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Examining the Evidence for Killer Whale Predation on Steller Sea Lions in British Columbia and Alaska

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Examining the Evidence for Killer Whale Predation on Steller Sea Lions in British Columbia and Alaska AquaticMammals 2003, 29.3, 325–334 Examining theevidence for killer whale predationon Stellersea lions in BritishColumbia and Alaska Kathy Heise1,Lance G.Barrett-Lennard 1,2,EvaSaulitis 3,CraigMatkin 3, and David Bain4 1Departmentof Zoology, University of British Columbia,Vancouver, B.C. V6T1Z4, Canada 2VancouverAquarium MarineScience Centre, Vancouver, B.C. V6B3X8, Canada 3NorthGulf Oceanic Society, Homer, Alaska 99603,USA 4SixFlags Marine World, Vallejo, California 94589, USA Abstract lionsin its stomach. This, combined with other recentwork indicating that killer whales could be Thediscovery of ippertags from 14 Steller sea responsiblefor the decline of sea otters over large lions (Eumetopiasjubatus )inthe stomach of adead areasof western Alaska (Estes et al., 1998), killerwhale ( Orcinusorca )in1992 focused attention promptedan examinationof the evidence for killer onthe possible role of killerwhale predation in the whalepredation on Steller sea lions. We usedtwo declineof Stellersea lions in western Alaska. In this methodsto examine this question: an analysis of the study,mariners in British Columbia and Alaska stomachcontents of dead killer whales, and a weresurveyed to determine the frequency and out- questionnairesurvey of mariners who had the comeof observed attacks on sea lions, the age opportunityto observe interactions between sea classesof sea lions taken, and the areas where lionsand killer whales. predatoryattacks occurred. The 126 survey respondentsdescribed 492 killer whale/ sealion Killerwhales interactions,of which at least 32 were fatal attacks Killerwhales often have been described as oppor- onthe sea lion. The greatest rate of observed tunisticpredators (Dahlheim, 1981, Matkin & predationoccurred in the Aleutian Islands. The Leatherwood,1986). In thelast twenty years; how- stomachcontents of dead and stranded whales also ever,long-term study of killer whale populations in wereexamined. Stomachs that were not empty variousgeographical areas has changed this view. containedonly shor marinemammal remains, but Oneof the most signi cant ndingshas been that notboth. This supports earlier evidence of dietary populationsare speci c intheir choice of prey and segregationbetween sh-eating resident and marine foragingstrategies (Baird, 1994; Barrett-Lennard mammal-eating transient killerwhales in Alaska. et al.,1996;Saulitis, 1993; Saulitis et al., 2000). Stellersea lion remains were found in two of 12 Long-termstudies in the eastern North Paci c have killerwhale stomachs examined from Alaska ledto the identi cation of sh-eating resident and between1990 and 2001. Stomach contents from two mammal-eating transient killerwhales (Bigg et al., oVshore killerwhales provided the rstdirect 1987; Ford et al.,1994,1998, 2000; Ford & Ellis, evidencethat this third form of killerwhale feeds on 1999;Matkin & Saulitis,1994; Matkin et al., 1999; sh. Barrett-Lennard& Ellis, 2001).Genetic analyses hasrevealed that in Alaskan waters there are at Keywords: Steller sea lions, Eumetopiasjubatus , leasttwo subpopulations of resident killer whale killerwhales, Orcinusorca ,predation,stomach con- (northernBritish Columbia residents and Alaska tents,harbour seals, Alaska, British Columbia, residents)and three subpopulations of transient questionnaire. killerwhale (Gulf of Alaska transients, west coast transientsand AT1 transients,Barrett-Lennard & Introduction Ellis,2001). The AT1 transientsare found primarily aroundPrince William Sound and Kenai Fjords Since1980, the Steller sea lion ( Eumetopiasjubatus ) andnumber approximately 11 animals (Matkin populationin the Aleutian Islands and the Gulf of et al.,1999). The west coast transients range Alaskahas declined by 80%(Trites & Larkin,1996, betweenCalifornia and southeast Alaska and over Ferrero et al.,2000).In 1992, a killerwhale ( Orcinus 200have been identi ed in the waters between orca)wasdiscovered with tags from 14 Steller sea Washingtonand Alaska (Ford & Ellis, 1999). ? 