50 



Fishery Bulletin 92(1). 1994 



Figure 3 



Age-length frequencies of walleye pollock (Ther 

 chalcogramma) based on otoliths in northern fur 

 [Callorhinus ursinus) gastrointestinal tracts by year. 



agra 

 seal 



and both shallow and deep diving along the conti- 

 nental slope. Dive pattern information is based on 

 time-depth recordings (Gentry et al., 1986; Loughlin 

 et al., 1987; Goebel et al., 1991), radio telemetry 

 (Loughlin et al., 1987), stomach volume estimates 

 (Mead, 1953; Taylor et al., 1955'; Spalding, 1964; 

 Wada, 1971; Kajimura, 1984), and stomach clear- 

 ance studies (Miller, 1978 5 ; Bigg, 1981 6 ; Bigg and 

 Fawcett, 1985; Murie and Lavigne, 1985). 



Based on fur seal and trawl collections in 

 this study and on distributional information 

 of prey (Smith, 1981; Dunn, 1983; Kubodera 

 and Jefferts, 1984; Lynde, 1984), shallow 

 diving fur seals over the continental shelf 

 concentrated on juvenile walleye pollock and 

 juvenile gonatid squid (Gonatus madokai-G. 

 middendorffi), while shallow divers off-shelf 

 targeted juvenile gonatid squid (Berryteuthis 

 magister-Gonalopsis borealis) and bathy- 

 lagid smelt. Daytime deep diving over the 

 continental shelf would be advantageous to 

 seals concentrating on prey (i.e., adult wall- 

 eye pollock) that tend to school at depth 

 during daytime hours and disperse as they 

 rise in the water column at night. Adult 

 gonatid squid probably occur in schools at 

 the bottom on the continental shelf and re- 

 main deep along the shelf edge during both 

 day and night. The location and degree of 

 concentration of prey may be closely associ- 

 ated with the hydrography of the foraging 

 region. The hydrography of the foraging re- 

 gion may have the most direct influence on 

 the diving patterns of fur seals. 



Hydrographic characteristics of the Bering 

 Sea continental shelf, include a two-layered 

 midshelf and a three-layered outer shelf 

 domain that may stratify and concentrate 

 prey by species and age in a vertical plane. 

 Nishiyama et al. (1986) proposed that ver- 

 tical stratification within the eastern Bering 

 Sea shelf serves as a "nursery layer" to con- 

 fine young-of-the-year pollock in the upper 

 40 m of the water column within the bound- 

 ary region between the upper and lower lay- 

 ers. Copepod nauplii are also concentrated 

 in this area, providing a ready source of food 

 for larval walleye pollock (Bailey et al., 

 1986). Wada ( 1971) determined that primary 

 foods of fur seals off the Sanriku Coast in Japan 

 consisted of migrating species closely related to 

 boundary regions, especially transition zone regions. 

 The horizontal temperature and salinity structures 

 that occur on either side of frontal regions within 

 our study area (Kinder and Schumacher, 1981 ) may 



4 Taylor. F II (' , M. Fuginaga, and F. Wilke. 1955. Distribu- 

 tion and food habits of the fur seals of the North Pacific Ocean 

 Rept. of Coop. Invest by the Govts, of Can., Japan, and the 

 U.S.A. Feb. -July, 86 p Available Alaska Fish. Sci. Cent.. 

 NOAA, NMFS, 7600 Sand Point Waj \K , BinC 15700, Seattle, 

 WA 98115-0070. 



' Miller, L. K. 1978. Energetics of the northern fur seal in rela- 

 tion to climate and food resources of the Bering Sea. Final Rep. 

 to U S. Mar. Mamm. Comm. MMC-75/08, 27p. 



K Bigg. M. A. 1981. Digestion rates of herring (Clupea harengus 

 pallasi i and squid iLali^o npalfsri-nsi in northern fur seals. 

 Submitted to the 24th Annual Meeting of the Standing Sci. 

 Comm., N. Pac. Fur Seal Comm., 6-10 April, Tokyo, Japan. 

 Available: Alaska Fish. Sci. Cent.. 7600 Sand Point Way NE., 

 BmC15700, Seattle, WA 98115-0070. 



