PREY OF THE STELLER SEA LION, 

 EUMETOPIAS JUBATUS, IN THE GULF OF ALASKA 



Kenneth W. Pitcher^ 



ABSTRACT 



Stomach contents of 250 Steller sea lions, Eumetopiasjubatus, coWected in the Gulf of Alaska consisted 

 by volume of 95.7'7f fishes, 4.29c cephalopods, <0.19c decapod crustaceans, <0.1% shelled-gastropods, 

 and <0.1% mammals. The 10 top-ranked prey were walleye pollock, Theragra chalcogramma; squids, 

 Gonatidae; Pacific herring, Clupea harengus pallasi; capelin, Mallotus villosus; Pacific cod, Gadus 

 macrocephalus: salmon, Oncorhynchus spp.; octopus. Octopus sp.; sculpins, Cottidae; flatfishes, 

 Pleuronectidae; and rockfishes, Scorpaenidae. Walleye pollock was the predominant prey, composing 

 about 5S9c of the total volume and occurring in 67"^ of the stomachs with food. Predation on capelin and 

 salmon appeared to be largely limited to spring and summer when these species were abundant in 

 nearshore waters. Utilization of walleye pollock by sea lions appeared to have increased between 

 1958-60 and 1975-78, perhaps because of an increase in the relative abundance of walleye pollock. 

 There was nearly complete overlap in the diet of sea lions and the harbor seal, Phoca vitulina richardsi. 

 Potential competition may have been ameliorated by differences in distribution, differing diving 

 capabilities, a more diverse diet for harbor seals and use of larger prey by sea lions. 



The importance of knowledge of diets of marine 

 mammals has become increasingly apparent with 

 the recent emphasis in offshore oil and gas devel- 

 opment and the resulting potential for reduction 

 or change in composition of prey resulting from 

 pollution (Evans and Rice 1974). These data are 

 also needed by both fisheries and marine mammal 

 managers, particularly since recent legislation 

 (The Marine Mammal Protection Act of 1972; 

 United States PL 92-522 and The Fishery Conser- 

 vation and Management Act of 1976; PL 94-265) 

 requires management based on ecosystem con- 

 cerns. 



Between 1975 and 1978 I studied prey utiliza- 

 tion by the SteWer s<ea\\ovi., Eumetopias jubatus , in 

 the Gulf of Alaska from Cape Suckling to Sanak 

 Island (Figure 1). This area contains an estimated 

 110,000 to 140,000 sea lions, 10 breeding rookeries, 

 and a mimimum of 50 hauling areas and is consid- 

 ered to be the center of abundance for the species 

 (Calkins and Pitcher^). 



Several prior studies of sea lion foods (Imler and 

 Sarber 1947; Mathisen et al. 1962; Thorsteinson 



and Lensink 1962; Fiscus and Baines 1966), al- 

 though limited in geographic and seasonal cover- 

 age, provide a base for comparisons of prey use 

 over time. Historical records of prey abundance 

 (Pereyra and Ronholt^) provide valuable insight 

 into prey utilization. The results of a concurrent 

 study of harbor seal, Phoca vitulina richardsi, 

 foods (Pitcher 1980) allowed me to compare prey 

 utilization of these two resident pinnipeds with 

 largely overlapping distributions. 



METHODS 



Between 1975 and 1978, 250 sea lions were col- 

 lected by shooting from nearshore waters, 

 rookeries, and hauling areas of the Gulf of Alaska 

 (Table 1). Stomach contents were removed in the 

 field, wrapped in muslin, and preserved in 10% 

 Formalin.'* In 15 cases, when large amounts of 

 freshly eaten prey occurred, the prey were 

 weighed, identified from external characteristics, 

 and disposed of in the field. Volume (cubic cen- 

 timeters) and weight (grams) of prey were as- 

 sumed to be equal in these samples (Fiscus and 



'Alaska Department of Fish and Game, 333 Raspberry Road, 

 Anchorage, AK 99502. 



^Calkins, D. G., and K. W. Pitcher. 1981. Population as- 

 sessment, ecology and trophic relationships of Steller sea lions in 

 the Gulf of Alaska. Outer Continental Shelf Environmental 

 Assessment Program Final Report. Juneau Project Office, P.O. 

 Boxl808, Juneau, AK 99802. . ^ /I o -> 



Manuscript accepted: April 1981. 



FISHERY BULLETIN: VOL. 79, NO. 3, 1981. 



^Pereyra,W.R.,andL.L.Ronholt. 1976. Baseline studies of 

 demersal resources of the northern Gulf of Alaska shelf and 

 slope. U.S. Dep. Commer., NOAA Processed Rep. NMFS 

 NWFC, 281 p. 



■'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



467 



