Prey switching of Dall's porpoise Phocoenoides dalli with population decline of Japanese pilchard Sardinops melanostictus around Hokkaido, Japan

Abstract

The stomach contents of 150 Dall's porpoises collected during 6 surveys in the Sea of Japan and Sea of Okhotsk around Hokkaido in 1988 to 1996 were examined. A total of 49 prey types, including 29 species, were identified. Epipelagic prey items were found in samples collected during midday, and mesopelagic and some benthopelagic prey species were found in samples collected in the morning. Although the size range of prey was great, most prey were small. The dominant prey species switched in both seas from the late 1980s to the early 1990s as the Sardinops melanostictus (Japanese pilchard) populations in the both seas declined. In the Sea of Japan, the diet of Dall's por- poises switched to ~heragra chalcogramma (walleye pollock), and in the Sea of Okhotsk, their diet switched to Engraulis japonicus (Japanese anchovy) and Berryteuthis magister (magistrate armhook squid). The utilization of new prey species did not always coincide with the fluctuation in catch sta- tistics for the prey in local fisheries. Dall's porpoises fed on many benthopelagic prey species when epipelagic prey species were not available, suggesting that epipelagic prey species are the preferred prey. The vertical distribution of prey presumably affects prey selection.