Icelandic killer whales Orcinus orca use a pulsed call suitable for manipulating the schooling behaviour of herring Clupea harengus

Icelandic and Norwegian killer whales feed on herring, after debilitating them with underwater tail slaps. We analysed sound recordings of Icelandic and Norwegian killer whales engaged in feeding and other behaviour. We describe a pulsed call made by Icelandic killer whales shortly before underwater tail slaps, which had an atypical low frequency (average peak frequency: 683 +/- 131 Hz), long duration (3.0 +/- 1.1 s) and high intensity (source level 169-192 dB pp re 1 mu Pa @ 1 m). The low-frequency emphasis of this call was below the most sensitive hearing range of killer whales, suggesting that the call may not be optimal for interspecific communication. However, herring could easily perceive the killer whale call since the frequency content is similar to the resonant frequency of their swim bladder as well as to the most sensitive frequency band of hearing in this species. Previous studies have shown that sound may cause schooling herring to cluster. A high density of herring in a school would increase the effectiveness of the underwater tail slaps. We suggest that some Icelandic killer whales use this low-frequency call to herd herring into dense schools immediately before delivering an underwater tail slap, thereby increasing their foraging success.