FISHERY BULLETIN: VOL. 84, NO. 4 



in stomachs of seals thereby inflating the importance 

 of squid (Scheffer 1950; Spalding 1964; Bigg and 

 Fawcett 1985). Recent experimental studies con- 

 firmed that squid beaks accumulate in fur seal 

 stomachs (Bigg and Fawcett 1985). 



To date, no reports have been published on the 

 diet of northern fur seals that take this bias into ac- 

 count. However, Bigg and Perez (1985) suggested 

 a method, called modified volume, which reduces the 

 bias and also accounts for differences in digestion 

 rates between fish and squid. In this method, 

 evidence of diet based on trace remains, such as 

 squid beaks and fish bones, is omitted in the 

 analyses, and a combination of the frequency of oc- 

 currence and volumetric methods is used to estab- 

 lish the relative importance of individual prey 

 species. 



We use the modified volume method in this report 

 to analyze data from the stomach contents collected 

 by the United States and Canada during 1958-74. 

 We will describe the annual diet of northern fur seals 

 in the eastern North Pacific and eastern Bering Sea 

 by region and subregion. We also incorporate the 

 energy content of important prey species to deter- 

 mine whether this might affect relative importance, 

 a procedure not tried previously with this seal. 



METHODS 



Lander (1980) and Kajimura (1984, 1985) de- 

 scribed the methods used to take northern fur seals 

 at sea during 1958-74 and to identify- and measure 

 the prey items found in their stomachs by volume 

 and frequency of occurrence. A total of 18,404 

 stomachs were collected of which 7,373 contained 

 food and an additional 3,326 had only trace remains. 

 Perez and Bigg (fn. 3) summarized the data on 

 volume and frequency of occurrence for all species 

 of northern fur seal prey by month and region. 



Perez and Bigg 4 and Bigg and Perez (1985) gave 

 a detailed discussion of the procedure used to cal- 

 culate modified volume values. First, prey species 

 represented in any stomach only by trace amounts 

 (<10 cc) were omitted. Second, the proportions of 

 total fish and total squid in the diet by subregion, 

 region, and month were then determined by non- 

 trace frequency of occurrence. Third, the ratio of 

 each species within only the fish category and within 



only the squid category was determined by volume. 

 The taxonomic groupings recorded in the original 

 data which overlapped each other were either pooled 

 with higher taxa or were proportionally divided 

 among component species depending upon which 

 level of taxa had the most data. This prevented food 

 groupings from being partially compared against 

 themselves. Next, the volumetric ratios for in- 

 dividual fish and squid species were adjusted to sum, 

 respectively, to the total proportion of fish and squid 

 in the diet. Finally, all values were readjusted to 

 total 100%. 



The relative importance of prey species has been 

 presented in this report in two ways: 1) modified 

 volume values for each region by month, and for 

 each subregion with data from all months pooled; 

 and 2) modified volume values for each region based 

 on combined months data which were weighted for 



Table 1 .—Estimated energy values (wet mass) for important north- 

 ern fur seal prey. C = bomb calorimetry combustion value; P = 

 proximate analysis value 1 ; muscle = edible portion only of raw 

 material; whole = raw material from entire specimen. 



4 Perez, M. A., and M. A. Bigg. 1981. Modified volume: a two- 

 step frequency-volume method for ranking food types found in 

 stomachs of northern fur seals. Unpubl. rep., 25 p. Northwest 

 and Alaska Fisheries Center, National Marine Mammal Labora- 

 tory, National Marine Fisheries Service, NOAA, 7600 Sand Point 

 Way N.E., Seattle, WA 98115. 



'Values were calculated with the following energy factors derived from Watt 

 and Merrill (1963): 9.50, 5.65 and 4.00 kcal/g respectively for fat, protein and 

 carbohydrate. 



2 Miller, L. K. 1978. Energetics of the northern fur seal in relation to 

 climate and food resources of the Bering Sea. U.S. Mar. Mammal Comm. 

 Rep. MMC-75/08, 27 p. 



3 Myctophidae and Paralepididae. 



4 Pleuronectidae. 



5 Perez, M. A., Natl. Mar. Mammal Lab., Northwest and Alaska Fish. Cent., 

 Natl. Mar. Fish. Serv., NOAA, 7600 Sand Point Way N.E., Seattle, WA 981 15, 

 unpubl. data, 1984. 



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