Nocturnal activity by transient killer whales at St. Paul Island, AK

Abstract

Observations of killer whales in the act of predation are rare because feeding bouts are infrequent in relation to sighting opportunities, and they apparently feed at night as well as during the day. A pop-up autonomous recording unit (ARU) was used to expand the observation window of transient (mammal-eating) killer whales by monitoring them continuously from 22 June to 12 July 2006 near a northern fur seal rookery at St. Paul Island, AK, a predation hot-spot in the Bering Sea. The ARU recorded at a sampling rate of 0.5–16<th>000 Hz. Killer whale vocalizations were detected on 19 of 20 days. Most vocalizations occurred from the end of civil twilight (00:26 on 1 July) through mid-morning. Fewest vocalizations occurred between 1800 and 2400. Vocal whales were most likely feeding, based on the paradigm that transients are mostly silent except when attacking prey. Call types are being identified and vocalizations are being correlated with visual observations and fur seal activity at the rookery. Nocturnal hunting is likely one important reason why it has been difficult to understand the foraging behavior of these elusive predators and must be considered when drawing conclusions about the roles transient killer whales play in marine ecosystems.