Sonar characteristics of the harbour porpoise (Phocoena phocoena): source levels and spectrum

Abstract

The international problem of incidental capture and consequent drowning of small cetaceans in passive fishing gear necessitates research to identify techniques which will minimize such interactions. Harbour porpoise (Phocoena phocoena) interactions with bottom-set gear are of considerable concern both in North America and in Europe. However, these animals are difficult to study and their apparent inability to avoid gillnets requires explanation. Few data on this animal's echolocation capabilities exist. Source levels (SL) and spectra have been determined for two young animals rescued from strandings. In the absence of direct acoustic measurement, the minimum sonar beam widths have been estimated using physical acoustic parameters with dimen-sions determined from autopsied animals. Individual harbour porpoises foraging in the Cromarty Firth in Scotland have been tracked at the surface with electronic theodolites and video, whilst their underwater acoustic emissions were monitored using an innovative acoustic detection system. The preliminary data suggest that these animals search for prey using a narrowbeam, narrowband, high-frequency sonar with a detection range, for single fish of an ingestible size, up to 30 m. These sonar characteristics appear to favour foraging close to either the sea surface or the bottom. The implications of this finding are discussed in the context of bottom-set gillnets.