Sinclair et al.: Prey selection by Callorhinus ursinus 



149 



Discussion 



The modal size distribution of walleye pollock in GI 

 contents of female and juvenile male fur seals re- 

 flected year-class strength projections of walleye 

 pollock (Fig. 7). Walleye pollock have highly variable 

 recruitment rates (Smith, 1981), and year-class 

 strength varied five-fold between 1977 and 1982 

 (Bakkala et al., 1987). Population estimates based 

 on bottom trawl and midwater acoustic surveys in 

 the eastern Bering Sea indicated that the 1980 year 

 class (age 1 in 1981) was about half the average 

 year-class size; the 1981 year class (age in 1981) 

 was the weakest observed prior to 1983; and the 

 1978 year class (age 3 in 1981) was the strongest 

 observed. The 1982 and 1984 year classes were 



strong and the 1985 year class was considered av- 

 erage (Bakkala et al., 1987). Similarly, walleye pol- 

 lock as prey in 1981 were primarily adults 3 and 4 

 years of age (from the 1977 and 1978 year class); in 

 1982, seals ate age-0 pollock exclusively; and in 

 1985, prey pollock were primarily from the 1984 

 year class. The concordance of pollock recruitment 

 and fur seal GI content analysis indicates that the 

 variable recruitment of walleye pollock affects prey 

 consumption by northern fur seals. 



The three basic dive patterns described for adult 

 females in the Bering Sea are shallow, pelagic night- 

 time diving (most commonly to 50—60 m); deep day- 

 and-night diving over the continental shelf (most 

 commonly to 175 m); and some combination of both, 

 including shallow diving over the continental shelf 



