Journal of Great Lakes Research 37 (2011) 584–587

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Size preferences and behaviors of native yellow perch foraging on invasiveround gobies

Michael J. Weber ⁎, John M. Dettmers 1, David H. Wahl, Sergiusz J. CzesnyDivision of Ecology and Conservation Sciences, Illinois Natural History Survey, and Department of Natural Resources and Environmental Sciences, University of Illinois,1816 South Oak Street Champaign, IL 61820, USA

⁎ Corresponding author at: Department of WildlifeDakota State University, Box 2140B Northern Plains Bios57007, USA. Tel.: +1 605 688 6121.

E-mail address: Michael.Weber@sdstate.edu (M.J. W1 Present address: Great Lakes Fishery Commission, 21

100 Ann Arbor, MI 48105, USA.

0380-1330/$ – see front matter © 2011 International Adoi:10.1016/j.jglr.2011.04.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 June 2010Accepted 22 March 2011Available online 12 June 2011

Communicated by: Geoffrey Steinhart

Index words:Yellow perchRound gobyPrey preferencePredationForaging behaviorsPrey profitability

Predation is one of the primary mechanisms that shape aquatic food webs and predator–prey interactions aretypically highly dependent on sizes of both forager and its prey. Round goby Neogobius melanostomus is arecent invader to the Great Lakes and can be an important prey item for native predators. However, predationpatterns on round gobies have received limited attention. We assessed size-specific predator–preyinteractions between invasive round gobies and native yellow perch Perca flavescens by comparing preypreferences for three size classes of adult yellow perch foraging on six size classes of round gobies. Smallyellow perch preferred the smallest round gobies available, medium sized yellow perch increased the range ofround goby sizes consumed but still preferred smaller prey, whereas large yellow perch consumed largerround gobies and excluded the smallest prey size. Yellow perch foraging behaviors indicated thatintermediate sizes of round gobies were struck at most frequently and that pursuit and handling timeincreased whereas capture efficiency and prey profitability decreased with round goby size. Our resultsindicate that predator–prey interactions between yellow perch and round gobies may be size dependent andheavily influenced by capture efficiency and prey profitability.

© 2011 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.

Introduction

Understanding processes regulating predator–prey interactions inaquatic communities remains a significant ecological challenge becausepredation may restructure prey population size-structure, abundance,or behavior. The most basic predator–prey interaction is diet selection.Predators generally prefer prey that are easier to locate, capture, andhandle (Juanes, 1994; Weber et al., 2010). A frequent case of dis-proportionate predation is size selectivity (Juanes, 1994; Turesson et al.,2002) with predators generally preferring prey of an ‘optimum size’above which capture becomes difficult and below which energeticprofitability is reduced (Juanes, 1994; Turesson et al., 2002).

Introduced and invasive species may have strong and often cas-cading influences on predator–prey interactions and foodweb structure(Madenjian et al., 2002). Round goby Neogobius melanostomus is one ofthe more recent invaders that have rapidly expanded in all five of theLaurentian Great Lakes (Charlebois et al., 1997). Round gobies maybenefit native piscivores (Steinhart et al., 2004) because their high

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abundances and extended spawning season (Charlebois et al., 1997)provide predators with a wide range of prey sizes throughout the year.Numerous piscivores in the Great Lakes, including lake trout Salvelinusnamaycush (Dietrich et al., 2006), smallmouth bass Micropterusdolomieu (Steinhart et al., 2004), burbot Lota lota (Johnson et al.,2005), walleye Sander vitreus (Johnson et al., 2005), and yellow perchPerca flavescens (Truemper et al., 2006; Weber et al., 2010) haveincluded round gobies in their diet. Experimental studies indicate thatyellowperch at times select round gobies over other prey items (Weberet al., 2010). However, round gobies may be more aggressive, posegreater capture difficulties, and provide lower energetic gain comparedto other prey (Weber et al., 2010) and thusmay interact differentlywithpredators compared to native prey.

