Opportunistic Feeding of the Northern Fur Seal, 



Callorhinus ursinus, in the Eastern North Pacific 



Ocean and Eastern Bering Sea 



HIROSHI KAJIMURA 1 



ABSTRACT 



The Pribilof Island population of northern fur seals, Callorhinus ursinus, feeds on a variety of prey throughout 

 its subarctic range from California to the Bering Sea. A total of S3 species of fish and 10 species of squid has been 

 identified from the stomachs of fur seals taken during 1958-74. Some fur seal prey species are commercially fished 

 while others are not of commercial importance but are important forage food for many other predators including 

 other marine mammals, seabirds, and fishes. The season and location are important considerations, as are the 

 migratory characteristics of forage species, when studying the diet of fur seals in the eastern Pacific. Evidence sug- 

 gests that fur seals are opportunistic feeders preying on the most available species throughout their range. 



INTRODUCTION 



The northern fur seal, Callorhinus ursinus, feeds on a vari- 

 ety of fishes and squids throughout its range in the eastern 

 North Pacific Ocean and the eastern Bering Sea. Small school- 

 ing fishes are usually the principal forage species over the con- 

 tinental shelf region and oceanic squids are important seaward 

 of the continental slope in deepwater areas. Fur seals are 

 found from California to the Bering Sea in nearly all months 

 of the year, with a few exceptions, and with peak abundance 

 varying by time and area. 



Evidence presented here suggests that fur seals are opportu- 

 nistic feeders preying on the most available species throughout 

 their range. The information used in reaching this conclusion 

 was obtained primarily for the area off California and the east- 

 ern Bering Sea, focusing on the principal forage foods of fur 

 seals as based on stomach content volume and the relative 

 abundance of fishery resources available in the area. The prin- 

 cipal forage species of fur seals for other areas between Cali- 

 fornia and the Bering Sea will also be discussed, but in lesser 

 detail. 



Background 



Following the purchase of Alaska by the United States in 

 1867, the most critical period for the survival of the northern 

 fur seal, Callorhinus ursinus, occurred between 1879 and 1909 

 when no effective international conservation agreement ex- 

 isted. During this period, almost 1 million fur seals (mostly 

 pregnant females) were taken at sea. Between 1889 and 1909 

 the number taken at sea was almost twice that taken on the 

 Pribilof Islands (the northern fur seal breeding islands of St. 

 Paul and St. George) in the eastern Bering Sea. Because of the 

 unrestricted harvest of females at sea, the Pribilof Island fur 

 seal population declined from about 2 million in 1880 to ap- 

 proximately 300,000 animals in 1909. In 1911 the United 



'Northwest and Alaska Fisheries Center National Marine Mammal Labora- 

 tory, National Marine Fisheries Service, NOAA, 7600 Sand Point Way NE., 

 Seattle, WA 98115. 



States, Japan, Russia, and Great Britain (for Canada) formed 

 the Convention for the Preservation and Protection of Fur 

 Seals which prohibited pelagic sealing. Since then under inter- 

 national management, the Pribilof Island fur seal population 

 has recovered and now numbers about 1.3 million animals or 

 nearly 80<7o of the world population of this species (Lander 

 and Kajimura 1982). The food of the northern fur seal was 

 first reported from seals collected in 1895 by Alexander (1896) 

 as a part of the Jordan Commission investigations. 



Present Treaty 



The present Interim Convention on Conservation of North 

 Pacific Fur Seals was signed February 1957 by Japan, Canada, 

 the U.S.S.R., and the United States, and protects fur seals as 

 did the Convention of 1911. The two broad objectives of the 

 1957 Convention are: 1) To achieve and sustain maximum pro- 

 ductivity of the fur seal resource and 2) to determine the rela- 

 tionships between fur seals and other living marine resources. 

 The terms of the present Convention during 1958-63 required 

 each member country to conduct pelagic research and collect a 

 minimum number, or "quota," of seals each year. At this 

 time, when most of the seals were collected, it was necessary to 

 find areas of seal concentration and to collect as many as pos- 

 sible to meet the quota required by the Convention. Effort was 

 therefore directed toward collecting seals — thereby sacrificing 

 the quality of systematic surveys to determine the density, dis- 

 tribution, or relative abundance of fur seals by time and area. 

 The quota required by the Convention was modified after 

 1963. Initially, hunting was carried out in areas where seals 

 were known to be seasonally abundant during the days of 

 pelagic sealing and research voyages prior to 1958; marginal 

 areas where seals were known to be scarce were generally 

 neglected. 



At present it is known that most fur seals found in the east- 

 ern North Pacific Ocean are from the Pribilof Islands (St. 

 Paul, St. George, and Sea Lion Rock; Otter and Walrus 

 Islands have no fur seal rookeries) in the eastern Bering Sea, 

 which are the principal breeding grounds of the northern fur 

 seal. Fur seals from the western Bering Sea (Commander 



