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 Ž shor marinemammal remains, but Oneof the most signiŽ cant Ž ndingshas been that notboth. This supports earlier evidence of dietary populationsare speciŽ c intheir choice of prey and segregationbetween Ž 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 PaciŽ c have killerwhale stomachs examined from Alaska ledto the identiŽ cation of Ž sh-eating resident and between1990 and 2001. Stomach contents from two mammal-eating transient killerwhales (Bigg et al., oVshore killerwhales provided the Ž 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; Ž 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 identiŽ 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 beenidentiŽ 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 identiŽ 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 Žshbecause 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 identiŽ ed to neonates(Olesiuk et al.,1990;Barrett-Lennard, species.Species identiŽ cations were conŽ rmed by unpublisheddata), although approximately 167 PaciŽc IdentiŽcations 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 PaciŽ c (Bick- andfrom unpublished photographs held by the ham et al.,1996).The eastern population is found PaciŽc 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 mercialŽ 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 Ž n. 19902 BeartrapBay, PWS AT1Transient(G,P) Empty. 19912 CapeSt. Elias Gulf of AK ? Sub-adultsea lion including skull, harbourseal, Dall’s porpoiseskull. 19923 MontagueIsland PWS GofAKTransient(G) 15 Steller sea lion tags, 480 sea lion whiskers, harbourseal claws (8 hind, 6 fore)and 20 harbour sealwhiskers, bullet,halibut hook, 29 small& 27large sea lion claws. 19934 CookInlet AK ? Regurgitated1 harbourseal  ipperand beluga skin andblubber while stranded. 20005 OrcaInlet PWS AT1Transient(G,P) Atleast 3 harbourseals, (1 adult female, 1 male, 1pupfemale), three harbour seal  ippertags (from 1adultfemale and 1 pup). 20015 HinchinbrookIsland PWS ? Harbourseal  ipper,fur andclaws from at least 2harbourseals, bull kelp ( Nereocystis )(over6 kg). 20016 IzembeckLagoon Bering Sea ? Bird feathers(4), harbour seal (fur, 2clawsand 1whisker), riverotter bones and fur, sandand smallrocks. Known oVshorewhales 19947 BarnesLake SEA OVshoremale (G,P) Crabshell, sculpin and eel grass. 19947 BarnesLake SEA OVshorefemale (G,P) Salmonidbones. Knownresident whale 19911 MontagueIsland PWS Resident(G) 2circlehooks with gangionand stainless steel snap,small pieces of plastic. Unkownwhale 19938 StPauls Island Bering Sea (unknown young male) 500 g bullkelp, 1 verylarge squid or mediumsized octopusbeak, 1 mediumsized squid beak, 1commonmurre.

*Methodof determining population identity in brackets: G =geneticanalysis. P= Photo-identiŽcation. In thecase of geneticidentiŽ cation it was possibleto determine if thewhale was aGulfof Alaska transient (G ofAK) or anAT transient(as per Matkin et al., 1999). Sources: 1.Kathy Heise and Lance Barrett-Lennard (University of British Columbia,Dept. of Zoology, Vancouver, B.C. V6T 1Z4). 2.Kate Wynne (Alaska Sea Grant Program, 118 Trident Way, Kodiak Island, AK 99615). 3.Eva Saulitis (North Gulf Oceanic Society, 60920 Mary Allen Ave., Homer, AK 99603). 4.David Bain (Six Flags Marine World, Vallejo,California 94589). 5.Craig Matkin (North Gulf Oceanic Society, 60920 Mary Allen Ave., Homer, AK 99603). 6.Donna Willoya and Liana Jack (Alaska Sea Otter and Steller Sea Lion Commission, 6239B St#204,Anchorage, AK 99518). 7.David Bain (address above) and Rich Ferrero (National Marine Mammal Lab, NMFS, Seattle,WA 98115). 8.Alan Springer and Mike Williams (Universityof Alaska,Fairbanks, AK 99775). totaltime mariners spent on the water. We asked andother marine mammal species. Interactions marinerswho witnessed interactions between sea betweenkiller whales and sea lions were separated lionsand killer whales to describe their observa- intotwo categories: predatory and non-predatory. tions,including details on the number of animals Anon-predatoryinteraction was one inwhich killer involved,the age class of thesea lions, the locations whalesand sea lions were observed swimming in whereinteractions were observed, and the length of closeproximity with no sign of aggression by the timethe interactions lasted. We didnot solicit killerwhales towards the sea lions. A predatory informationon interactions between killer whales interactioninvolved the killer whales behaving 328 K. Heise et al.