Yellow perch is an abundant predator within nearshore areas of theGreat Lakes and may be affected by round goby introduction andexpansion. Yellow perch in the Great Lakes consume a wide variety offish prey, including the families cyprinidae, clupeidae, percidae, cottidae,and osmeridae (e.g., spottail shiners Notropis hudsonius, alewives Alosapseudoharengus, sculpinsCottus spp.;Wells, 1980; Truemper et al., 2006;Weber et al., 2010). However, abundances of several native fishes havedeclined following the invasion of round gobies (i.e., sculpins, rainbowsmelt Osmerus mordax, johnny darter Etheostoma nigrum; Janssen andJude, 2001; Lauer et al., 2004; Truemper et al., 2006). Yellow perch areoften considered opportunistic foraging generalists with consumptionpatternsbasedonprey availability (Knightet al., 1984;Graeb et al., 2005;

. Published by Elsevier B.V. All rights reserved.

Fig. 1. Percentage (mean±1 standard error) of round goby size class consumed bysmall (top left), medium (middle left), and large (bottom left) yellow perch duringlaboratory experiments. The solid horizontal line represents the expected sizedistribution of prey consumed. Preferences of small (top right), medium (middleright), and large (bottom right) yellow perch feeding on round gobies as indicated bychi-square adjusted residuals. Residuals above the horizontal dashed line at 2 indicatepositive selection whereas residuals below the horizontal dashed line at −2 indicatenegative selection.

585M.J. Weber et al. / Journal of Great Lakes Research 37 (2011) 584–587

Truemper et al., 2006), although they can display prey preferences(Weber et al., 2010). Thus, shifts in prey assemblages may have im-portant implications for yellow perch foraging and growth.

Due to their relatively recent introduction to theGreat Lakes, limitedinformation exists about predator–prey interactions between yellowperch and round gobies (but see Truemper and Lauer, 2005; Truemperet al., 2006; Weber et al., 2010). Yellow perch may preferentiallyconsume smaller round gobies than predicted by gape limitations(Truemper and Lauer, 2005); however, it is not known if field dietpatterns are a result of differences in prey availability, behavior, or both.Deciphering size-related predation patterns on round gobies willincrease the understanding of predator–prey interactions in additionto the transfer of energy, nutrients, and contaminants (Bunnell et al.,2005). In this study, we used laboratory experiments to quantify yellowperch size preferences on round gobies and to quantify the componentsof these predator–prey interactions (i.e., strikes, captures, pursuit, andhandling time) to determine if they are size-related.

Methods

Small (240–259 mm total length; TL), medium (260–289 mm TL),and large (290–310 mm TL) adult yellow perch were obtained from alocal aquaculture facility where they had been maintained on naturalprey (i.e., invertebrates and fishes). Yellow perch were held in an 800-lraceway threemonths prior to initiation of experiments and acclimatedto readily consume 40–120 mm round gobies. Water temperatureswere regulated by constantwater flow fromLakeMichigan (mean±SE;16.8±0.6 °C) and ambient light was provided by overhead skylights(14 h light:10 h dark).

Yellow perch size preference was examined in round flow throughfiberglass tanks (1020-l, 1-m diameter) with a central standpipe.Trials consisted of a single predator and six round gobies, one fromeach of six size classes [50–55, 60–65, 70–75, 80–85, 90–95, and 100–105 mm TL, hereafter referred to by minimum length (i.e. 50–55 mmwill be 50 mm)]. All size classes were within yellow perch gapelimitations (Truemper and Lauer, 2005). Round gobies were collectedby seine from Lake Michigan, Winthrop Harbor, Illinois. A singleyellow perch was held without food for 24 h to standardize hungerlevels before being released into the pool to acclimate for 2 h. Afterpredator acclimation, six round gobies were released into the middleof the pool. No prey was attacked within the first several minutes,providing time to reach the bottom of the pool and adjust to theirsurroundings. Trials were monitored hourly until prey were con-sumed (mean trial duration=1 h). Multiple round gobies were con-sumed in five trials (9% of all trials). We used 10 individual predatorsof each size class and completed 14 trials with small, 18 trials withmedium, and 20 trials with large yellow perch (≤2 trials per indi-vidual predator). Additional replicates were completed to increasepower for medium and large size classes. Size distributions of con-sumed prey were analyzed using chi-square tests to determinedifferences from random consumption. Chi-square tests also pro-duced expected consumption percentages and adjusted residuals tocompare yellow perch consumption patterns with random consump-tion to indicate sizes of round gobies preferred (positive residual N2)or avoided (negative residual b−2; Agresti, 1996).

