son with those taken from trawl-caught fishes. The 

 maximum length of nondegraded otoliths was 

 measured to the nearest 0.1 mm using vernier 

 calipers. When more than 20 otoliths occurred in a 

 single stomach, a subsample of 20 was measured. 



Very few otoliths were found in the stomachs of 

 ribbon, Phocafasciata, and spotted, P. largha, seals. 

 For those species, additional otoliths were obtained 

 from small intestines which were split along their 

 entire length and examined for parasitological 

 studies. There was no significant difference between 

 sizes of otoliths obtained from stomachs and intes- 

 tines of ribbon seals (Frost and Lowry 1980). Too few 

 otoliths were retrieved from spotted seal stomachs 

 to test their sizes relative to otoliths from intestines. 

 However, otoliths from intestines were of the same 

 general size range and condition as those from 

 stomachs. We therefore pooled the measurements 

 of otoliths from stomachs and intestines. 



The fork lengths and weights of walleye pollock 

 consumed were estimated from equations in Frost 

 and Lowry (1981a). 



Results 



We measured a total of 2,060 otoliths from 117 in- 

 dividual marine mammals belong to 5 species (Table 

 1). Most of the otoliths were from the stomachs and 

 small intestines of 19 ribbon seals and 90 Steller sea 

 lions, Eumetopias jubatus. Ribbon seals, spotted 

 seals, and a minke whale fed primarily on walleye 

 pollock <20 cm long (Table 2, Fig. 1). Harbor seals, 

 Phoca vitulina richardsi, fed on a wide size range 

 of pollock, including equal numbers of fishes 8-15 cm 

 and 20-35 cm long and a few individuals 45-56 cm 

 in length. Most pollock eaten by sea lions (76%) were 

 20 cm or longer. Young sea lions (<4 yr) collected 

 in 1981 (all were males) ate significantly smaller fish 

 (x = 22.4 cm, n = 37) than did older animals (x = 

 26.9 cm, n = 51; P < 0.005). 



There were some differences in sizes of pollock 

 consumed at different localities and in different 

 years. The sizes of pollock eaten by harbor seals col- 

 lected at Otter Island in 1979 ranged from 10.3 to 

 56.3 cm (i = 31.8 cm), while those eaten by a seal 

 collected at Port Heiden in 1981 were all <12.6 cm 

 long (x = 10.6 cm). Two sea lions collected in 1976 

 and 1979 near the Pribilof Islands had eaten pollock 

 averaging 46.9 cm in length (range 18.4-61.4 cm), 

 while those collected in 1981 to the west had eaten 

 substantially smaller pollock averaging 25.2 cm in 

 length (range 8.3-64.2 cm). In Figure 1, the smaller 

 size mode corresponds to 1981 collections and the 

 larger mode to those from 1976 and 1979. In 1981 



sea lions collected in the central Bering had eaten 

 larger pollock than those off the Kamchatka Penin- 

 sula (x = 26.8 cm vs. 23.5 cm; P < 0.001). This was 

 not attributable to different age or size composition 

 of the samples, since the difference was apparent 

 for older sea lions (>5 yr; x = 21.8 cm vs. 25.6 cm; 

 P < 0.01) as well as the samples as a whole, and the 

 mean age and standard length of all sea lions >5 yr 

 in the Kamchatka sample (x age = 9.1 yr, x SL = 

 297 cm, n = 27) was greater than that of the cen- 

 tral Bering sample (x age = 8.2 yr, x SL = 282 cm, 

 n = 25). 



Discussion 



Of the marine mammal species we examined, rib- 

 bon seals, spotted seals, and a minke whale ate 

 almost exclusively small pollock, whereas Steller sea 

 lions and harbor seals ate pollock of a wide range 

 of sizes. There are few other data available on the 

 sizes of pollock consumed by marine mammals in the 

 Bering Sea. Nemoto (1959) indicated that the length 

 of pollock eaten by fin whales, Balaenoptera physa- 

 lus, never exceeded 30 cm, while larger pollock were 

 sometimes eaten by humpback whales, Megaptera 

 navaeangliae. Fiscus et al. (1964) reported that in 

 1962 northern fur seals, Callorhinus ursinus, ate 

 mostly whole pollock <30-35 cm long. McAlister et 

 al. 3 found intact pollock in fur seal stomachs collected 

 in the eastern Bering Sea, July- September 1974, to 

 range from 10 to 35 cm, with a mean length of 19.3 

 cm. Most specimens were between 16 and 21 cm 

 long. In 1981, Loughlin 4 collected fur seals north of 

 Unalaska Island and found the average size of 

 pollock consumed to be 30.4 cm. Antonelis 5 found 

 that bearded seals, Erignathus barbatus, collected 

 near St. Matthew Island in the central Bering Sea 

 had eaten only small pollock (x length = 8.2 cm). 



It is unknown whether the consumption patterns 

 described above are a result of actual size selection 

 of prey or if they result from coincidental distribu- 

 tion of predators and prey size classes. The overall 

 density of pollock and distribution by age classes are 

 far from uniform in the southern Bering Sea (Smith 

 1981; Bakkala and Alton 6 ). The sizes of fishes con- 



3 McAlister, W. B., G. A. Sanger, and M. A. Perez. 1976. Pre- 

 liminary estimates of pinniped-finfish relationships in the Bering 

 Sea. Unpubl. background paper, 19th meeting North Pac. Fur Seal 

 Comm., Moscow, 1976. 



4 T. R. Loughlin, National Marine Mammal Laboratory, 7600 Sand 

 Point Way N.E., Seattle, WA 98115, pers. commun. November 1983. 



5 G. Antonelis, National Marine Mammal Laboratory, 7600 Sand 

 Point Way N.E., Seattle, WA 98115, pers. commun. December 1983. 



6 Bakkala, R., and M. Alton. 1983. Evaluation of demersal trawl 

 survey data for assessing the condition of eastern Bering Sea 



193 



