Norwegian killer whale movements reflect their different prey types

Abstract

Norwegian killer whales (Orcinus orca) are thought to be generalists that feed primarily on fish, but some individuals have been observed targeting pinnipeds. In the study reported here, field observations of foraging behaviours formed the basis of a priori classification as either seal-eaters or fish-eaters. Concurrent collection of photographic identification and biopsies for stable isotope analysis were used to validate prey choice classification. We found through satellite tracking that whales classified as seal-eaters took different paths south after leaving the northern fjords seemingly optimized for pinniped predation. Specifically, we found that seal-eaters took paths that tightly followed the coastline, remaining on average 6.9 ± 10.7 km (mean ± SD, n = 315) from the coast, whereas fish-eaters moved offshore along the continental shelf, travelling on average 45.1 ± 30.2 km (n = 1534) from the coast. We also found that, compared to fish-eaters, seal-eaters displayed more movements directed towards harbour seal haul-outs (p = 0.001). As expected, our data suggest that the fish-eaters feed primarily on fish, whilst seal-eaters appear to opportunistically use diverse foraging strategies optimized for either fish or seals based on availability and preference. Our findings demonstrate that tracking data can elucidate Norwegian killer whale movements associated with different prey types and selection.