Next, we completed behavioral observations of yellow perch for-aging to evaluate mechanisms influencing size preferences. A singlemedium yellow perch (260–289 mm) was acclimated to the pool for2 h after which three round gobies of the same size class werereleased. Round gobies from the 50–90 mm size classes were tested.Behaviors of yellow perch foraging on 100 mm round gobies were notevaluated because they were not consumed during size preferenceexperiments. Again, no prey was attacked within the first severalminutes, providing time to reach the bottom of the pool and adjust totheir surroundings. Predatory behaviors (strikes, captures, pursuit,and handling time) were observed for 15 min, after which the expe-

riment was terminated (Weber et al., 2010). Pursuit time was thecombination of time spent following and in pursuit of prey. Handlingtime was the time from capture until typical eye, respiratory, andswimming motions of the predator resumed (Weber et al., 2010). Atleast 10 replicates were completed with round gobies of 50, 60, and70 mm; 7 replicates were completed for 80 mm round gobies and 3replicates were completed for 90 mm round gobies. An individualyellow perchwas only used once for a single trial of each prey size anda minimum of 48 h elapsed between trials on any individual fish.

Based on behavioral observations, we calculated capture efficiencyfor each size class of prey as the number of captures divided by thenumber of strikes.We also determined optimal prey sizes by calculatingprey profitability as the ratio of joules gained per unit handling time(Turesson et al., 2002) as determined by a round goby length–weightrelationship (Truemper and Lauer, 2005). Predatory behaviors (strikes,captures, capture efficiency, and pursuit and handling time) and preyprofitability were log-transformed before comparing across prey sizesusing ANOVA. Differences among prey size classes within a singlepredatory behavior were identified with Fisher's LSD mean separationtest. Level of significance for all tests was set at α=0.05.

Results

Small yellow perch preferred small round gobies with size pre-ferences increasing with predator size. Small yellow perch consumedround gobies 20–33% (mean=24%) of their total length. Small yellowperch clearly selected the smallest round goby size class (χ2=34.06,df=5, Pb0.01), with a large difference between the percent of 50 mmround gobies consumed and expected consumption, but consumedround gobies up to 80 mm (Fig. 1, top left panel). A large positiveresidualN2 further indicated that 50 mm and 70 mm gobies werepreferred whereas round gobies≥90 mm were avoided (Fig. 1, top

586 M.J. Weber et al. / Journal of Great Lakes Research 37 (2011) 584–587

right panel). Medium yellow perch consumed round gobies 17–33%(mean=24%) of their total length. Medium yellow perch also selectedfor small prey sizes (χ2=12.93, df=5, P=0.02) but consumedsimilar proportions of 50, 60, and 70 mm round gobies (Fig. 1, middleleft panel). Residuals also indicated that medium yellow perchpreferred 50–70 mm round gobies and avoided 100 mm round gobies(Fig. 1, middle right panel). Large yellow perch consumed roundgobies 20–32% (mean=26%) of their total length. Large yellow perchconsumed fewer 50 mm round gobies than expected by excludingthem as prey but consumed more 90 mm round gobies than expected(χ2=14.73, df=5, P=0.01; Fig. 1, bottom left panel). Residuals alsoindicated that 70 mm and 90 mm round gobies were preferredwhereas the smallest and largest round gobies were avoided (Fig. 1,bottom right panel). All yellow perch size classes excluded 100 mmround gobies from their diet.

Prey size affected the foraging behaviors of medium yellow perch(260–289 mm). Pursuit time was less for 50 mm round gobies thanfor the four largest size classes (ANOVA, F4,40=6.85, Pb0.01; Fig. 2a).Yellow perch struck at 70 mm round gobies more than either larger orsmaller size classes whereas the largest and smallest size classes ofround gobies were struck at least frequently (ANOVA , F4,40=6.87,Pb0.01; Fig. 2b). The three smallest size classes of round gobies (50,60, and 70 mm) were captured more often than the larger two sizeclasses (80 and 90 mm; ANOVA, F4,40=12.70, Pb0.01; Fig. 2c).Differences in strikes and captures across round goby size classesresulted in differential capture efficiency. Capture efficiency washighest for 50 and 60 mm round gobies but decreased for larger sizeclasses (ANOVA, F4,40=16.67, Pb0.01; Fig. 2d). Handling time differedamong all prey size classes and increased with prey size (ANOVA,F4,40=91.94, Pb0.01; Fig. 2e). Prey profitability was greater for 50, 60,and 70 mm round gobies and was lower for 80 and 90 mm roundgobies (ANOVA, F4,40=6.93, Pb0.01; Fig. 2f).