Figure 1. Thestudy area showing place names in the text and the locations where killer whale stomachcontents were recovered (see Table 1). In two cases(Barnes Lake, southeast Alaska, and on MontagueIsland, Prince William Sound), killer whales were found in close proximity, which is why only10 strandingsappear on themap. Gulf of Alaskatransients have been seen in southeast Alaska, butnot in British Columbia,and their range into o Vshorewaters is unknown(Ford & Ellis 1999). TheAT1 transients are generally found around Prince William Sound. The 144 Wlongitudemarks thedividing line between the western (declining) and eastern populations of Steller sea lions.

aggressivelytowards the sea lions by chasing or personalcommunication 1).Geneticanalyses of attackingthem. We expressedthe interaction sevencarcasses revealed that two animals were observationrate using the following index: oVshores,onean Alaskan resident from AB pod, andfour animals were transients (Barrett-Lennard, InteractionIndex = 2000).Genetic analyses were attempted on thetwo Numberof interactions observed whalesrecovered in 2001, but the samples were too Totalnumber of observer hours for all years #105 degradedfor successful extraction of DNA. Har- bourseal parts were found in all seven of the stomachsthat contained marine mammal remains, Results andSteller sea lion parts were found in two. Fifteen Stellersea lion  ippertags (including two with the Stomachcontents samenumber) from 14 Steller sea lions were found Table1 liststhe stomach contents of 12killerwhale inthe stomach of a killerwhale found dead on carcassesthat washed ashore in Alaska between MontagueIsland, Prince William Sound in 1992. 1990and 2001, and the locations of thesecarcasses All sealions had been tagged on MarmotIsland in isshown in Figure 1. Four killer whales were 1987(4), 1988 (9) and 1990 (1) as part of a individuallyidentiŽ able from photographs; two as transients (AT1and AT 19fromthe AT1 transient 1GraemeEllis, PaciŽc BiologicalStation, Department of subpopulation)and two as oVshores (GraemeEllis, Fisheriesand Oceans, Nanaimo, BC. April2001. Evidencefor killer whale predation on Steller sea lions 329

Table 2. Interactionsbetween killer whales and sea lions between 1935 and 1993 as reported by126questionnaire respondents.

Numberof killer whales/ sealion interactions witnessed 492 Numberof non-aggressive interactions (%) 441 (89.6%) Numberof reported non-lethal attacks by killer whales (%) 19 (3.9%) Numberof reported kills ofsea lions by killer whales (%) 32 (6.5%) Numberof interactions observed/ 100000 h ofsea time 12.3 Numberof non-predatory interactions observed/ 100000 h ofsea time 11.0 Numberof non-lethal attacks observed/ 100000 h ofseatime 0.5 Numberof lethal attacks observed/ 100000 h ofseatime 0.8 Numberof killer whale sightings observed/ 100000 h ofseatime 1100 Mediannumber of killer whale sightings/ year/observer(range 0– 150) 10.0 Mediankiller whale group size for allsightings (range 1 to45) 7.5 Mediankiller whale group size for allpredatory interactions 4.0 (range2 to20)

Table 3. Observer eVort andnumber of interactionsreported in each area between 1935 and 1993.

