FISHERY BULLETIN: VOL. 79, NO. 3 



Three studies of sea lion foods in which a total of 

 135 stomachs containing food were examined were 

 conducted in the Gulf of Alaska between 1958 and 

 1960 (Mathisen et al. 1962; Thorsteinson and Len- 

 sink 1962; Fiscus and Baines 1966). Major prey 

 included shelled mollusks; cephalopods; Pacific 

 sand lance, Ammodytes hexapterus; rockfishes; 

 and smelts. Because geographic and seasonal 

 composition of these samples and my collections 

 were not strictly comparable (previous collections 

 were nearly all near rookeries during the breeding 

 season, while I sampled throughout much of the 

 year at a wide range of locations) strict compari- 

 sons of the data are not possible. However, one 

 major difference was apparent; walleye pollock, 

 the predominant prey in my sample, was not found 

 in the earlier studies. Concurrent with this appar- 

 ent increase of walleye pollock in the sea lion diet 

 has been an increase in walleye pollock abundance 

 in the Gulf of Alaska. Between 1961 and 1973-75 

 walleye pollock increased from 5 to 45% by weight 

 of total demersal fish stocks and was found to be 

 the predominant species (Pereyra and Ronholt 

 footnote 3). 



One additional collection of seven sea lions was 

 made in 1945 (Imler and Sarber 1947). Walleye 

 pollock and flatfishes were the major foods. 



Harbor seals and Steller sea lions are the only 

 abundant pinnipeds resident in nearshore regions 

 of the Gulf of Alaska. Food habit studies of both 

 species were conducted concurrently; both sea 

 lions and harbor seals (Pitcher 1980) frequently 

 were collected on the same trips. This resulted in 

 relatively comparable geographic and seasonal 

 coverage (Table 1). Results of the two studies were 

 similar (Table 5) with nearly complete overlap of 

 principal prey. Spearman rank correlation analy- 

 sis showed a significant positive correlation (rs = 

 0.67, P<0.01) between the rankings of principal 

 prey eaten by both sea lions and harbor seals. The 

 percentage of cephalopods eaten by both predators 

 was similar; however, sea lions ate more squids 

 while harbor seals consumed more octopus. Wall- 

 eye pollock was the top-ranked prey of both sea 

 lions and harbor seals; however, the percentage of 

 occurrence was nearly twice as high for sea lions 

 (66.7%) as for harbor seals (34.9%). Eulachon, 

 Thaleicthys pacificus, and Pacific sand lance were 

 both evident components of the harbor seal diet 

 (occurring in 8.2% and 7.1% of the stomachs, re- 

 spectively) but were not recorded as food of sea 

 lions during this study. Most eulachon occurrences 

 were from harbor seals collected in freshwater and 



estuarine habitats of the Copper River Delta 

 (Pitcher 1980) where no sea lions occurred. 



Although use of prey by sea lions and harbor 

 seals was similar, several factors may have 

 ameliorated potential competition. Mean length of 

 walleye pollock eaten by sea lions was signifi- 

 cantly greater it = 32 A, P<0.001) than for those 

 eaten by harbor seals, based on otoliths recovered 

 from stomachs (Table 2). This may indicate a ten- 

 dency towards use of larger prey by sea lions. Al- 

 though distribution of the two species often over- 

 laps in the Gulf of Alaska, sea lions range farther 

 offshore (Fiscus et al.^). In addition, harbor seals 

 often use freshwater and estuarine habitats rarely 

 used by sea lions in the Gulf of Alaska. Harbor seals 

 can probably stay submerged for considerably long- 

 er periods than sea lions (R. Eisner ) which may 

 allow them to more efficiently utilize cryptic and 

 solitary prey such as octopus and flatfishes. Al- 

 though use of principal prey between the two 

 species was similar, harbor seals had a more di- 

 verse diet. They preyed upon a minimum of 31 

 species (Pitcher 1980) compared with 20 for sea 

 lions. Both sea lions and harbor seals appeared to 

 modify their diets according to prey availability. 

 Several lines of evidence led to this hypothesis. 

 Walleye pollock, the predominant prey of sea lions 

 and harbor seals, was the most abundant species of 

 demersal fish in the area. Similar seasonal and 

 geographic variations in the diets of both species 

 were found which probably reflected use of abun- 

 dant and readily available prey at that time and 

 location. There were apparent changes over time 

 in the relative composition of the sea lion diet 

 (primarily walleye pollock) which appeared to cor- 

 relate with changes in prey abundance. Also, re- 

 ports in the literature indicated use of different 

 prey in other geographic regions (Spalding 1964; 

 Fiscus and Baines 1966). 



Four of the five, top-ranked prey of both sea lions 

 and harbor seals (Table 5) were off-bottom school- 

 ing species. Many of the important prey reported 

 in other studies of Steller sea lion foods also fit into 

 this category and include Pacific herring; smelts; 



^Fiscus, C. H., H. W. Braham, R. W. Mercer, R. D. Everitt, B. D. 

 Krogman, P. D. McGuire, C. E. Peterson, R. M. Sonntag, and D. E. 

 Withrow. 1976. Seasonal distribution and relative abundance 

 of marine mammals in the Gulf of Alaska. In Environmental 

 assessment of the Alaskan Continental Shelf, Vol. 1, p. 19-264. 

 Principal investigators reports for October- December 1976. En- 

 vironmental Research Laboratories, NOAA, Boulder. Colo. 



•"R. Eisner, Professor of Physiology, Institute of Marine Sci- 

 ence, University of Alaska, Fairbanks, AK 99701, pers. commun. 

 January 1980. 



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