOR, . F. 1978. Predator-preynteractionsn twoschoolingishesCaram ignobilis nd Stolephoruapurpureus. nimal Behavior 26:760-777.MITTELBACB,. G. 1981. Foragingefficiency ndbodysize:a studyof optimaldiet and habitatuseby bluegills.Ecology 2:1370-1386.NELL,S. R. ST.J., AND . M. CULLEN. 974. Exper-iments n whether choolingy heirpreyaffectsthe hunting behaviorof cephalopods nd fishpredators. ournalof Zoology London)172:549-569.

NYBERC,. W. 1971.Preycapturen the argemouthbass. American Midland Naturalist 86:128-144.OZIMEK, ., A. PREJS, NDK. PREJS. 976. Biomassand distribution f underground artsof Pota-mogetonerfoiliatusand Potamogetonucens nMikolajskieake,Poland.AquaticBotany :309-346.


13/13


PYRE,G. H., H. R. PULLIAM,ANDE. L. CH^RNOV.1977.Optimal oraging:a selectiveeviewof theoryandtests. The Quarterly Review of Biology 52:137-154.RaD^ROV, . V. 1973. Schoolingn the ecologyoffishes. ranslated rom Russian y H. Mills.JohnWiley and Sons,New York, New York, USA.S^IRI,M. K., NDJ. C. T^SH. 1979. Use of coveranddispersalby crayfish to reduce predation bylargemouthbass.Pages44-48 in D. L. Johnsonand R. A. Stein,editors.Responsef fish o hab-itat structure n standingwater. North CentralDivision, American FisheriesSociety,SpecialPublication , Bethesda,Maryland,USA.SEGHERS,. H. 1974. Schooling ehaviorn the gup-py (Poeciliaeticulata): n evolutionary esponseto predation.Evolution28:486-489.SHAW, . 1978. Schoolingishes.AmericanScientist66:166-175.SHELDON,R. B., ANDC. W. BOYLEN.1977. Maximumdepth inhabited by aquatic vascular plants.American Midland Naturalist 97:248-254.

SMYTHE, . 1970. On the existence f "pursuit n-vitation" signals n mammals.American Natu-ralist 104:491-494.STEIN,R. a. 1977. Selective redation,optimal or-aging, and the predator-prey interaction be-

tween ishand crayfish. cology 8:1237-1253.STEIN,R. A., AND . J. MAGNUSON.976. Behavioralresponse f crayfish o a fish predator.Ecology57:751-761.TAYLOR, .j. 1976. Value of clumpingprey and theevolutionary response to ambush predators.American Naturalist 110:13-29.WERNER,. E. 1974.The fishsize,preysize,handlingtime relation in several sunfishes and some im-

plications. Journal of the Fisheries ResearchBoard of Canada 31:1531-1536.WERNER, . E., D. J. HALL, D. R. LAUGHLIN, . J.W^GNER, . A. WILSANN,^NDF. C. FUNR. 1977.

Habitat partitioning n a freshwater ish com-munity.Journal of the FisheriesResearchBoardof Canada 34:360-370.

Documents

Predador-Prey Interactions Fishes