Proceedingsof the International Symposiumon IVorth PacificFlatfish AlaskaSea Grant College Program ~ AK-SG-.95-04,l 995 Food Habitsof Three Congeneric Flatfishes: Yellowfin Sole Pleuranectesasper!, Rock Sole P b.ilineatus!,and AlaskaPlaice P q.uadrituberculatus! in the EasternBering Sea GeoffreyM. Langand PatriciaA. Livingston Alaska FisheriesScience Center, IVationaI Man'ne FisheriesService Seattle, Washington USA Bruce S. Miller FisheriesResearch Institute, University of Washington Seattle, Washington USA Abstract The stomach contents were identified from a total of 9,096 yellowfin sole Pleuronecresasper!, 586 rock sole P.bill neatus!,and 513Alaska plaice P.quadritubercuiatus! collected from the easternBering Sea from 1984to 1988.In terms of percentby weight,yellowfin solehad the broadest diet of the three species, consuming similar amounts of benthopelagicprey such as clams,other marineworms sipunculans, echiurids, and priapuiids!, and polychaetes. Rocksole preyed prima- rily on polychaetes,other marine worms,and fish in smalleramounts. Alaskaplaice exhibited the narrowestdiet selection,consisting of polychaetesand other marineworms nearlyexclusively. Slight dietary variationwas seenwith respectto predator size and depth stratafor each of the species. The diets of Alaska plaice and rock solewere similar due to their relianceupon polychaetes,while yellowfinsole differed from these two because of the variety of prey items in its diet. In addition, the 226 lang et al. Food Habits of Three F/atfishes in the Easrern Bering Sea Table 1. Habitat characteristicsof yellowfin sole,Pleurottectes asper, rock sole, P.bilineatus, and Alaskaplaice, P. quadritttberculatus,in the eastern Bering Sea from Allen 1984 and Allen and Smith 1988!. Ye!1owfin sole Rocksole Alaska plaice Bottom type Sand soft! Gravel coarse] Sand soft! Depth range m! 10-600 0-575 6-475 Depth optimum m] 50-100 50-100 50-100 Geographic range Korea-B.C., Korea- Peter the Great Canada California Bay-Gulf of Alaska Life zone Inner-outer shelf Inner-outer shelf Inner-outer shelf Zoogeography Arctic- NW NE boreal Arctic-N boreal Pacific Pacific boreal Pacific Refuge Bury Bury Bury areas of highest abundance of each of these fish were somewhat geographically separate, It appears that competition for similar prey items between species was low, Despite slight differences in their gross feeding inorphology, these three fish were all well-suited for benthic-oriented invertebrate predation. Introduction Yellowfin sole Pleuronectes asper!, rock sole P. bilineatus!, and Alaska plaice P.quadrituberculatus! are small-mouthed f atfish abundant in the eastern Bering Sea. The three species are siinilar in their general habitat characteristics Table 1! and in their distribution throughout the eastern Bering Sea Fig. 1!, although their individual areas of highest abundance are slightly separate. Both yellowfin sole and rock sole support commercial fisheries, with catches of 149,569 metric tons t! Wilderbuer 1993! and 56,998 t Wilderbuer and Walters 1993! in 1992, respectively. Additionally, 18,985 t of Aiaska plaice were caught in 1992 Waiters and Wilderbuer 1993]. The diet of yellowfin sole consists of clams, crustaceans, poly- chaetes, and other benthic invertebrates Haflinger and McRoy 1983, Wakabayashi 1986, 1989, Allen 1984, Livingston et al. 1986!. Food habits investigations have shown that rock sole consume polychaetes, echiurids, and gamrnarid amphipods Brodeur and Livingston 1988, Proceedingsof the International Symposium on IelorthPacific F!atfssh 227 i+ ~, sTeewrreKE i r ! 60 56 56 54 1 BOW 174 166 162 156 Figurei. Areasof highestdensity of yelloufin sole,rock sole, and Alaska plaice in the eastern Bering Sea. Corcobado-Onate 1991!. Previous studies on Alaska plaice have shown that polychaetesmake up the majority of their diet, althoughmol- lusks, echinoderms, and benthic crustaceans are also consumed Moiseev 1953,Skalkin 1963,Mineva 1964,Zhang 1988,Allen 1984!. These studieshad small sizesand liinited spatialand/or temporal coverage. Comparativefood habit studiesof thesespecies were performed byAllen 984! and Zhang988!. Allen 984! found that eventhough yellowfin sole,rock sole,and Alaskaplaice had similar feedingmor- phologiesand co-occurredin 65%of the samplesfrom the rniddle shelf of the easternBering Sea, they had dissimilar diets.Zhang 988! found as much as 80% diet overlapbetween Alaska plaice and rock solewhen prey were classifiedto the phylum level;yellowfin soledid not show as inuch overlap with either species.These studies present 5Otneinsight into the trophic relationshipsof theSethree species, However, they were limited to very small 1-12 fish per species! ZZB lang et al, Food Habits of Three Flatfishes in the Eastern Bering Sea sample sizes and did not account for variation in the diet due to size of fish or location. Since thesefish constituteone-quarter of the exploitableground- fish biomassin the easternBering Sea North PacificFishery Manage- ment Council NPFMC! 