Typeof Interaction Observer Number Location Non-predatoryHarass Kill hours ofobservers

California,Washington, and Oregon 0 0 0 59 962 7 British Columbia 168 1012 1 023130 29 SoutheastAlaska 102 3 6 865 592 27 Gulfof Alaska 112 4101 409636 40 Bering Sea 2 2 1 321 255 18 Aleutians 57 0 3 105 360 5 Total 441 1932 3 784935 126

long-termstudy 2.Two ofthe tags were consecu- werecommercial Ž shers,24 were tourboat opera- tivelynumbered. 3 Ifthesea lions were eaten shortly tors,and the remaining were considered as ‘others’, beforethe killer whale’ s death,the maximum age of including oatplane pilots and recreational mostof the sea lions was four or Ž veyears when boaters.Mariner experience on the water ranged consumed.All ofthe tags were equally encrusted from1 to58 years, with a medianof 14 years, witha blackishsubstance that had to be scraped-o V 138days per year, and 10 hperday, and a meanof forthe numbers to be read. The killer whale 14.3years, 156 days per year, 10.3 h/ day.The stomachrecovered in the summer of 2000 in Prince peakof sighting activity occurred in July. Table 2 WilliamSound contained tags from two female summarizesthe attacks and kills (predatory inter- harbourseals that were tagged earlier in the year in actions)reported by respondents. Because the thesame area. One seal weighed 25.9 kg and the datafor the number of killer whale sightings and other48.9 kg at the time of tagging 4. groupsizes were strongly skewed, we reportthe medianresults, as well asthe range. Observers saw smallgroups of killerwhales most often, but occa- Surveyresponses sionallylarger groups of up to 45 animals were We received126 completed questionnaires from seen.Table 3 summarizesthe regional geographic mariners.Fifty respondents were researchers, 38 distributionof respondents from California to 2TomLoughlin, National Marine Mammal Laboratory, Alaskaand the number of interactionswitnessed in NMFS, Seattle,WA, andDon Calkins, Alaska Sea Life each area. Centre,Seward, AK. April2001. Of 492reported interactions between killer whalesand Steller sea lions, 441 (89.6%) were 3 TagNumbers 108, 174, 240, 305, 412, 429, 430, 439, 485, non-predatoryin nature. Those non-predatory 507,545, 589, 630, and 806. interactionsincluded two cases of sea lions harass- 4KathyFrost, AlaskaDepartment of Fish andGame, ingkiller whales. Although predatory attacks were Fairbanks,Alaska. November 2000. reportedfor only 10% of allinteractions, over 60% 330 K. Heise et al.

Table 4. Reportedage classes of southern sea lions inArgentina (Hoelzel, 1991) and of Steller sealions in British Columbiaand Alaska attacked by killer whales (this study).

Southernsea lions Stellersea lions Non-lethal Non-lethal attacks Kills attacks Kills

Pups 127 (54%) 82 (99%) 0 (0%) 2 (6%) Subadults 13 (6%) 1 (1%) 3 (16%) 5 (16%) Adults 96 (41%) 0 (0%) 11 (58%) 16 (50%) Not Stated 5 (26%) 9 (28%) Total 236 83 19 32

