Importance of walleye pollock (Theragra chalcogramma) to wintering ribbon seals (Phoca fasciata) in Nemuro Strait, Hokkaido Japan

抄録

Many ribbon seals (Phoca fasciata) appear in winter in Nemuro Strait, eastern Hokkaido. We propose that their presence is partly related to an exploitation of the large numbers of spawning walleye pollock (Theragra chalcogramma). To investigate feeding habits of ribbon seals in winter, we sampled stomachs from seals for prey composition and size taken in the strait from February–April 1996–1998. Nine fish and five squid species were identified from sixty-four seal stomachs. Walleye pollock and magistrate armhook squid (Berryteuthis magister) were predominant in the diet of seals each year. The index of relative importance of prey showed that walleye pollock was the most dominant prey item. Most pollock eaten by seals had reached maturity. Prey distribution suggested that ribbon seals foraged in the intermediate-bottom layer of the continental slope water. No differences were found between prey composition and size of male and female ribbon seals. Subadult and adult ribbon seals consumed more pollock and less squid than young seals, which also tended to consume smaller pollock. Pinpoint lanternfish (Lampanyctus regalis) and saffron cod (Eleginus gracilis) occurred only in stomachs of adults and young, respectively. Foraging technique and diving ability may explain the differences in prey composition and size among seal age classes.