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Suspected Surplus Killing of Harbor Seal Pups by Killer Whales

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Suspected Surplus Killing of Harbor Seal Pups by Killer Whales 150 NORTHWESTERN NATURALIST 86(3) SOWLS AL, DEGANGE AR, NELSON JW, LESTER GS. com (PJC); Carter Biological Consulting, 1015 1980. Catalog of California seabird colonies. Hampshire Road, Victoria, British Columbia V8S Washington, DC: US Fish and Wildlife Service, 4S8 Canada (HRC); US Fish and Wildlife Service, Biological Services Program, FWS/OBS-78/78. San Francisco Bay National Wildlife Refuge Com- 253 p. plex, PO Box 524, Newark, California 94560 USA TAKEKAWA JE, CARTER HR, HARVEY TE. 1990. De- cline of the common murre in central California, (GJM, MWP); Present Address (MWP): US Fish 1980±1986. In: SG Sealy, editor. Auks at sea. Stud- and Wildlife Service, Red Rock Lakes National ies in Avian Biology 14:149±163. Wildlife Refuge, 27820 South Centennial Road, Lima, Montana 59739 USA. Submitted 9 March Department of Wildlife, Humboldt State University, 2005, accepted 27 July 2005. Corresponding Editor: Arcata, California 95521 USA; philpcapitolo@hotmail. CJ Ralph. NORTHWESTERN NATURALIST 86:150±154 WINTER 2005 SUSPECTED SURPLUS KILLING OF HARBOR SEAL PUPS (PHOCA VITULINA) BY KILLER WHALES (ORCINUS ORCA) JOSEPH KGAYDOS,STEPHEN RAVERTY,ROBIN WBAIRD, AND RICHARD WOSBORNE Key words: harbor seal, killer whale, Orci- novel mortality pattern in harbor seals (Phoca nus orca, Phoca vitulina, surplus killing, preda- vitulina) that strongly suggests 1 or more in- tion, disease, San Juan Islands, Washington dividuals from 1 of these ecotypes killed seal pups for reasons other than consumption. Within the inland waters of Washington State As part of an ongoing disease-screening pro- and southern British Columbia Province, 3 dis- ject, complete postmortem examinations were tinct ecotypes of killer whales (Orcinus orca) oc- performed on dead marine mammals in suit- cur. The better known ``resident'' and ``tran- able condition from San Juan County, Washing- sient'' populations each display unique genetic ton (48.68N, 123.08W). Necropsies were orient- (Hoelzel and others 2002), dietary (Baird and ed toward determining the cause of morbidity Dill 1995; Ford and others 1998), behavioral or mortality and to survey for speci®c patho- (Baird 2000), vocal (Ford 1990) and morpholog- gens. Routinely complete examinations were ical differences (Baird and Stacey 1988). The performed and gross observations were record- resident pods, also known as the ``southern res- ed. Tissue samples were preserved in 10% neu- ident'' population, eat primarily ®sh and occur tral buffered formalin, processed, and evalu- in large stable groups (Bigg and others 1990). ated microscopically. Kidney and liver were Transients feed primarily on other marine screened for heavy metals, and other ancillary mammals and occur in smaller and less stable tests such as aerobic bacterial culture, poly- groups (Baird and Dill 1995, 1996; Baird and merase chain reaction, and attempted virus iso- Whitehead 2000; Baird 2000). The 3rd popula- lation were performed as indicated. tion, which is seen occasionally in the area, is Since 1977, The Whale Museum (Friday Har- called the ``offshore'' ecotype (Wiles 2004). bor, Washington) has maintained a database of These killer whales are believed to be primarily public and scienti®c sightings of killer whales ®sh eaters (Hoelzel and others 2002) that are and other cetaceans in Washington and south- smaller in size than other ectotypes and genet- ern British Columbia inland waters. In 1981, ically distinct from both residents and tran- the marine mammal sighting network was ex- sients, although more closely related to the res- panded to include stranding data for San Juan ident killer whales. We recently identi®ed a County. These databases provided stranding WINTER 2005 GENERAL NOTES 151 TABLE 1. Characteristics and gross lesions of harbor seal pup carcasses found in San Juan County, WA, September and November, 2002. Seal pups were killed but were largely uneaten. Lacerated Puncture Date found Gender Decapitation ¯ippers wounds Whale ecotypes sighted 3 Sep male partial unilateral none residents, transients, offshores 13 Sep female complete no yes residents, transients, offshores 1Nov female partial bilateral yes residents, transients 10 Nov female complete bilateral yes residents, transients 10 Nov male partial no yes residents, transients data as well as information on the presence of (Table 1). Aside from the nutritional status of 1 killer whale ecotypes within the region. All animal, gross and microscopic examination, sighting and stranding data are recorded by trace mineral analysis of liver and kidney, and species, date, location, and by quadrants of ap- aerobic bacterial culture of multiple organs did proximately 2 km2 (Heimlich-Boran 1986, 1988; not reveal underlying disease, suggesting that Felleman and others 1991; Olson 1998). The ob- seals were healthy at the time of predation. servers contributing to the killer whale data- Serrated lacerations consistent with shark base include the untrained general public, long bites were not seen on any of the carcasses. time