1993! and are iinportant to the comtnercial trawl fishery,it is essential to studytheir life history characteristics and, in particular,their trophic dynainics. Knowledgeof their indi- vidual food habits, as well as their interaction with each other and the ecosystem,becomes a criticalelement as fisheriesmanagement takes a multi-species approach. In addition, there exists a need to under- standhow three congenerswith similar morphology,distributions, and trophic characteristicsutilize the samebasic resource. Therefore, a comprehensive,detailed investigation of the food habits of these three fish and of their dietary relationships is needed. The purpose of this study is to ! present, compare, and contrast the diets of these three flatfish; ! determine which factors are important in depicting each diet; and ! discussthe trophic and ecologicalstatus of each species. Methods and Materials Stomach samples were collected from 9,096 yellowfin sole in the eastern Bering Seafrom May 1984through September 1988;513 Alaska plaice and 586 rock sole stomach samples were also collected between Juneand August of 1984, 1987,and 1988 Table 2!, Additional samplescollected during the 1989-1991eastern Bering Searesource assessmentsurveys were usedfor canonicalcorrespondence analysis CCA! Table 2!. Sampleswere collectedaboard Nationai Marine Fisheries Service NMFS!, Resource Assessment and Conservation Engineering RACE! Division research cruises and by biological observersfrom the ResearchEcology aiid FisheriesManagement REFM! Division Fisheries Observer Program on commercial fishing vessels. Fish were randomly selected from a subsampleof the catch for stoinach sampling. Stomachs were collected from fish that showed no signsof either net feeding e,g. presence of freshprey in the mouth, esophagus,or gill chamber!or regurgitation e.g. presenceof digested prey in the mouth or gill chamber, or a flaccid or water-filled stom- ach!.Acceptable stoinachs were excisedat seaand preserved in 10% buffered formalin. Predator length, sex, spawning condition, haul location, date, and time were recordedfor each specimen collected. Proceedingsof the internarional Symposiumon North Pacific Flatfish 229 Table 2. Number of yellowfin sole,rock sole,and Alaskaplaice stomach samplesanalyzed from the easternBering Sea for the periods 1984-1988 and 1989-1991. Predator species and Number of samples sample period With food Empty Tot Yellowfin sole 1984-1988 6,133 2,963 9,096 1989-1.991 2,444 682 3,126 Total 8,577 3,645 12,222 Rock soie ]984-1988 470 113 568 1989-1991 1,095 147 1,257 Total 1,565 260 1,825 Alaska plaice 1984-1988 449 64 513 1989-t991 818 61 879 Total 1,267 125 1,392 Sainpleswere transported to the laboratoryand storedin a 70% ethanol solution. In the laboratory, contents of individual stomachs were analyzed. Preyitems weresorted and identified to the lowestpractical taxon, counted, and weighed to the nearest milligram, To examine and compare the dietsof these three species, it was necessaryto consolidatethe diet into a smallnumber of common prey categories.The following eight prey types were chosen because they representedthe sevenmost common prey plus a catch-all miscellaneouscategory. This combinationof eight categoriesalso resultedin the lowestpercentage of miscellaneousprey of the combi- nations that were tried. The eight prey categories were: Polychaetes polychaete annelid worms Clams bivalve inoliusks Gammarids gammarid amphipods Decapods decapodscrustaceans, shrimp, and crab Marine worm echiurids, priapulids, sipunculans Echinoderrns sand dollars, brittle stars, sea cucumbers, sea stars 230 Lang et al. Food Habits of Three Flatfishes in the Eastern Bering Sea Fish teleos t fish Miscellaneous all other prey types, including unidentified prey. Size categories,by fork length, for yellowfinsole < 20 cm, 20-30 cm, > 30 cm!, rock sole < 20 cm, 20-35 cm, > 35 cm!, Alaska plaice < 20 crn, 20-40 cm, > 40 cm! were deterinined based upon the most obvious ontogenetic changes seen in the diet. Three depth zones, < 50 m, 50-100 m and 100-200rn, were used for dietary analysis.These depth zones were chosen due to the distinct oceanographic and hydrographic characteristics that separatethe three areas Favorite et al, 1976, Kinder and Shurnacher 1981!. These depth zones were further subdivided into six strata by dividing the area into northwest and southeast sections. Strata 10 and 20 represent the southeast and northwest sections of the < 50 in depth zone, respectively, strata 30 and 40 the 50-100 m depth zone, and strata 50 and 60