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Fishery Bulletin 92(1), 1994 



ing Scheffer (1950b). In the analysis of data, males 

 and females of all ages were treated as one group 

 because of small sample sizes. 



The highest number of either upper or lower 

 cephalopod beaks and left or right otoliths was re- 

 corded as the maximum number of each species 

 present. If deterioration made some left and right 

 otoliths of a species indistinguishable, they were 

 counted and the total was divided by 2. The fre- 

 quency of occurrence and number of individuals 

 from each prey taxon was calculated for each seal. 



The fork length (FL) of pollock and dorsal mantle 

 length (DML) of squid was measured directly when 

 whole prey were present in the stomachs. In the 

 absence of whole prey, body size was estimated by 

 measurement of otoliths and beaks. The maximum 

 length of pollock otoliths and lower rostral length 

 (LRL) of gonatid squid beaks were measured to the 

 nearest 0.05 mm with vernier calipers. Squid DML's 

 were estimated by comparison of LRL measure- 

 ments to the LRL/DML relationship of 51 gonatid 

 squid caught in trawls conducted in the vicinity of 

 seal collections. Walleye pollock fork lengths were 

 estimated by regression against otolith length (Frost 

 and Lowry, 1981). For otoliths measuring: 



> 10.0mm,(FL) Y = 3. 175X - 9.770 ( R = 0.968) 

 < 10.0mm, (FL) Y = 2.246X- 0.5 10 (/? = 0.981). 



Walleye pollock ages were estimated from these 

 lengths based on length-age relation described by 

 Smith (1981) and Walline (1983) for walleye pollock 

 from the Bering Sea. 



Otoliths may dissolve or erode to varying degrees 

 depending on their size and duration in fur seal 

 stomachs. We evaluated the bias introduced in FL 

 estimates due to eroded otoliths by assigning 

 otoliths to four condition categories (excellent, good, 

 fair, and poor) based on amount of wear. After qual- 

 ity categorization, the maximum lengths of otoliths 

 (except those in "poor" condition) were measured for 

 estimation of body length by regression, and length 

 frequencies of each category were determined inde- 

 pendently. 



Cephalopod beaks are more resistant to digestion 

 than otoliths and were typically identifiable. Beaks 

 with chipped, worn, or broken rostra were rare and 

 were not measured. Cephalopod beaks were identi- 

 fied to species when possible, but most were catego- 

 rized into two groups referred to as Gonatopsis bo- 

 realis-Berryteuthis magister or Gonatus madokai- 

 Gonatus middendorffi . The two individual species 

 within each group can be separated based on their 

 external morphology and statolith structure, but 



cannot presently be separated based on beak struc- 

 ture alone (Clarke, 1986). 



Trawl collections of potential seal prey 



Trawls were conducted throughout the study area 

 from the Miller Freeman between 1900 and 0600 

 hours within the vicinity of seal collections (Fig. 1). 

 Both bottom and midwater trawls were conducted 

 to provide a relative measure of the availability and 

 size of potential fur seal prey species. Bottom trawls 

 were made at 52-498 m (.v=139 m) depths with an 

 83/112 Eastern bottom trawl (17-m width, 2.3-m 

 height mouth opening; 3.2-cm codend liner mesh; 

 360-mesh circumference; 200-mesh depth; 30-m 

 bridle). Thirty-nine bottom trawls were conducted 

 in 1981 (14 October-4 November), 51 in 1982 (24 

 September-8 October), and 26 in 1985 (5 August- 

 22 August). Seven 1985 trawls were made beyond 

 maximum recorded dive depths of adult female seals 

 (257 m; Ponganis et al., 1992). They were included 

 in analyses because the species and size offish and 

 squid caught were consistent with those caught by 

 bottom trawl within seal dive depths. 



Collection and sorting methods and calculation of 

 bottom trawl catch per unit of effort (CPUE) values 

 followed Smith and Bakkala (1982). The total bot- 

 tom trawl catch was randomly split into a sample 

 of about 2500 kg. Individual species of fishes were 

 identified and weighed (wet) and CPUE (no./ha) was 

 estimated based on distance trawled. In 1981 and 

 1982, cephalopods were classified as squid or octo- 

 pus and discarded. In 1985, all cephalopods were 

 identified, sexed, weighed, and frozen whole. Beaks 

 were extracted and stored in 70% isopropyl alcohol. 



Sex and age determination and body length mea- 

 surements were made on a subsample of up to 200 

 walleye pollock from each trawl. Fork lengths were 

 measured to the nearest centimeter. Saccular 

 otoliths were collected for age determination (Smith 

 and Bakkala, 1982) and stored in 70% isopropyl 

 alcohol. Walleye pollock CPUE was calculated by age 

 and body length. For purposes of this study, age- 

 length frequencies for male and female walleye pol- 

 lock were combined for each of the three years. 



Midwater trawls were made in 1985 with a Dia- 

 mond midwater net (n=8) ( 10-16 fm mouth opening; 

 3.2-cm codend liner mesh; 354-mesh circumference; 

 and 160-mesh depth with 2-m bridles) and a 

 Marinovich herring trawl (rc = 15) (6.1-m width, 6.1- 

 m height mouth opening; 1-cm codend liner mesh; 

 150-mesh circumference; and 350-mesh depth with 

 10-m bridles). Specific trawling positions were cho- 

 sen within the vicinity of northern fur seal collec- 

 tion areas based on the presence of fish or squid as 