2003 EAAM 326 K. Heise et al. Approximately60 Gulf of Alaska transients have commonlyreported in other parts of the world beenidenti ed (Ford & Ellis, 1999),although this (e.g.,Lopez & Lopez,1985; Guinet, 1991; Hoelzel; numberis expected to increase as aresultof current 1991).However, data have not been systematically research eVortin thearea(Barrett-Lennard, unpub- collectedon killer whale predation on Steller sea lisheddata). A thirdpoorly-known population of at lions.In this study, we surveyedmariners to obtain least200 killer whales, referred to as oVshores informationon the frequency with which killer hasalso been identi ed (Ford et al., 2000; whalesand sea lions were observed in proximity, Barrett-Lennard& Ellis 2001).Their range is not theoutcome of observed attacks on sea lions, the known,and their diet is thought to include ageclasses of sea lions taken and the areas where shbecause they travel in large groups and are predatory-typeattacks were observed. We usedthis acousticallyactive. informationor ‘ecologicalknowledge’ (Huntington, Inthis study, we reporton the stomach contents 2000)as amethodof acquiring information on the recoveredfrom 12 killer whales in Alaska from extentto which killer whales prey on Steller sea 1990to 2001.Opportunities to examinekiller whale lions. stomachcontents occur relatively infrequently, per- hapspartly due to low mortality rates (annual mortalityrates for resident killer whales range from Materialsand Methods 0.011/yearfor adult females to 0.039/yearfor adult males,Olesiuk et al.,1990).In addition, killer whale Stomachcontents carcassesgenerally sink (Zenkovich, 1938). From Thestomach contents of 12 killer whales stranded 1973to 2000 in British Columbia, only 24 killer inAlaska between 1990 and 2001 were examined whalecarcasses were recovered, of which eight were forfragments of prey that could be identi ed to neonates(Olesiuk et al.,1990; Barrett-Lennard, species.Species identi cations were con rmed by unpublisheddata), although approximately 167 Pacic Identications Inc. of Victoria, B.C. the residentwhales and an unknown number of tran- lateFrancis (Bud) Fay (University of Alaska, sientsdied over this same period (Ford et al., 2000). Fairbanks),Elaine Humphries (University of Todate, stomach contents have only been recov- BritishColumbia, Vancouver) and William A. eredfrom one known transient in BritishColumbia Walker(Natural History Museum of Los Angeles (Ford et al.,1998,Barrett-Lennard unpublished County,Los Angeles). We attemptedto identify data). eachdead killer whale using photographs from previouslypublished catalogues (Bigg et al., 1987; Stellersea lions Dahlheim et al.,1997;Ford et al.,1994,2000; Ford Thereare two genetically distinct populations of &Ellis,1999; Heise et al.,1992;Matkin et al., 1999) Stellersea lions in the eastern North Paci c (Bick- andfrom unpublished photographs held by the ham et al.,1996).The eastern population is found Pacic BiologicalStation (PBS), Department of fromCalifornia to Cape Suckling (144 W, Fig. 1) Fisheriesand Oceans, Nanaimo, B.C. andthe andgenerally has been increasing (Trites & Larkin NorthGulf Oceanic Society, Homer, Alaska. 1996,Calkins et al.,1999).The western Alaskan Geneticanalyses were successful on sevenof the 12 population,which has shown a dramaticdecline killerwhale carcasses following methods described overthe past 30 years (Loughlin et al.,1992;Trites inBarrett-Lennard (2000), to determine whether &Larkin,1996; Sease et al.,2001)is foundwest of thewhales were from the resident, transient, or CapeSuckling. Numerous hypotheses have been oVshorepopulation. The minimum number of advancedto explain the decline, including shooting pinnipedprey was calculated by counting the orentanglement (Trites & Larkin,1996), reduction numberof teeth, claws, and whiskers recovered. inthe quantity or qualityof food(Trites & Larkin, We estimateda minimumof 70 whiskers (>5 cm 1992,Castellini, 1993; Merrick et al.,1997;Rosen & long)per animal for both harbour seals ( Phoca Trites,2000), disease and parasites (Spraker vitulina)andsea lions, based on specimens held by et al.,1993),and an overall decline in the carrying PBSand whisker counts published in Scammon capacityof the Bering Sea (National Research (1874). Council,1996; Trites et al.,1999).It is also possible thatpredation may have caused the decline, al- Mariner’s survey thoughprior to this study this hypothesis received We distributeda four-pagequestionnaire in 1993 littleattention. and1994 to approximately 250 mariners in British Observationsof attacks on and kills of sea lions Columbiaand Alaska, including researchers, com- bykiller whales have been documented throughout mercial shermen,and tour boat operators. The Alaskaand British Columbia (e.g., Tomilin, 1957; resultsof those surveys were compiled to produce Rice, 1968;Harbo 1975; Ford & Ellis, 1999),and anaccount of the number of interactions observed killerwhale predation on otariids (eared seals) is betweensea lions and killer whales relative to the Evidencefor killer whale predation on Steller sea lions 327 Table 1. Stomachcontents of killer whales from Alaska (Area: PWS =PrinceWilliam Sound; SEA =SoutheastAlaska, AK=Alaska). Year Area Population* StomachContents Knowntransient and/ or whalewith stomachcontaining marine mammal remains 19901 Culross Island AT1Transient Bones,whiskers andhair from adult and juvenile PWS (G) male harbourseal, 1 Dall’s porpoisedorsal n. 19902 BeartrapBay, PWS AT1Transient(G,P) Empty. 19912 CapeSt. Elias Gulf of AK ? Sub-adultsea lion including skull, harbourseal, Dall’s porpoiseskull.
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