werelethal for the sea lion. The duration of the thatkiller whales were not seen in the area at the predatoryattack, combined with the time taken to timeof tagging. It ispossiblethat the sea lion pups consumethe sea lion, ranged from 1– 2 h.The dispersedfrom the rookery as a group,which was majorityof attacks and kills reported were on small thenattacked by the killer whale somewhere adultsea lions (n =27).Only two pup kills were betweenMarmot Island and Prince William Sound. reported(Table 4). Pinniped researchers rarely Oneof the sea lions, tagged in 1990, was at most observedpredatory interactions between killer twoyears old at the time it was killed by the killer whalesand Steller sea lions, and only one fatal whale. attackwas reported, by a pinnipedresearcher in Harbourseals were the predominant prey item thePriblof Islands. The sea lion was consuming a foundin all seven killer whales stomachs that furseal pup when it was attacked by the killer containedmarine mammal remains, and they are whales. likelya moreimportant prey item for killer whales thanare Steller sea lions. In areviewof killerwhale interactionswith marine mammals from around the Discussion world,harbour seals were the most commonly Dietsof killer whales reportedprey of killer whales in thenorthern hemi- Givenrecent concern over the decline of Stellersea sphere (JeVerson et al.,1991).Most predation by lionsin western Alaska, it is noteworthy that only westcoast transients witnessed by mariners from twoof the twelve killer whale’ stomachs examined FrederickSound, Alaska to WashingtonState also inthis study contained Steller sea lion remains. Of involvedharbour seals (58%, Ford et al., (1998). specialinterest were the 15  ippertags from 14 Only9% were kills of sealions (both California and Stellersea lions that were recovered from a whale Steller).The most common marine mammal prey of thatwas genetically identiŽ ed as a Gulfof Alaska killerwhales in Alaska reported by Matkin & transient (Table1). Two ofthe tags had the same Saulitis(1994) were harbour seals and beluga number.All ofthe tags were equally encrusted with whales (Delphinapterusleucas ).Possiblehunting ablacksubstance (possibly due to a chemical specializationby transient killer whales on harbour reactionwith stomach acids) suggesting that the sea sealshas been observed in southern British Colum- lionswere not killed recently. Two oftherecovered bia,where certain groups of transients appear to tagswere consecutively numbered, which is inter- foragespeciŽ cally for them (Baird & Dill,1995), estinggiven that 800 sea lion pups were tagged in largelyignoring the Steller and California sea lions totalon Marmot Island 5.However,it is not poss- thathaul-out in the same area. A similarsituation ibleto conŽ rm whether the sea lions were eaten by couldoccur in Alaska. Members of the AT1 tran- thekiller whale shortly after tagging, or whether sientgroup, primarily seen in southwestern Prince theywere taken individually or as a groupsome- WilliamSound and Kenai Fjords, seem to forage timelater. Pups normally remain on shore for the primarilyfor Dall’ s porpoises( Phocoenoidesdalli ) Žrstmonth of life.T. Loughlin(personal communi- andharbour seals (Saulitis et al.,2000).They cation6)reportedthat few sea lions went into the frequentlypass by Steller sea lions without initiat- waterimmediately after the tagging and branding ingany obvious interactions. The Gulf of Alaska processin 1987 and 1988 on Marmot Island, and transientsare less commonly seen in Prince William Sound,but some of its members (the AC group, 5,6TomLoughlin, National Marine Mammal Laboratory, basedon Heise et al.,1992)have been observed NMFS, Seattle,WA, April 2001. attackingSteller sea lions there (R. Corcoran, Evidencefor killer whale predation on Steller sea lions 331

