![]() ![]() Migrating chum salmon ( Oncorhynchus keta), the second largest of the Pacific salmonids, are important prey during autumn although Chinook are targeted preferentially when available. Resident killer whales are primarily salmonid predators that show strong selectivity for Chinook salmon ( Oncorhynchus tshawytscha), probably because of this species' comparatively large size, high lipid content, and year-round availability in the whales' coastal habitat ( Ford & Ellis 2006). Despite overlapping ranges, the two populations do not mix. Distinct northern and southern populations of resident killer whales (219 and 84 whales, respectively in 2005) are found in the region. Population demographics, including births and mortalities, can thus be assessed accurately each year. These whales live in stable societies comprised of matrilineal groups from which individuals rarely, if ever, disperse. Resident killer whales that frequent coastal waters of British Columbia, Canada and Washington State, USA, have been the focus of annual field studies since the early 1970s, based on photo-identification of individuals from natural markings ( Bigg et al. In this study, we examined potential resource limitation in two populations of resident killer whales in relation to the fluctuating availability of their principal prey species over a 25-year period. Such ecological specialization has important implications for population regulation of these top predators through bottom-up processes, particularly if they have limited ability to switch to alternative prey types. Similarly specialized mammal- and fish-feeding ecotypes of killer whales have been reported in Antarctic waters ( LeDuc et al. ![]() ![]() The poorly known offshore killer whales appear to be fish feeders, though stable isotope and fatty acid profiles from tissue samples suggest a diet distinct from that of residents ( Herman et al. Resident killer whales feed on fishes, particularly salmon, but do not prey on marine mammals, while transient killer whales prey on marine mammals but do not feed on fishes ( Ford et al. ![]() The coastal waters of the northeastern Pacific Ocean are home to three genetically distinct and socially isolated forms of killer whales, known as residents, transients and offshores ( Ford et al. However, different populations of killer whales can have remarkably specialized foraging behaviours and diets, and these populations may coexist sympatrically. As a species, it feeds on a diverse array of prey types, including many species of marine fishes, invertebrates, mammals, turtles and birds ( Ford 2009). The killer whale ( Orcinus orca) is the oceans' apex non-human predator. Other ecologically specialized killer whale populations may be similarly constrained to a narrow range of prey species by culturally inherited foraging strategies, and thus are limited in their ability to adapt rapidly to changing prey availability. Our results suggest that, although these killer whales may consume a variety of fish species, they are highly specialized and dependent on this single salmonid species to an extent that it is a limiting factor in their population dynamics. Here we show, using 25 years of demographic data from two populations of fish-eating killer whales in the northeastern Pacific Ocean, that population trends are driven largely by changes in survival, and that survival rates are strongly correlated with the availability of their principal prey species, Chinook salmon ( Oncorhynchus tshawytscha). Although the possible top-down effects of killer whale predation on populations of their prey have received much recent attention, little is known of how the abundance of these predators may be limited by bottom-up processes. Killer whales ( Orcinus orca) are large predators that occupy the top trophic position in the world's oceans and as such may have important roles in marine ecosystem dynamics. ![]()
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