PEREZ and BIGG: DIET OF NORTHERN FUR SEALS 



same species of prey were important by region and 

 month as reported in the current study. This was 

 true for the numerous annual and intermittent sum- 

 mary reports prepared by the United States and 

 Canada for the North Pacific Fur Seal Commission 

 during 1958-74. However, they are not reviewed 

 here because they typically described diet for a par- 

 ticular year or 2-6 yr period and were based on 

 subsets of the samples that we used. In other 

 studies, Stroud et al. (1981) and Kajimura (1984, 

 1985) mentioned, although did not demonstrate, that 

 squids were the main food species in offshore areas, 

 whereas fishes were the most important inshore. 

 This observation was confirmed in our findings. The 

 phenomenon appears to exist throughout the feed- 

 ing range of northern fur seals off western North 

 America. 



Also, as found in our study, Taylor et al. (1955) 

 and Kajimura (1985) reported that the main food for 

 the northern fur seal off California was northern an- 

 chovy. Similarly, Clemens and Wilby (1933), 

 Clemens et al. (1936), Schultz and Rafn (1936), May 

 (1937), Wilke and Kenyon (1952), Spalding (1964), 

 and Kajimura (1985) all indicated that Pacific her- 

 ring was the primary prey between Washington and 

 southeastern Alaska. Taylor et al. (1955) and Kaji- 

 mura (1985) found that capelin was prominent in the 

 diet off Kodiak Island; and Lucas (1899), Wilke and 

 Kenyon (1952), and Kajimura (1985) found that 

 walleye pollock was the most significant species in 

 the eastern Bering Sea; and Wilke and Kenyon 

 (1957) reported that capelin was important to north- 

 ern fur seals near Unimak Pass. 



However, there were some differences between 

 the results of earlier research and the current analy- 

 sis. Taylor et al. (1955) stated that 1) jacksmelt, 

 Atherinopsis calif orniensis, was second in impor- 

 tance for northern fur seals off California rather 

 than insignificant as we reported; 2) salmon was the 

 main food off Oregon rather than a minor diet item; 

 3) walleye pollock was more important than Pacific 

 herring off Washington; and 4) Pacific sand lance 

 was rarely foraged off Kodiak Island rather than 

 eaten almost as frequently as capelin. Kenyon (1956) 

 found Pacific sandfish, Trichodon trichodon, to be 

 the most commonly consumed food of seals which 

 were taken on rookeries of the Pribilof Islands, 

 whereas the current study found that it was rarely 

 eaten. Most of these differences probably resulted 

 from small sample sizes of earlier studies or dis- 

 similar measures of importance. Also, some differ- 

 ences in diet will result from interannual variability 

 in prey abundance and movement patterns owing 

 to environmental conditions or other factors. 



Factors other than just the relative importance 

 by region and month must be taken into account 

 when determining the significance of each prey 

 species to the seal. Robbins (1983) stated that the 

 nutritional value of food should also be considered. 

 For example, food species with high caloric values 

 will be more important than those with low caloric 

 values because the amount of food required for 

 metabolic functions depends to some extent upon 

 the energy content of that food. However, high 

 energy foods are more valuable only when they are 

 not more difficult to capture and do not contain 

 more indigestible or toxic substances than lower 

 energy content species. These detrimental factors 

 do not appear to be involved when considering the 

 most important foods eaten by northern fur seals 

 in the eastern North Pacific Ocean. Northern an- 

 chovy and Pacific herring were already the most im- 

 portant prey species even without accounting for 

 their energy content. But because they also had rela- 

 tively high energy values, their importance in- 

 creased in the seal's diet. Thus, relative importance 

 with an adjustment for energy content appears to 

 be a better measure of diet than when energy con- 

 tent is not incorporated. 



Another factor to consider is the proportion of the 

 year that the northern fur seal population spends 

 in each locality. Each prey species in the total an- 

 nual diet should be weighted by the importance of 

 each subregion and region where the prey is eaten. 

 This weighting requires understanding the route 

 and timing of migration, and the changes in local 

 seasonal abundance of northern fur seals. The 

 general pattern of migration for the Pribilof Islands 

 stock is well known (Baker et al. 1970; Fiscus 1978; 

 Bigg 1982 6 ). Essentially all population components, 

 except most 1-2 yr-olds, are thought to occur in the 

 eastern Bering Sea during June-July to October 

 where they pup, mate, nurse, and rest on the Pribilof 

 Islands. Most 1-2 yr-olds remain in the North Pacific 

 Ocean during this time. The stock leaves the east- 

 ern Bering Sea in November-December and travels 

 mainly to the coastal areas between southeastern 

 Alaska and California, with the largest number ap- 

 parently going to California by January. Most males 

 remain in Alaskan waters, and seals aged 1-2 yr re- 

 main offshore. The return migration starts in March- 

 April with most seals arriving in the northern Gulf 

 of Alaska by May. However, while this general pat- 



6 Bigg, M. A. 1982. Migration of northern fur seals in the 

 eastern North Pacific and eastern Bering Sea: an analysis using 

 effort and population composition data. Unpubl. rep., 77 p. De- 

 partment of Fisheries and Oceans, Pacific Biological Station, 

 Nanaimo, British Columbia V9R 5K6, Canada. 



967 



