Table 2.— Summary of sizes of walleye pollock consumed by marine mammals in the Bering 



Sea. 



'The weight of the mean length fish does not correspond to the mean weight of fishes consumed due 

 to the exponential nature of the length-weight relationship for fishes and the distribution of lengths of 

 fishes consumed. 



sumed generally agree with the basic distribution 

 pattern for pollock in that sea lions collected near 

 the continental slope ate many large pollock, while 

 ribbon and spotted seals collected north of St. Mat- 

 thew Island ate almost entirely small pollock. 

 However, concurrent sampling of prey in stomachs 

 and those available in the environment suggest that 

 some selection does occur. Fur seals were found to 

 eat smaller pollock than those caught in otter trawls 

 taken nearby (x length = 30.4 cm in seals, 38.3 cm 

 in trawls), while sea lions appeared to select larger 

 fishes (x length = 29.9 cm in sea lions, 25.5 cm in 

 trawls) (Loughlin fn. 4). Such comparisons must be 

 interpreted with caution since demersal trawl 

 samples underestimate the abundance of young 

 pollock, most of which occur several meters off the 

 bottom (Traynor 7 ). 



Other information also indicates that marine mam- 

 mals sometimes select fishes of certain size classes. 

 The sizes of arctic cod, Boreogadus saida, caught in 

 otter trawls in the northern Bering Sea were com- 

 pared with the estimated lengths of fishes eaten by 

 spotted and ribbon seals collected in the same area 

 and time period (Frost and Lowry 1980; Bukhtiyarov 

 et al. 1984). While the distribution of trawl-caught 

 fishes was distinctly bimodal, seals ate predominant- 

 ly fishes of the larger size classes. Saffron cod, 

 Eleginus gracilis, eaten by adult white whales, Del- 

 phinapterus leucas, in the Kotzebue Sound region 

 of the southern Chukchi Sea were larger than those 

 eaten by younger animals collected at the same loca- 

 tion on the same dates (Seaman et al. 1982). We ob- 

 tained similar results in this study for young versus 

 old sea lions. Pitcher (1981) found that pollock eaten 

 by sea lions were significantly longer (x = 29.8 cm) 



pollock. Unpubl. Rep., 43 p. Northwest and Alaska Fisheries 

 Center, NMFS, NOAA, Seattle, WA. 



7 Traynor, J. J. 1983. Midwater pollock (Theragra chalcogram- 

 ma) abundance estimation in the eastern Bering Sea. Unpubl. 

 Rep., 7 p. Northwest and Alaska Fisheries Center, NMFS NOAA 

 Seattle, WA. 



194 



than those eaten by harbor seals (x = 19.2 cm; P < 

 0.001) collected in the same general locations in the 

 Gulf of Alaska. 



The factors involved in the apparent size selection 

 of prey are poorly known for marine mammals. A 

 strict relationship between the size of predators and 

 the size of their prey is not to be expected in such 

 behaviorally complex and morphologically diverse 

 animals. For example, the prey of ringed seals, Phoca 

 hispida, range in length from 1 cm (euphausiids) to 

 at least 121 cm (wolffish, Anarhichas sp.) (Frost and 

 Lowry 1981c). The largest animal we examined in 

 this study, a minke whale 7.3 m long, ate uniformly 

 small pollock. Age-related differences in sizes of 

 fishes eaten by sea lions and belukha whales are 

 more likely due to morphological and behavioral 

 development than to size relationships per se. 

 Although size may affect a sea lion's ability to catch 

 large pollock, and old sea lions are larger than young 

 ones (i SL = 212 cm for sea lions age 1-4 yr, n = 

 33 vs. x SL = 289 cm for those >5 yr, n = 52), the 

 size range of pollock eaten by both young and old 

 sea lions was similar. The largest pollock (64 cm) 

 represented in our samples was eaten by a 215 cm 

 long, 3-yr-old sea lion which indicates that physical 

 differences due strictly to predator size are not the 

 sole factor influencing preference for a particular 

 prey siza Aspects of feeding strategy, including size 

 selectivity, are the result of a complex and inter- 

 acting suite of morphological, physiological, and 

 behavioral adaptations which allow an organism to ' 

 gather food in the most efficient manner (Schoener 

 1971). 



Size-specific feeding may have important conse- 

 quences for predators. For example, the length of 

 1-yr-old pollock fluctuates markedly among years, as ! 

 does the numerical abundance of the first year class. 

 In 1976 abundance was low (729 million individuals 

 in the NMFS Bering Sea survey area) and fishes 

 were small (x = 11.6 cm), while in 1974 abundance 

 was high (2,840 million individuals) and fishes were 



