FISHERY BULLETIN: VOL. 81. NO. 1 



and arrowtooth flounder); and Pacific ocean 

 perch, Sebastes alutus. Pacific herring and wall- 

 eye pollock (hereafter referred to as herring and 

 pollock) are the most important of these species 

 in the diet of fur seals in the Bering Sea, and have 

 been heavily fished (as have yellowfin sole, hali- 

 but, and Pacific ocean perch). The intensity of 

 fishing on herring and pollock suggests the pos- 

 sibility of fur seal stock depletion due to de- 

 creased food abundance, although stock deple- 

 tions can also have other causes. 



Figure 2, adapted from Pereyraetal. (footnote 

 5) and Favorite et al. (footnote 6), gives the total 

 catch for pollock and herring as well as an index 

 of relative abundance (CPUE) based on research 

 trawl surveys conducted by the International Pa- 

 cific Halibut Commission, the National Marine 

 Fisheries Service (NMFS), and the Japanese 

 Fishery Agency. 



Pollock stocks have been heavily fished since 

 1964, with peak yields coming in the early 1970's. 

 A steady increase in CPUE between 1964 and 

 1968 may have been due in part to improvements 

 in fishing gear and tactics, but must also have 

 been due to higher levels of recruitment of young 

 fish (Pruter 1973), possibly because of reduced 

 cannibalism. Pruter ( 1973) pointed out that, since 

 only a few age groups of pollock are utilized in 



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any given year, poor recruitment could have a 

 disastrous effect on the fishery. 



Herring harvest in the Bering Sea before 1968 

 was mostly west of long. 170°W. However, when 

 stocks there declined, effort was shifted to 

 the eastern Bering Sea, where the stocks were 

 heavily exploited for 3 yr before abundance levels 

 fell. 



Relating stock abundances to fur seal food 

 availability requires examining the overlap be- 

 tween Pribilof rookery feeding grounds and the 

 area of the fishery. Since both fur seals and fish- 

 ermen concentrated on areas of high fish density, 

 we might expect competition for those fish spe- 

 cies they both pursue. 



Herring is a preferred food of fur seals, and 

 evidence for heavy feeding on herring by fur 

 seals in the Bering Sea was obtained from stom- 

 ach samples taken in 1964 (Perez 11 ). Since no 

 large herring fishery exists in the eastern Bering 

 Sea, we cannot be sure whether 1964 was a year 

 of herring abundance or the high diet incidence 

 of herring that year was just a local effect. Fur 

 seals heavily exploit herring off Washington 

 (Kajimura et al. footnote 3) where they are usu- 

 ally abundant. Heavy feeding on herring by fur 

 seals has also been observed near Sitka, Alaska 

 (Wilke and Kenyon 1957). 



Schooling species such as herring, pollock, 

 and squid provide a spatially heterogeneous, or 

 patchy, feeding environment, making it difficult 

 to interpret feeding patterns by average stomach 

 content data. Pollock populations are patchy and 

 mobile (Pereyra et al. footnote 5). The distribu- 

 tion of pollock between 1965 and 1970, generally 

 warmer years, was more concentrated on the in- 

 ner shelf than in the relatively colder years, 

 1971-75 (Pereyra et al. footnote 5). However, the 

 region of the lower shelf between the Pribilof 

 Islands and Unimak Island has consistently pro- 

 vided a large proportion of the Japanese catch of 

 pollock throughout the history of the fishery in 

 all months of the year (Pereyra et al. footnote 5). 

 Thus, it may be that the fishery and the fur seal 

 are most closely in competition for the pollock on 

 the outer shelf. While fur seals are capable of 

 taking relatively large prey, most pollock taken 

 seem to be in the 6-20 cm range, while the fishery 

 takes fish averaging 35 to 40 cm (Salveson and 

 Alton 1976). 



Figure 2.— Catch and relative abundance of walleye pollock 

 and Pacific herring in the eastern Bering Sea. Adapted from 

 Pereyra et al. (text footnote 5) and Favorite et al. (text footnote 

 6). 



U M. A. Perez, Northwest and Alaska Fisheries Center Na- 

 tional Marine Mammal Laboratory, National Marine Fisher- 

 ies Service, NOAA, 7600 Sand Point Way NE., Seattle, WA 

 98115, pers. commun. May 1980. 



124 



