Sinclair et al.: Prey selection by Callorhinus ursinus 



147 



indicated on 38 kHz echosounders and a chromoscope. 

 Midwater towing depths measured by an attached 

 transducer ranged from 22 to 340 m (x=143 m). 



All species of fish and cephalopods collected in 

 midwater trawls were identified and counted. The 

 CPUE and frequency of occurrence of each species, 

 LRL and sex of gonatid squid, and walleye pollock 

 frequency of occurrence by age and length were cal- 

 culated separately for each trawl type. 



Comparison of seal diet and trawl collections 



The Odds Ratio (Fleiss, 1981) was used to compare 

 prey availability (as determined by midwater and 

 bottom trawls) with selection of prey by fur seals for 

 each sample year: 



= 



pV 



where pi = % of diet comprised by a given prey 

 taxon, 

 ql = % of diet comprised by all other prey 



taxon, 

 p2 = % of food complex in environment com- 

 prised by a taxa, and 

 q2 = %> of food complex in environment com- 

 prised by all other taxa. 

 Values were calculated for number of each prey 

 species and percent frequency of occurrence among 

 seals, and CPUE values (no./ha) for each trawl type. 

 Values for p2 and ql were also calculated for the 

 trawl types combined in order to provide a compre- 

 hensive description of the water column. The natu- 

 ral log of the calculated Odds Ratio represents ei- 

 ther positive or negative selection- The Odds Ratio 

 was chosen because, unlike other electivity indices, 

 the significance of the distance of calculated values 

 from zero (null hypothesis that prey were consumed 

 non-selectively) can be tested with the Z-statistic 

 (Gabriel, 1978). 



In order to quantify the degree of overlap in the 

 composition of bottom trawls, midwater trawls and 

 fur seal GI contents, percent similarity (PS) values 

 (Langton, 1982) were calculated: 



PS =100-0.5^a -b, 



where a = %> number of a given prey for seals, and 

 b = % number of the same prey for trawls. 



Results 



Fur seal diet 



Eighty-three fur seals were collected. Ten of the 17 

 GI tracts collected in 1981 were empty and were 



excluded from the analysis. Of the 73 animals in- 

 cluded in the analysis, 13 were juvenile males, 3 

 were juvenile females and 57 were adult females. 

 Most fur seals were collected over depths less than 

 200 m within the outer shelf domain (Fig. 1). 



Fish represented 89% and cephalopods 11% of 

 prey numbers for all three sample years combined. 

 One-hundred percent of the GI tracts had fish re- 

 mains and 82% of all samples contained walleye 

 pollock. A total of 2,658 walleye pollock otoliths were 

 measured. In all years combined, juvenile walleye 

 pollock (3-20 cm FL) were the most numerous and 

 frequently occurring prey species. Sixty-five percent 

 of prey walleye pollock were from the 0-age group 

 (3-13 cm FL) and 31% were from age group 1 (13- 

 20 cm FL). Only 4% of prey pollock were from age 

 group 2 (20 + cm FL) and older. 



Gonatid squids occurred in 36% of the samples, 

 but in comparison with pollock, they were not con- 

 sumed in large numbers (Fig. 2). Gonatus madokai- 

 G. middendorffi and Gonatopsis borealis-Berry- 

 teuthis magister were the second most frequently 

 occurring prey in all years combined. Seventy-nine 

 percent of the 389 beaks measured were from squid 

 5-12 cm DML. 



Northern smoothtongue (Leuroglossus schmidti), 

 a bathylagid deepsea smelt, was the second most 

 numerous fish prey overall (Fig. 2) even though it 

 was found only in 1985 (Table 1). Northern smooth- 

 tongue composed a higher percentage of the total 

 number offish than walleye pollock >2 years old for 

 all sample years combined. Atka mackerel 

 (Pleurogrammus monopterygius) composed 23.9% of 

 the 1981 prey sample and was present in five of 

 seven stomachs collected in 1981 that had prey re- 

 mains, but the species was identified from the prey 

 remains of only one other individual among the six 

 collected in the same area in 1982 (Table 1). 



Although walleye pollock were eaten by fur seals 

 in all 3 years, marked differences in age and body 

 size were found between years (Table 1; Fig. 3). In 

 1981, the few walleye pollock otoliths found were 

 from fish 3-4 years of age. Fur seal GI tracts con- 

 tained primarily age-0 pollock in 1982 and age-1 

 pollock in 1985. Exclusion of otoliths that were in 

 fair condition caused a downward shift in modal FL 

 frequencies of 1 to 2 cm, but did not change our es- 

 timation of the age categories of pollock eaten by 

 fur seals. 



The species of forage fishes and squids consumed 

 by fur seals varied between samples taken on and 

 off the continental shelf (200 m) (Fig. 4). The GI 

 tracts of fur seals collected over oceanic and conti- 

 nental slope regions contained primarily northern 

 smoothtongue and squids, especially Gonatopsis 