C.Thoma,T. Edwards,personal communication 7). thewhales were there (Bain, personal observation), Theone Gulf of Alaska transient identiŽ ed in this yetwere apparently not eaten by the two whales studyhad Steller sea lion remains in its stomach that died. (Table 1). Evidencefrom stomach content analysis of killer Ourresults support Bigg et al.’s(1987)segrega- whalesaround the world provides evidence that tionof killer whales into at least two forms, those feedingspecialization of killer whales is common; thateat Ž sh( residents)andthose that eat marine virtuallyall stomach contents reported contained mammals (transients).Fishremains were not found eithermarine mammals or Ž sh,but not both inany of thestomachs containing marine mammal (Zenkovich,1938; Tomilin, 1957; Nishiwaki & remains.However, the relative importance of prey Handa,1958; Betesheva, 1961; Rice, 1968;Jonsgard speciesdetermined from stomach content analysis &Lyshoel,1970). Evidence from Soviet whaling shouldbe interpreted cautiously, because parts of dataalso suggest segregation between Ž sh-eating thebody such as skin,  esh,bones, claws, and andmammal-eating killer whales in the Antarctic whiskerscan be digested and expelled at di Verent (Berzin& Vladimirov,1982). Of 785killer whales rates.If prey are not swallowed whole, the parts collected,629 (80%) were of a smaller‘ yellow’form eatenmay in uence the analysis. For example, the foundnear shore and 156 (20%) were of a larger whalerecovered from Culross Island in 1990 had ‘white’form found further o Vshore.Ninety-nine thetail  ukeand patches of skin from a Dall’s percentof the stomach contents from the yellow porpoisein its stomach, yet contained no porpoise killerwhales were Ž sh,and 90% of the stomach bones.Had digestion continued much further, it is contentsfrom the white animals were marine mam- unlikelywe wouldhave identiŽ ed porpoise as a mals.Ivashin (1981 in Mikhalev et al., (1981)) fooditem. Interestingly, Tomilin (1957) reported reportedon 362 killer whale stomachs. Sixty per- thatkiller whales often ate only the  ukeportion of centcontained only Ž sh,30% contained minke porpoises.Prey sharing amongst killer whales, as whale (Balaenopteraacutorostrata )remains,5% describedby Guinet et al.(2000)and Pitman et al. containedsquid, and 4% contained pinnipeds. (2003),may also in uence which portions of a carcassare consumed by a killerwhale. The whale Mariner’s survey inthis study recovered from Culross Island had seal Surprisingly,few of the questionnaire respondents skinin its stomach, yet on other occasions, killer witnessedpredatory attacks by killer whales on whaleshave been observed removing and discard- Stellersea lions, and many reports were second or ingthe skin of harbour seals before consuming third-handaccounts (Table 2). The highest number them(Barrett-Lennard, Heise, unpublished data). ofkills reported by an observer was four, and Interpretationof the data is further complicated thisindividual worked along the west coast of becausethe cause of death could not be determined VancouverIsland, and had spent almost 300 000 h forany of the whales and it is possible that the onthe water and had over 1700 sightings of killer animalswere not feeding normally at the time of whalesover the course of hiscareer. Mariners spent death.However, our results are consistent with anaverage of 8100 h onthe water for each obser- observedkills by transients in British Columbia vationof a killerwhale/ sealion interaction and reportedby Ford & Ellis (1999). 125000 h foreach observation of a fatalattack on a Thefeeding ecology of the third form of killer sealion (Table 2). These averages include responses whalesknown as oVshores isless well understood. fromresearchers conducting studies on eitherkiller Ford et al.(2000)suggested that o Vshorekiller whalesor sea lions, who were well-situated to see whalesfeed principally on Ž sh,because they travel interactions.At thetime of the survey, the Ž ve inlarge groups and are acoustically active. The authorsof this paper had spent a totalof approxi- stomachcontents recovered from the two o Vshore mately155 000 h onthe water searching for and killerwhales in this study provide concrete evidence observingkiller whales, and none saw a fatalattack thatthey do eat Ž sh.One whale had salmon bones ona Stellersea lion. Collectively, we observed28 inits stomach, and the other had sculpin (family non-predatoryinteractions and only one case of Cottidae),as well assome pieces of crab shell and harassmentof sealions by killer whales. eelgrass.These whales were part of a largergroup Thequestionnaire results indicated that the often whales that were trapped in Barnes Lake, majorityof attacks and kills witnessed by mariners Alaskafor six to ten weeks before they died. involvedadult sea lions (Table 4). However,the fact Harbourseals were also present in the lake while thatthese records are based on opportunistic obser- vationsof predation,rather than on detailedobser- vationsof killer whale foraging behaviour, could havebiased the study in favour of predation on 7RichCorcoran, Solomon Gulch Fish Hatchery,Valdez, adultsea lions since such attacks are highly visible. AK.April2001. Killerwhales caused a greatdeal of splashing 332 K. Heise et al. duringattacks of adultsea lions, by breaching on or leonine).Inview of the low number of