Studying killer whale predation in the field: A sound approach to detecting kills.

Abstract

Killer whales are top predators in many marine ecosystems and play an important role in regulating marine mammal populations. Studies investigating the feeding ecology of mammal-eating killer whales have been compromised by our inability to consistently identify kills using visual observation. This study aimed to identify predation events using acoustic rather than visual cues. Recordings were made during focal follows of transient killer whales in British Columbia and Alaska using towable hydrophones. During ten follows, kills were confirmed using traditional methods and these recordings were scanned for echolocation clicks, whistles, pulsed calls, and characteristic sounds apparently generated during prey handling. All acoustic analyses were done blind to the behavior context. A discriminant function analysis showed that kills could be identified from the acoustic record with high certainty, primarily because rates of all sound types increased after a kill. Prey-handling sounds proved to be the best indicator that an attack was successful. An analysis of structural parameters of prey-handling sounds and estimate of their source level is also presented. These results show that acoustic cues can improve our ability to detect feeding events. Acoustic monitoring should therefore be incorporated into any field study aiming to quantify predation by mammal-eating killer whales.