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Fishery Bulletin 104(3) 



than scats (Jobling and Breiby, 

 1986; Gales et al., 1993) and 

 have largely been excluded from 

 pinniped diet analyses. Spews 

 from northern fur seals are pres- 

 ent on summer breeding islands, 

 but diet studies that are based 

 on spews are limited (Kiyota et 

 al., 1999). 



Our study is a comparative 

 evaluation of diet based on scat 

 and spew samples collected from 

 northern fur seal breeding rook- 

 eries on two islands in the east- 

 ern Bering Sea. Prey remains 

 in scat and spew samples were 

 compared for prey species com- 

 position, and the age or size of 

 walleye pollock (Theragra chal- 

 cogramma) and of two species of 

 gonatid squids (Gonatopsis bo- 

 realis and Berryteuthis magister) 

 were estimated. Finally, we com- 

 pared age classes of walleye pol- 

 lock found in scats and spewings 

 to age classes of walleye pollock 

 caught by the commercial trawl 

 fishery in management regions 

 adjacent to the Pribilof Islands. 



Figure 1 



Location of St. George Island and St. Paul Island, within the Pribilof Islands, 

 Alaska. Shaded area indicates walleye pollock iTheragra chalcogramma) com- 

 mercial trawl fishery management areas surrounding the Phibilof Islands. 



Materials and methods 



Sample collection and processing 



Scats and spews were collected opportunistically from 

 rookeries on the Pribilof Islands (St. George and St. 

 Paul), Alaska (Fig. 1), during the breeding season 

 between 1990 and 2000. Samples collected from rook- 

 eries during this time (late July through September) 

 were considered to be primarily from females of breeding 

 age (Antonelis et al., 1997). Each scat and spew sample 

 was placed in a plastic bag and frozen until it was 

 analyzed in the laboratory. Samples were thawed and 

 soaked in a mild emulsifying soap solution then rinsed 

 through nested sieves of 4.75, 1.4, 1.0, and 0.5-mm mesh. 

 Bones, otoliths, beaks, and eye lenses were recovered 

 from the sieved samples and stored for analysis. Bones, 

 otoliths, and eye lenses were stored dry in vials and 

 beaks were stored in vials containing 50% isopropyl 

 alcohol (Antonelis et al., 1997). Recovered diagnostic 

 fish bones, otoliths, and cephalopod beaks were identi- 

 fied to the lowest possible taxon by comparing them to 

 a reference collection. Distinctions, based on the mor- 

 phological features of beaks, between some cephalopod 

 species within the family Gonatidae were not possible. 

 Therefore, identifications of four squid species known 

 to occur in northern fur seal diet were categorized into 

 two groups following Sinclair et al. (1994) and Antonelis 

 et al. (1997) and are referred to as Gb-Bm (Gonatopsis 



horealis and Berryteuthis magister] and Gm-Gm (G. 

 madokai and G. niiddendorfii). Additionally, distinctions 

 between four gonatid species (Eogonatus tinro. G. herryi, 

 G. pyros, and G. onyx), were difficult to determine from 

 the morphological features of beak and, collectively, are 

 referred to as "gonatid group I" in the present study. 



Prey indices 



Individual prey species and prey groups were analyzed 

 according to their frequency of occurrence (FO) in scats 

 and spews for both islands. The percent frequency of 

 occurrence (F0%) was calculated by dividing the number 

 of scats or spews containing a specific prey species or 

 group by the total number of scats or spews containing 

 identifiable prey remains. Data from each year were 

 pooled for F0% calculations because of low sample sizes 

 in some years. Prey species or groups with a F0% 26% 

 when rounded to the nearest integer, for either island or 

 sample type, were considered to be primary prey. 



The minimum number of individuals (MNI) of each 

 prey type was calculated for each sample, and summed 

 over all samples. The cephalopod MNI was estimated 

 by using the maximum count of either upper or lower 

 beaks in each sample. Fish MNI was estimated by us- 

 ing the maximum count in each sample from left or 

 right otoliths, plus half of the otoliths for which a side 

 (right or left) could not be determined (Antonelis et al., 

 1997). Percent MNI (MNI'7f) was then calculated by 



