Dietary specialization in two sympatric populations of killer whales (Orcinus orca) in coastal British Columbia and adjacent waters

Abstract

Two forms of killer whale (Orcinus orca), resident and transient, occur sympatrically in coastal waters of British Columbia, Washington State, and southeastern Alaska. The two forms do not mix, and differ in seasonal distribution, social structure, and behaviour. These distinctions have been attributed to apparent differences in diet, although no comprehensive comparative analysis of the diets of the two forms had been undertaken. Here we present such an analysis, based on field observations of predation and on the stomach contents of stranded killer whales collected over a 20-year period. In total, 22 species of fish and 1 species of squid were documented in the diet of resident-type killer whales; 12 of these are previously unrecorded as prey of O. orca. Despite the diversity of fish species taken, resident whales have a clear preference for salmon prey. In field observations of feeding, 96% of fish taken were salmonids. Six species of salmonids were identified from prey fragments, with chinook salmon (Oncorhynchus tshawytscha) being the most common. The stomach contents of stranded residents also indicated a preference for chinook salmon. On rare occasions, resident whales were seen to harass marine mammals, but no kills were confirmed and no mammalian remains were found in the stomachs of stranded residents. Transient killer whales were observed to prey only on pinnipeds, cetaceans, and seabirds. Six mammal species were taken, with over half of observed attacks involving harbour seals (Phoca vitulina). Seabirds do not appear to represent a significant prey resource. This study thus reveals the existence of strikingly divergent prey preferences of resident and transient killer whales, which are reflected in distinctive foraging strategies and related sociobiological traits of these sympatric populations.

Phylogenomics of the killer whale indicates ecotype divergence in sympatry

“…Ecotypes may also exhibit differences in social structure, morphology, behavior and vocal signatures (see for review de Bruyn et al (2013)). In the North Pacific, the resident and transient ecotypes occupy largely sympatric distribution ranges (Ford et al, 2000), but specialize on very different prey resources (fish and marine mammals, respectively; Ford et al, 1998;Krahn et al, 2007), are genetically differentiated (Hoelzel et al, 1998(Hoelzel et al, , 2002(Hoelzel et al, , 2007, and exhibit different social organization (Ford et al, 2000), mating systems (Pilot et al, 2010) and vocal behavior (Yurk et al, 2002;Deecke et al, 2005). Less is known about the 'offshore' ecotype, however, our data indicate that we need to consider their differentiation in sympatry as well.…”

Use of carbon and nitrogen stable isotopes to study the feeding ecology of small coastal cetacean populations in southern Brazil

“…In the literature, there are several studies where differences were found in feeding habits of populations of the same species living in distinct habitats: killer whales (Orcinus orca) in the Pacific Northwest on the border between the US and Canada (Ford et al 1998); sea lions (Eumetopias jubatus) in Alaska (Kurle & Gudmundson 2007); and dolphin species in Australia (Owen et al 2011).…”

“…Understanding the feeding ecology of specific species is considered essential in order to implement effective conservation measures (Owen et al 2011). Franciscana individuals from a separate population found in the nearshore area, located outside of the bay, were also included in the analysis in order to check for variation in feeding strategies of the same species in relation to habitat differences (Ford et al 1998). The diet of these populations was characterized to complement the findings of a recent study on their feeding strategies, which analyzed the stomach contents of specimens collected in Babitonga Bay (Cremer et al 2012).…”

Orchestration : the movement and vocal behavior of free-ranging Norwegian killer whales (Orcinus orca)

“…They are characterized by stable social groups, populationspecific foraging strategies and vocal repertoires that are likely to be transmitted through learning. Their feeding behaviors include hunting fishes, cephalopods, sea turtles, sea birds, mustelids, pinnipeds and cetaceans (e.g., Martinez & Klinghammer, 1970;Christensen, 1978;Smith et al, 1981;Hoelzel, 1991;Jefferson et al, 1991;Baird et al, 1992;Similä & Ugarte, 1993;Matkin & Saulitis, 1994;Baird & Dill, 1995;Baird & Dill, 1996;Fertl et al, 1996;Similä et al, 1996;Ford et al, 1998;Baird & Whitehead, 2000;Ford & Ellis, 2006;Simon et al, 2006). The diversity of hunting strategies parallels the diversity of prey on which they feed.…”