PEREZ and MOONEY: LACTATING NORTHERN FUR SEALS 



Table 3.— Relative dietary importance, energy value and average daily consumption of prey by individual 

 lactating and nonlactating female northern fur seals (age >4 yr) in the eastern Bering Sea during 

 July-September. 



'Percent modified volume of stomach contents data collected during 1958-74. 



2 For some species, data were derived from results of proximate analyses on muscle tissue composition using energetic 

 density factors of 9.50, 5.65 and 4.00 kcal/g (gross energy), respectively for fat, protein, and carbohydrate (Watt and Merrill 

 1963). Data for other species were based on bomb calorimetry analyses of whole specimens. 



3 Derived by multiplying columns 1 and 2, and summing to 100%. 



4 Based on proximate analysis data for Pacific herring, Clupea harengus pallasi, in Bigg et al. (1978). 



5 Based on proximate analysis data for salmonids (Salmonidae); Pacific sand lance, Ammodytes hexapterus; and flounders 

 (Pleuronectidae) in Sidwell (1981). 



6 Based on data from heat of combustion in analyses of whole fish specimens of capelin, Mallotus villosus, and walleye 

 pollock, Theragra chalcogramma [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.] 



'Based on proximate analysis data for deepsea smelt (Bathylagidae) in Childress and Nygaard (1973). 



8 Based on proximate analysis data for Atka mackerel, Pleurogrammus monopterygius, in Kizevetter (1971). 



9 Average value of prey species in diet adjusted by their relative dietary importance. 



10 Perez, M. A. 1984. Unpubl. data. Northwest and Alaska Fish. Cent. Natl. Mar. Mammal Lab., Natl. Mar. Fish. Serv., 

 NOAA, 7600 Sand Point Way NE., Seattle, WA 98115. 



8,960 kcal and 4,940 kcal for the average, individual 

 lactating and nonlactating adult female northern fur 

 seal during July-September (Table 3). Average 

 postpartum females not in a lactating state would 

 have a daily energy consumption requirement of 

 5,430 kcal or feeding rate of 11.0% (3.89 kg) of total 

 body mass. 



Table 3 also provides estimates for each food item 

 of the total energy and biomass consumed daily by 

 the average individual adult female. Lactating 

 females each consume about 6,220 kcal/d gross 

 energy (4.3 kg/d) of fish and 2,740 kcal/d gross 

 energy (2.2 kg/d) of squid, and each nonlactating in- 

 dividual consumes about 3,430 kcal/d gross energy 

 (2.4 kg/d) offish and 1,510 kcal/d gross energy (1.2 

 kg/d) of squid. Female northern fur seals are not 

 able to feed every day, and thus estimated consump- 

 tion for the average foraging day is 8,980 kcal/d 

 gross energy (6.1 kg/d) of fish and 3,950 kcal/d gross 

 energy (3.1 kg/d) of squid by lactating seals, and 

 4,530 kcal/d gross energy (3.1 kg/d) of fish and 1,990 

 kcal/d gross energy (1.6 kg/d) of squid by nonlac- 

 tating females. 



We also calculated estimates of the total energy 

 in biomass consumed by all adult females during 

 July-September in the eastern Bering Sea (Table 4). 



Because the northern fur seal population has been 

 declining in recent years (North Pacific Fur Seal 

 Commission 1984) we used 80% of the estimated 

 population values given by Lander (1981): 2.61 x 

 10 5 pregnant/postpartum and 1.19 x 10 5 nonpreg- 

 nant adult females (age ^4 yr). We assumed all of 

 these seals are present in the eastern Bering Sea 

 during this period. Because 5% of the pups born on 

 St. Paul Island, Pribilof Islands, between 1975 and 

 1982 died on the rookeries during July and August 

 (Kozloff 1985), we modified our calculations to 

 reflect the number of postpartum females which are 

 nonlactating. We thus estimated a total of 2.48 x 

 10 5 lactating and 1.32 x 10 5 nonlactating adult 

 females (age >A yr). Multiplying individual estimates 

 by these population totals, lactating females con- 

 sume an estimated collective total of 204.5 x 10 9 

 kcal gross energy (146.5 x 10 3 1) and nonlactating 

 females consume an estimated collective total of 

 60.2 x 10 9 kcal gross energy (43.1 x 10 3 1) of food. 

 Therefore, all adult female northern fur seals con- 

 sume an estimated collective biomass of 189.6 x 

 10 3 1 with a gross energy value of 264.7 x 10 9 kcal 

 during July-September, of which 69.4% of this 

 energy (183.7 x 10 9 kcal; 125.9 x 10 3 t) are fish 

 and 30.6% (81.0 x 10 9 kcal; 63.7 x 10 3 1) are squid. 



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