interactions nearthe sea lions, and by slashing at them with witnessedin the eastern North PaciŽ c, we recom- theirtail  ukes.This conspicuous activity generally mendthat researchers make a concertede Vort in lastedover 1 handwas therefore likely to attract thefuture to note the behaviour of killer whales theattention of passing mariners. By comparison, aroundSteller sea lions, particularly in areas where attacksof younger sea lions were probably much sealion populations are declining. Observations lessobvious. Harbour seals, which are similar in shouldinclude scans of the water surface after sizeto small sea lions, are usually killed under water seeingkiller whales mill in the area (to look for bykiller whales. Blood, oil and/ orfragments of bloodor blubberfragments) and if possible identi- blubberare usually the only evidence of a fatal Žcationphotographs of killer whales should be attackon harbour seals. made. Such eVortscould provide valuable insight We receivedkiller whale identiŽ cation photo- intothe question of whether or not transient killer graphsfrom several respondents and in some cases whalesare responsible for the decline of Steller sea identiŽed the whales concerned. All whalesident- lionsin western Alaska. iŽed in attacks on sea lions or other marine mam- malswere transient killer whales. The median group sizefor all killer whale sightings was 7.5; however, Acknowledgments themedian size of groups that attacked Steller sea Sincerethanks are given to the many mariners who lionswas four (Table 2). In British Columbia, this generouslyresponded to our survey, and to Rich diVerencecould re ect the di Verencebetween the Ferrero,Alan Springer, Mike Williams, Kate averagesizes of resident (5– 50) and transient (1– 7) Wynne,Dan Monson, Donna Willoya and Liana killerwhale groups (Bigg et al.,1987).In southern Jackfor graciously donating information on the Alaska,transients seldom travel in groups larger stomachcontents of strandedkiller whales. We also thanŽ veanimals (Matkin et al., 1999). thankthe late Bud Fay, Elaine Humphries, William Pinnipedresearchers who have spent time on sea A. Walker,and PaciŽ c IdentiŽcations (Victoria, lionrookeries rarely witnessed killer whale attacks. BC) foridentifying stomach contents for us, as well Itis possible that transient whales foraging near sea asDon Calkins and Tom Loughlin for providing lionhaul-outs and rookeries may be particularly informationon Steller sea lion tags, and Kathy diYcultto observe, since they hunt by stealth to Frostfor supplying information on harbour seal avoidthe risk of alerting their prey (Barrett- tags.We aregrateful to Steve Raverty who pro- Lennard et al.,1996).Indeed, the only pinniped videdsuggestions on the possible causes of researcherto observe a fatalattack on a Stellersea encrustedmaterial on the sea lion tags and to Doug lionwas on a furseal rookery at the time, and the Sandilandsfor Figure 1. We appreciatethe e Vorts sealion was preoccupied consuming a furseal pup ofGraeme Ellis, who photo-identiŽ ed four of the whenit was attacked by the killer whales. killerwhale carcasses. We arevery grateful for the Thepattern of killer whale predation on species Žnancialsupport of the North Gulf Oceanic similarto Steller sea lions, and the remains of Society,and the North PaciŽ c UniversitiesMarine sub-adultsea lions found in twoof the twelve killer MammalConsortium while conducting this work. whalestomachs, suggest that a higherproportion of We alsothank Don Calkins, Jeanette Thomas and pupsand juveniles may be killed than are re ected ananonymous reviewer for their comments on this inthe questionnaire data. More research is needed manuscript. todetermine whether killer whale predation on Stellersea lions has signiŽ cant population level eVects.We suspectthat killer whale predation on LiteratureCited sealion pups and juveniles peaks while animals are Baird,R. W. (1994)Foraging behaviour and ecology of congregatedat rookery sites. The peak of sighting transient killerwhales ( Orcinusorca ).Ph.D.Thesis. activityof killer whales and of observer activity SimonFraser University. Vancouver. 157 pp. occurredin July, coinciding with the period when Baird,R. W.&L.M. Dill. (1995)Occurrence and mostSteller sea lion pups leave the rookeries behaviourof transient killer whales: seasonal and (Sandegren,1970). Several observers reported that pod-speciŽc variability,foraging behaviour and prey killerwhales spent more time near haul-out and handling. CanadianJournal of Zoology 73, 1300–1311. near-shoreareas during the pupping season than Barrett-Lennard,L. G.(2000)Population structure and duringthe rest of the year. Researchers in other matingpatterns of killer whales (Orcinus orca) are areasand on otherspecies have reported that pups revealedby DNAanalysis. Ph.D. Thesis, Universityof British Columbia,Vancouver, B.C. Canada.97 pp. aremost frequently taken by killer whales (Baird, Barrett-Lennard,L. G.,Ford, J. K.B.&Heise,K. (1996) 1994harbour seals; Hoelzel, 1991 southern sea lions Themixed blessing of echolocation:di Verencesin sonar Otaria avescens ;Lopez& Lopez,1985 southern useby Ž sh-eatingand mammal-eating killer whales. sealions and southern elephant seals Mirounga AnimalBehaviour 51, 553–565. Evidencefor killer whale predation on Steller sea lions 333

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