Estimation of diet of male northern fur seals (Callorhinus ursinus) based on analysis of fecal and regurgitated materials

Hard part remains of foods in feces (scats) and regurgitated pellets (vomits) were analyzed to estimate species and size of prey consumed by male bachelor northern fur seals (Callorhinus ursinus). Of the 107 scats and six vomits sampled at Polovina Cliffs hauling ground on St. Paul island, Alaska, in June-August 1996, 19 scats (17.8 %) and six vomits (100 %) contained undigested hard parts. Otoliths of walleye pollock Theragra chalcogramma (n = 47), Pacific sandfish Trichodon trichodon (n = 7), flatfish (n = 1), beaks of gonatid squids (four upper beaks and five lower beaks), and bones of walleye pollock, Pacific sandfish and other unidentified fish were recovered from the scats. Vomits contained pollock otoliths (n = 65) and bones of pollock and salmonids. Percentage of occurrence of the major prey items in the 19 scats were 89.5 % for walleye pollock, 10.5 % for Pacific sandfish, and 10.5 % for gonatid squids. These results coincided with the past knowledge that northern fur seals primarily prey on walleye pollock in the Bering Sea with supplementary consumption of gonatid squids and small forage fish. Vomits contained otoliths of adult walleye pollock (24-48 cm FL), contrary to scats which contained otoliths of both juvenile and adult pollock (7-43 cm FL). Number of pollock otoliths contained per one scat/vomit was larger in vomits (average 12.2) than in scats (average 3.1), and estimated number and weight of pollock consumed per one scat/vomit were also greater in vomits. No relation was found between scat weight and number of pollock otoliths contained. The difference in size and species composition of food remains between scats and vomits was supposed to result from size-selective accumulation of bulky hard parts in stomachs followed by occasional regurgitation as vomits. For accurate estimation of fur seal diet based on fecal and regurgitated materials, basic data should be collected on the excretion pattern of food particles into scats and vomits in relation to prey species and size