Discussion

Yellow perch displayed preferences for particular sizes of roundgobies that maximized capture efficiency and prey profitability. Small

Fig. 2. Pursuit time(a), strikes (b), captures (c), captureefficiency(d), handling time(e) andprey profitability (f; mean±1 standard error) for medium yellow perch (260–289 mm)foraging on 50, 60, 70, 80, and 90 mm round gobies during 15-min trials. Within a panel,values with different letters indicate significant differences among prey size classes(ANOVA, LSD, Pb0.05). Round gobies 100 mmwere not tested (NA).

yellow perch were the most selective foragers, preferentially con-suming the smallest round gobies whereas large yellow perch showedthe least preference for prey size, consuming four size classes of roundgoby at relatively equal proportions, but excluded the smallest roundgobies. Prey sizes preferred by yellow perch were similar to thoseconsumed in Lake Michigan (Truemper and Lauer, 2005). Thus,fluctuations in prey size distribution may have the greatest effect onsmaller yellow perch that are selective for small-bodied prey, whereaslarger yellow perch that are less size selective may be less affected byprey size distribution (Tonn and Paszkowski, 1986).

Prey size regulates both pre- and post-capture constraints thatdictate eventual prey consumption. Capture efficiency, pursuit andhandling time, and prey profitability are all highly dependent on preysize, with small prey generally being the most vulnerable (Juanes,1994). Yellow perch are active predators with small gapes and areaffected by both pre- and post-capture constraints (Weber et al.,2010). Yellow perch in our experiments could have consumed largerround gobies based on gape limitations (Truemper and Lauer, 2005).However, capture efficiency and prey profitability declined whereaspursuit and handling time increased with round goby size. Thesefindings are congruent with other piscivores that are constrained byprey behaviors, not gape limitations (Juanes, 1994).

Preference for small-bodied prey by yellow perch has beenhypothesized to be a function of random attack rates and non-randomcapture success (Paszkowski and Tonn, 1994). Our behavioral ob-servations revealed that both attacks and captures by yellow perch onround gobies were non-random. A dome-shaped relationship existedbetween round goby size and yellow perch attacks, with mid-sizedround gobies attacked most frequently. Round gobies 50 to 70 mm insize were captured with similar frequency by yellow perch and moreoften than 80 or 90 mm round gobies. Fewer strikes were directedtoward larger prey and their reduced capture success likely resultedfrom increased pursuit time. Capture efficiency was highest for smallround gobies in our experiments and rapidly declined for larger preysizes. Capture efficiency was likely partially responsible for prey sizepreferences; however, it could not fully explain our observed pre-ference patterns. Yellow perch at times attack preymore frequently tocompensate for low capture efficiency (Graeb et al., 2005). Preypreference patterns we observed nearly mirrored prey profitability,suggesting yellow perch selected for prey that maximized energeticgain. Therefore, predator behavior and choice likely interact with preybehavior and vulnerability to determine predator diets.

Our results build on the existing understanding of predator–preysize dynamics and piscivore foraging behaviors. Three sizes of yellowperch broadly consumed several size classes of round goby prey, withsmall yellow perch preferring the smallest fish and the smallest rangeof sizes. In contrast, large yellow perch excluded the smallest roundgobies, a rare behavior among piscivorous fishes, but instead preferredlarger prey. Both pre- and post-capture constraints suggest thatdifferences in prey vulnerability and profitability dictate yellow perchprey preferences. Instead of yellow perch being generalist foragers aspreviously described (e.g., Knight et al., 1984; Truemper et al., 2006),our results suggest that yellow perch can have prey preferences thatare based on prey profitability. This mechanistic understanding ofyellow perch consumption provides greater insight into field obser-vations of yellow perch diets.

Acknowledgements

We thank W. Brofka, R. Redman, S. Creque, B. Ruebush, J.Pinkerton, D. Lichti, D. Zapf, M. Kneuer, and the rest of the staff atthe Lake Michigan Biological Station for help in the laboratory. Wealso thank M. Diana and staff of the Kaskaskia Biological Station forlogistical support in obtaining fish for our experiments. Funding forthis project was provided by the Illinois Natural History Survey and

587M.J. Weber et al. / Journal of Great Lakes Research 37 (2011) 584–587

Federal Aid in Sport Fish Restoration Act under project F-123-Radministered through the Illinois Department of Natural Resources.

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