148 



Fishery Bulletin 92f 1). 1994 



10 20 



Percent number/frequency 

 30 40 50 60 70 



walleye pollock 

 (Theragra chalcogramma) 



gonatid squid 



(Gonatidae) 



Atka mackerel 



(Pleurogrammus monopterygius) 



northern smoothtongue 

 (Leuroglossus schmidti) 



Salmoniformes 

 fOsmendae) 



Percent of total number of 

 prey, all years 



Percent frequency of 

 occurrence of prey, all years 



Figure 2 



Percent of total number and frequency of occurrence of primary prey in northern fur seal 

 (Callorhinus ursinus) gastrointestinal tracts for sample years 1981, 1982, and 1985 combined. 

 Species shown include the top three prey from each sample year. 



borealis-Berryteuthis magister. Seals collected over 

 the continental shelf contained the remains of wall- 

 eye pollock of all ages and squids, especially Gonatus 

 madokai-G. middendorffi. Adult walleye pollock, 

 although rare in stomach contents, were found in 

 greatest frequency in fur seals collected from the 

 outer domain of the continental shelf. Juvenile wall- 

 eye pollock were consumed primarily over the 

 midshelf and outer domain. Atka mackerel was 

 found only in samples collected over the outer shelf 

 domain north of Unimak Island. 



Comparisons with trawl samples 



Of the five top-ranked species collected in bottom 

 trawls, only walleye pollock was found in fur seal 

 GI contents (Figs. 2 and 5). Walleye pollock from 

 bottom trawls ranged from 1 to over 12 years of age 

 and had mean body lengths of 38.9 cm (3-4 years 

 old) in 1981, 39.7 cm (4-5 years old) in 1982, and 

 44 cm (5-6 years old) in 1985 (Fig. 6). All but four 

 of the cephalopods caught in 1985 bottom trawls 

 were Berryteuthis magister ranging from 17.5 to 31.2 

 cm DML ( x = 2 1.6). As in the seal samples, B. 

 magister was collected in trawls conducted over the 

 outer continental shelf domain along the 200-m 

 contour, or over the continental slope between 200 

 and 1000 m. Otherwise, the bottom trawl catch for 

 all three years was so dissimilar to the midwater 

 trawl catch (Figs. 5 and 6) and fur seal GI contents 

 (Fig. 2) that electivity computations were not mean- 

 ingful (Odds Ratio=0). 



Calculation of the Odds Ratio and Z-statistic on 

 1985 data with midwater and bottom trawl catch 

 combined showed statistically significant positive 

 selection by fur seals for age-0 pollock (P=0.0002), 

 age-1 pollock (P<0.0001), northern smoothtongue 

 (P<0.0001), and gonatid squid (P=0.02). Negative 

 selection for adult walleye pollock was suggested but 

 was not statistically significant (P=0.13). 



A similarity index of 81% was calculated for spe- 

 cies composition and prey size in the 1985 GI 

 samples and midwater trawls. Fur seals fed on three 

 of the four top-ranked species caught in midwater 

 trawls (Figs. 2 and 5). Midwater trawls and seals 

 caught predominantly juvenile walleye pollock. 

 Gonatid squids (Gonatus madokai, G. middendorffi, 

 and Gonatopsis borealis) had low CPUE values but 

 were second in frequency of occurrence in both fur 

 seal GI tracts and midwater trawls. The modal 

 length of walleye pollock and gonatid squids was 5- 

 20 cm in both midwater trawl and GI samples in 

 1985. Few adult walleye pollock and no large squid 

 were collected in midwater trawls or seal GI samples. 



Seals and midwater trawls caught the same prey 

 species at the same general locations on and off the 

 continental shelf (Fig. 4). As in GI contents, age-0 

 and age-1 walleye pollock were collected in 

 midwater trawls made on the middle and outer shelf 

 and near the continental slope. Gonatopsis borealis 

 were found on the continental slope and near-slope. 

 Gonatus madokai and G. middendorffi were found 

 throughout the sampling area, but primarily on the 

 outer continental shelf and near-slope sampling areas. 



