PEREZ and MOONEY: LACTATING NORTHERN FUR SEALS 



thews 1951, 1952) and over 80% of the female's 

 stored energy reserves are used to feed their pup 

 (Fedak and Anderson 1982). 



We conducted similar analyses of data comparing 

 pregnant and nonpregnant adult females (age >4 

 yr) during June-July, but we did not find any sig- 

 nificant difference in relative feeding rates. We, 

 therefore, conclude that the onset of the lactation 

 process, and not pregnancy, initiates increased 

 feeding behavior in parturient fur seals. Pregnant 

 northern fur seals presumably consume more food 

 than required by nonpregnant females (i.e., more 

 than that simply required as a function of body 

 mass). This would be necessary for growth of the 

 fetus, especially during winter and spring months 

 when they are in the North Pacific. This conclusion 

 was based on a preliminary examination of the 

 pelagic fur seal data, although the results were not 

 statistically conclusive. Female northern fur seals 

 probably also store energy in fat reserves for the 

 stresses of birth and the first week of lactation. 

 Nevertheless, any additional food intake required 

 by pregnant females is substantially less than that 

 of lactating seals. 



We believe lactating females may reduce their 

 need for additional food intake during the last month 

 prior to weaning of pups because we did not find 

 a significant difference in food consumption between 

 lactating and nonlactating females during October; 

 however, data were few. Weaning does not occur 

 until late October or early November when females 

 abandon their pups; the mean date of weaning is 

 about 2 November (Peterson 1968). It should be 

 noted that births occur over at least a 30-d period 

 (Peterson 1968), and weaning of individual pups will 

 likewise occur over a similar time frame. It is thus 

 possible that pups born earlier will quit nursing 

 earlier than those born later in the season. The total 

 lactation period is about 3-5 mo. Therefore, the 

 feeding rate relationships and energy estimates 

 presented in this paper should typify those during 

 the first three months of lactation only, and not 

 necessarily during July-September. 



We assumed that all postpartum females taken 

 during 1958-74 were lactating. We believe that this 

 assumption does not significantly affect our results 

 because only a small percentage of the postpartum 

 females fail to lactate or terminate lactation for one 

 reason or another (such as still birth or death of the 

 pup). Therefore, our estimate of the difference in 

 consumption between lactating and nonlactating 

 females is a conservative indicator of the magnitude 

 of this ratio. This is because inclusion of postpartum 

 females that did not lactate would have decreased 



the mean value of stomach contents for the lactating 

 group. 



Individual northern fur seals show variations in 

 their feeding locations. Differences may occur over 

 location and time. For example, lactating females 

 may travel great distances, e.g., at least 160 km 

 from the Pribilof Islands (Perez 4 ), during their sea 

 trips in search of food, and they may dive up to 200 

 m (Gentry et al. in press) to catch prey. There are, 

 of course, differences in availability (e.g., walleye 

 pollock, Theragra chalcogramma, Smith and Bak- 

 kala 1982) and energetic density (e.g., Pacific 

 herring, Clupea harengus pallasi, Bigg et al. 1978; 

 deepsea smelt, Bathylagidae, Childress and Nygaard 

 1973) of prey by season, region, and depth. The 95% 

 C.I. for the importance of fish biomass in the fur 

 seal diet in the Bering Sea is 64.0-68.6% (Perez and 

 Bigg 5 ). Therefore, the estimated quantity of fish and 

 squid consumed, and their relative energy contribu- 

 tion, may vary ±5%. 



It should also be stressed that the estimates pre- 

 sented in this paper also depend heavily on metabolic 

 rate information for adult females which we ob- 

 tained from the literature. Individual variations 

 among seals will cause differences in results ob- 

 tained from several experiments, and future 

 research may provide somewhat different metabolic 

 rates. Should feeding rates be revised substantial- 

 ly, then the magnitude of energetic estimates from 

 these data will be affected in a corresponding direc- 

 tion. However, the relative ratio of food consump- 

 tion between lactating and nonlactating females dur- 

 ing the breeding season will be unaffected, and 

 remain about 1.6. We suggest the need for further 

 studies on feeding behavior and energetics of lac- 

 tating females and pups prior to weaning. 



ACKNOWLEDGMENTS 



We thank Michael Bigg and Peter Olesiuk of the 

 Pacific Biological Station, Nanaimo, B.C.; Daniel 

 Costa of the University of California at Santa Cruz, 

 and Roger Gentry and the late Mark Keyes of the 

 National Marine Mammal Laboratory, Seattle, WA, 

 for valuable information on the biology and behavior 

 of fur seals. We also thank Charles Fowler and 



"Perez, M. A., Northwest and Alaska Fisheries Center National 

 Marine Mammal Laboratory, National Marine Fisheries Service, 

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

 observ., 1984. 



6 Perez, M. A., and M. A. Bigg. 1984. Food habits of northern 

 fur seals (Callorhinus ursinus) off western North America. Un- 

 publ. rep., 67 p. Northwest and Alaska Fish. Cent. Natl. Mar. 

 Mammal Lab., Natl. Mar. Fish. Serv., NOAA, 7600 Sand Point 

 Way NE, Seattle, WA 98115. 



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