Abstract

Dall’s porpoises, Phocoenoides dalli, were often incidentally caught in the Japanese salmon gillnet fishery in the North Pacific. In order to investigate the reasons for entanglement, their auditory characteristics and capabilities and their responses to gillnets were examined. Dall’s porpoises emit short high frequency pulses ranging from 135 to 149kHz, with a pulse width of 50 to 60μs and a source level of 165 to 175dB re 1μPa. When chased toward a gillnet in open sea, they have been observed to change their swimming direction to avoid it by either swimming along it or diving underneath it. They are thus capable of avoiding entanglement. Estimated target strengths of a float, leadline, lead and netting were -25, -33 -39, and -55dB, respectively. Approximate estimates of Dall’s porpoise’s detection ranges for the leadline and netting were found to be 30 and 8m, respectively. Active and passive acoustic devices were tested with the aim of reducing the incidental catch. Four types of sound generator (SG-1 to 4) in the frequency range of 20 to 150kHz were developed on the basis of the frequency components of clicks and observed responses to sounds. Air-tube threads to increase the net target strength were also used. Incidental catches were monitored on the fishing ground and catch decrease rates (DRs) estimated. The DRs of the sound generators (with the exception of SG-4) were 3-16% and the DR in the case of the gillnet with three air-tube threads in the centre portion was 8-20%. As for SG-4, entanglement was concentrated in the portion of the net where SG-4 was not attached and the sound wave was weak. The target strengths of a rope, vinyl string and blister sheet are much larger than that of the netting. Experimental operations using gillnets equipped with these reflectors were conducted on the fishing ground. The detection abilities of other cetaceans, such as the harbour porpoise (Phocoena phocoena), white whale (Delphinapterus leucus) and the bottlenose dolphin (Tursiops truncatus) were also examined.