(…)In the Bering Sea, marine mammals, particularly Dall’s porpoises, Phocoenoides dalli, get incidentally entangled in gillnets of the Japanese mothership salmon fishery. The maximum total number of Dall’s porpoises to be caught in a year was fixed by U.S. law. In order to develop techniques and devices to reduce the incidental catch in the gillnets, it is necessary to learn why and how they get entangled.
In order to determine whether Dall’s porpoises can detect and avoid the gillnets, we recorded their clicks and observed their reactions to the gillnets in the open sea.
It is difficult to catch Dall’s porpoises alive and maintain them in captivity for a long time. However, a closely related species the Harbor porpoises (Phocoena phocoena) are easily caught alive in the Japanese coastal seas and are kept in several aquariums. So we attempted to examine the echolocation ability of Dall’s porpoise through experiments using Harbor porpoises which belong to the same family as the Dall’s porpoise. The Harbor porpoise emitted clicks of high frequency(more than 100 kHz) to detect objects. Detection of fine wire and discrimination of the height of a metal cylinder were studied to estimate the echolocation ability of the Harbor porpoise. The audiogram, transmitting directivity, and minimum audible angle of the Harbor porpoise were reported. Thus, there is considerably more information on clicks and echolocation abilities of Harbor porpoises, but their reactions to gillnets have not yet been observed. Therefore, we observed their reactions to gillnets in a pool and estimated their detectable ranges for the gillnet from waveform characteristics of their clicks, reflectivities of the gillnet materials and their hearing ability. (…)