shoreline residents, trained marine natu- Margins of the partially to completely truncat- ralists, and marine mammalogists with exten- ed torsos or heads were sharply delineated and sive photo-identi®cation experience. Observa- variably hemorrhagic. Based on wound mea- tions submitted by the untrained general pub- surements and comparison with archived killer lic are only included in the database when they whale skulls (The Whale Museum, unpubl. are veri®ed through an interview or have a con- data), the dimensions and spacing of the semi- vincing description of the species that can be elliptical pattern of puncture wounds seen on 4 corroborated with at least 1 other sighting. of the 5 carcasses were consistent with the Stranding data are collected by a group of mandibular span and dentition of killer trained volunteers. whales. Sighting records con®rm the presence Between 3 September and 10 November 2002, of resident (all 3 pods; J, K. and L) and transient 13 harbor seal pups were found dead on beach- killer whales in the region during both periods es in San Juan County during 2 distinct time and the presence of offshores only during the spans: from September 3 to September 13 and 1st time period (Table 1). Consequently, we hy- November 1 to November 10 (Table 1). Five car- pothesize that 1 or more killer whales were casses (3 females and 2 males) had similar likely responsible for all 5 killings. gross lesions indicating predation without con- Between 1982 and 2002, 281 dead harbor sumption (Table 1). Speci®cally, carcasses had seals were reported in San Juan County (The various combinations of the following gross le- Whale Museum, unpubl. data), with no injuries sions: partial or complete decapitation, lacera- of this type found. We hypothesize these events tions to the hind ¯ippers, and 1- to 3-cm-di- represent a novel pattern of killing without in- ameter, semi-elliptical abrasions and occasion- tent to eat and that 1 or more transient killer al punctures on the thorax and/or abdomen whales were most likely responsible. In this re- (Fig. 1 and Table 1). In 1 animal, abdominal vis- gion transient killer whales are the main pred- cera herniated from an abdominal puncture, ator of harbor seals and seals are the whales' and 2 additional animals had large amounts of most important prey (Baird and Dill 1996). Ad- blood in their chest cavities (hemothorax). The ditionally, the pattern of these seals being acute hemorrhage noted in several animals killed in 2 short and distinct periods matches suggested that trauma was antemortem and the travel patterns of transient whales that of- the proximate cause of death. Four carcasses ten make relatively brief appearances in the in- were in good nutritional condition as evi- land waters of Washington and British Colum- denced by adequate subcutaneous and visceral bia and then are not seen for sometimes years adipose stores and only the seal pup found on at a time (Baird and Dill 1995). If 1 or more 1 November, 2002 was moderately emaciated transients were responsible for these mortality 152 NORTHWESTERN NATURALIST 86(3) FIGURE 1. Photograph of a harbor seal pup (2002- SJ036) with lesions suggestive of killer whale predation without consumption. Note the missing head, arrows pointing to the puncture wounds arranged in a semi- elliptical pattern, and the lacerated hind ¯ippers. Photo courtesy of KC Balcomb III, Center for Whale Re- search, Friday Harbor, Washington. events, this marks a distinct pattern variation whales in the region. The killing of these har- from prior observations. Previous observations bor seal pups appears to be for reasons other by Baird and Dill (1996) detected only 4 in- than consumption. Evidence of bite wounds to stances when .1% of a pinneped carcass was the head and ¯ippers of all animals and to the apparently abandoned by transient killer thorax and abdomen of 4 of the 5 animals in- whales. In the 5 cases we identi®ed, the entire dicates that whales had ample opportunity to carcasses, excluding all or part of the head, consume these seals, but did not do so. Simi- were abandoned. The cases observed by Baird larly, Stacey and others (1990) observed tran- and Dill (1996) were found in August and Sep- sients killing or wounding seabirds in the same tember, the period when harbor seal pups in study area without consuming them. the area are being weaned, are foraging on Because they were sighted during only 1 of their own, and are most vulnerable to preda- the 2 time periods in which these seal carcasses tion. Food intake rates for transients are more were found and because they are believed to be than double during this period compared to ®sh eaters (Hoelzel and others 2002), it is un- the remainder of the year, and prey handling likely that offshore ecotype killer whales were times are also approximately double during responsible for these killings. Although resi- this period, presumably because the whales are dent killer whales feed primarily on ®sh (Ford more than meeting their energetic needs and and others 1998), they have been reported to extended prey handling is serving some other harass, but not kill harbor seals and porpoises purpose (Baird and Dill 1995).
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