152 



Fishery Bulletin 92(1), 1994 



400 



300- 



1 



1 200 



$ 



E 



100- 



1985 Trawls 



| Midwater 

 □ Bottom 



N-4140 



Jll^l 



10 



20 25 30 

 Fork length (cm) 



J I 



45 



_L_L 



50 



2 3 



Age (years) 



6 7 



Figure 6 



Age-length frequencies of walleye pollock (Therag 

 chalcogramma) collected in bottom and midwater trawls 



19H5. 



also form boundaries that concentrate prey. Diving 

 depths of 175 m coincide with the depth break of the 

 outer continental shelf. Diving depths of 50-60 m 

 coincide with the depth break of the frontal systems 

 between the midshelf and inner shelf. 



Previous analyses of fur seal diet in the eastern 

 Bering Sea were based primarily on a sample of 

 3,530 stomachs collected pelagically in 1960, 1962- 

 64, 1968, 1973, and 1974 (North Pacific Fur Seal 

 Commission Reports 1962, ' 1975, 2 and 1980 3 ; Fiscus 

 et al. 1 D64; Fiscus et al. 1965; Fiscus and Kajimura 

 1965;. i:tviews of the pelagic data cite walleye pol- 

 lock (Kajimura, 1985; Perez and Bigg, 1986), Pacific 

 herring (Clupea pallasi), capelin (Mallotus villosus), 

 Atka mackerel, gonatid squids (Gonatus spp., 

 Berryteuthis magister and Gonatopsis borealis), and 

 intermittently, northern smoothtongue (Kajimura, 

 1984) as principal fur seal prey in the eastern 

 Bering Sea. Published reports and reviews of fur 

 seal feeding habits prior to the pelagic collections 

 (1892-1950's) also described walleye pollock, cape- 

 lin, gonatid squid, and bathylagid smelt as primary 

 prey in seal spewings or stomachs (Scheffer, 1950a; 

 Wilke and Kenyon, 1952; Wilke and Kenyon, 1957). 



In terms of prey species composition, the 

 summer diet of female and juvenile male 

 northern fur seals does not appear to have 

 changed dramatically since the turn of the 

 century. Pollock and gonatid squid are still 

 the predominant prey of northern fur seals 

 in the eastern Bering Sea. More subtle 

 changes, such as a decrease in pollock size 

 may have occurred (Smith, 1981; Swartz- 

 man and Haar, 1983) and could play a criti- 

 cal role in foraging success of northern fur 

 seals. Unfortunately, records of prey size 

 in historical fur seal diet studies are incom- 

 plete. 



It should be noted that Pacific herring 

 and capelin were absent from fur seal di- 

 ets in this study, despite collections in ar- 

 eas where they occurred as important prey 

 in the past. Fluctuation in the population 

 status of Pacific herring and capelin in the 

 Bering Sea has been attributed to the spo- 

 radic and localized nature of their abun- 

 dance (Turner, 1886; Meek, 1916; Favorite 

 et al. 1977 7 ; Lowe 1991 8 ), overharvesting 

 and displacement by walleye pollock 

 (Wespestad and Barton, 1981; Swartzman 

 and Haar, 1983; Wespestad and Fried, 

 1983; Bakkala et al., 1987), and/or environ- 

 mental change such as the pronounced 

 warming in the Gulf of Alaska and Bering 

 Sea over the past decade (Royer, 1989). The 

 absence of these previously important prey may be 

 critical to seals during successive years of weak 

 walleye pollock year-class abundance. 



Fur seals select juvenile walleye pollock as prey 

 despite a wide availability of other prey types within 

 their dive range. Fur seals may select their prey by 

 size and schooling behavior, whether the prey are 

 myctophids in oceanic waters off Japan (Wada, 

 1971); Pacific herring, capelin, market squid iLoligo 

 opalescens) and Pacific whiting (=Pacific hake, 

 Merluccius productus) in the eastern North Pacific 

 (Kajimura, 1984; Perez and Bigg, 1986); or walleye 

 pollock in the eastern Bering Sea (Kajimura, 1984). 

 The most consistent prey characteristic between 

 feeding studies across the northern fur seal range 



7 Favorite, F., T. Laevastu, and R. R. Straty. 1977. Oceanogra- 

 phy of the northeastern Pacific Ocean and eastern Bering Sea, 

 and relations to various living marine resources. NWAFC Proc. 

 Rep. 280p. Alaska Fish. Sci. Cent., NMFS, NOAA, 7600 Sand 

 Point Way NE., Bin C 15700, Seattle, WA 98115-0070, 280p. 



R Lowe, S. A. 1991. Atka mackerel. In Stock assessment and fish- 

 ery evaluation report for the groundfish resources of the Bering 

 Sea/ Aleutian Islands region as projected for 1992, p. 11-2 to 

 11-40. North Pacific Fishery Management Council, P.O. Box 

 103136, Anchorage, AK 99510. 



