First record of killer whales (Orcinus orca) feeding on Atlantic salmon (Salmo salar) in northern Norway suggest a multi-prey feeding type

Abstract

Killer whales (Orcinus orca) are widely distributed throughout the world’s oceans, where they forage on a large variety of different prey species ranging from fish to marine mammals and birds (Forney & Wade, 2007). It has been shown that feeding ecology is of central importance to the killer whales’ social network. Food availability has a direct influence on group structure, and it is under debate to what extent the sociality of killer whales is flexible enough to adapt to local ecological conditions (Beck et al., 2012; Foster et al., 2012). In the north-east Pacific some highly specialized sympatric populations have evolved, with resident groups foraging on salmonids, transient groups feeding on marine mammals, and offshore groups feeding on fish such as sharks (Ford et al., 1998). Often these specialized populations show high site and time fidelity, such as the northern and southern resident lines in the north-east Pacific which feed primarily on Chinook salmon (Oncorhynchus tshawytscha) (Ford & Ellis, 2006). The survival of these killer whales seems to depend on the Chinook salmon’s year-round abundance (Ford et al., 2010). In the north-east Atlantic at least two different populations have been recently identified, including a herring feeding population in Norway and Iceland (Foote et al., 2011). In addition, two ecotypes of North Atlantic killer whales with morphological differences were determined: Type 1, with severe apical tooth wear, a generalist type with a length of up to 6.6 m, presumably feeding on fish and to some extent on seals; and Type 2, with no apical tooth wear, a highly specialized type with a length of up to 8.5 m, presumably feeding on other whales (Foote et al., 2009). Further, marine mammal–feeding killer whales were described from Scottish inshore waters (Bolt et al., 2009). In Irish waters, killer whales mostly feed on fish; stomach contents revealed a diet of siphonostomatoid copepods (Cecrops latreilli), ocean sunfish (Mola mola), mullet (Chelon labrosus) and salmon (Salmo salar) (Ryan & Wilson, 2003; McHugh et al., 2007, Ryan & Holmes, 2012), and in the 1970s a killer whale was observed hunting salmon in the Lough Foyle Estuary (Wilson & Pitcher, 1979). In the waters around the British Isles, herring and salmon were identified as prey species (Evans, 1988). Thus, we are only starting to understand the variety of the North Atlantic killer whales’ feeding ecology, especially the degree of prey specializations. Killer whales studied in Norway feed mostly on herring, with tagged animals showing high site and time fidelity to Fig. 1. Salmon caught by killer whale in Oksfjord, northern Norway, 2011. Photograph kindly provided by Geir Notnes.