Diet of Steller sea lions off the coast of Rausu, Hokkaido, Japan

抄録

Stomachs were examined from 67 Steller sea lions (Eumetopias jubatus) killed by hunters off the coast of Rausu in the Nemuro Strait, Hokkaido, from January to March, 1994-1996, to determine feeding habits. Five stomachs were empty. Prey species were identified by using the hard parts of fishes and cephalopods (e. g., fish bones, otoliths and cephalopod beaks). The most common prey was walleye pollock (Theragra chalcogramma). Other frequently occurring prey species were Pacific cod (Gadus macrocephalus), saffron cod (Eleginus gracilis), cephalopods, and flatfishes (Pleuronectidae). The percent weight of each of these species differed by year and month. These differences may be related to catch by commercial fisheries, as the consumption of walleye pollock decreased during years when few were caught by commercial fishermen. Length of fish prey was estimated using a formula derived from the relationship between otolith or subopercle size and fish length. Lower rostrum length of squid beaks was used to estimate mantle length by deriving a similar formula. Estimated body length of gadids ranged from 5 to 100 cm, and mantle length of squids ranged from 5 to 25 cm. There was no significant relationship between sea lion body length and estimated prey size.