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Killer whale movements on the Norwegian shelf are associated with herring density

Emma F. Vogel*, Martin Biuw, Marie-Anne Blanchet, Ian D. Jonsen, Evert Mul, Espen Johnsen, Solfrid Saœtre Hjøllo, Morten Tange Olsen, Rune Dietz, Audun Rikardsen

*Corresponding author:

ABSTRACT: Killer whales Orcinus orca have a cosmopolitan distribution with a broad diet ranging from fish to marine mammals. In Norway, killer whales are regularly observed feeding on overwintering Norwegian spring spawning (NSS) herring Clupea harengus inside the fjords. However, their offshore foraging behavior and distribution is less well understood. In particular, it is not known to what degree they rely on the NSS herring stock when the herring move to deeper offshore waters. Satellite telemetry data from 29 male killer whales was analyzed to assess whether their offshore foraging behaviour is linked to herring distribution. Unlike most marine predator–prey studies that use indirect proxies for prey abundance and distribution, our study utilized two herring density estimates based on (1) direct observations from acoustic trawl survey data and (2) simulations from a fully coupled ecosystem model. Mixed effects models were used to infer the effect of herring density and light intensity on whales’ movement patterns. Our results suggest that killer whales follow NSS herring over more than 1000 km from their inshore overwintering areas to offshore spawning grounds. All whales changed from fast directed, to slow non-directed movement when herring density increased, although individuals had different propensities towards movement. Our data indicated that whales continue to feed on herring along the Norwegian shelf. We conclude that NSS herring constitutes an important prey resource for at least some killer whales in the northeastern Atlantic, not only during the herring overwintering, but also subsequently throughout the herring spawning migration period.