Vocal behaviour of Norwegian killer whales during carousel and seiner foraging on spring-spawning herring

Norwegian killer whales (Orcinus orca) use different techniques to forage on spring-spawning herring. Two of the commonly observed techniques are carousel feeding, a cooperative feeding method, and seiner feeding, a noncooperative method. During seiner foraging, large groups of whales forage on herring discards around the nets or on discarded by-catch of fishing boats. Very little is known about the acoustic behaviour during these foraging contexts. The aim of this study was to examine possible differences in killer whale acoustic behaviour during both foraging contexts using simple sound analysis techniques. Calling, echolocation, and tail-slap activities were measured and compared between foraging contexts. Of these, calling and tail-slap activities were higher during carousel feeding, whereas echolocation activity increased with the number of individuals, irrespective of foraging context. No call types were used exclusively during a particular foraging context. A difference in mean occurrence of one call type was detected; call type N21 occurred more often during seiner foraging than during carousel foraging contexts. We suggest that the sequence of call types, rather than the use of isolated call types, is of greater importance in the coordination of group movements during carousel foraging.

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