FISHERY BULLETIN: VOL. 84, NO. 2 



Table 4.— Estimated energy value and consumption of fish and squid by the total population of lactating 

 and nonlactating female northern fur seals (age >4 yr) during July-September (92 days). 



Prey 



Lactating females 



Nonlactating females 



Individual 

 average 

 Energy consumption 

 (kcal/g) 1 (kg/d) 1 



Total seasonal 



consumption by 



population (2.48 x 10 5 ) 



Biomass Energy 



(x 10 3 t) (x 10 9 kcal) 



Individual 



average 



consumption 



(kg/d)' 



Total seasonal 



consumption by 



population (1.32 x 10 5 )2 



Biomass Energy 



(x 10 3 t) (x 10 9 kcal) 



'From Table 3. 



includes postpartum females that fail to lactate. 



DISCUSSION 



The food consumption data presented in Table 2 

 were based on partially digested stomach contents, 

 and thus underestimate the actual feeding rates of 

 adult female northern fur seals. It is apparent from 

 these data that lactating seals obtain most of their 

 energy needs by filling their stomachs slightly more 

 than the nonlactating seals early in the day and by 

 eating additional food later in the day. Any female, 

 whether lactating or not, may eat more than once 

 during the day, as captive northern fur seals often 

 do (Spotte 1980). Females must feed more than once 

 during the 24-h period (on those days when they are 

 able to feed) to meet their daily food requirements 

 because the maximum observed stomach contents 

 by percentage of body mass during July-September 

 1958-74 were 13.8 and 8.2%, respectively, for lac- 

 tating and nonlactating females (Perez 3 ), which are 

 less than their predicted feeding rates. In addition, 

 digestion does vary among individual seals and with 

 the type and amount of prey eaten (Bigg and Faw- 

 cett 1985). However, the data in Table 2 should be 

 typical of the relative relationship between lactating 

 and nonlactating females if actual feeding rates 

 could be measured for free-ranging seals. 



Lactating northern fur seals were estimated to 

 consume 8,960 kcal/d (gross energy), of which 3,520 

 kcal/d (gross energy) represent the additional intake 

 of food related to lactation. Energy expenditures for 

 maintenance and routine activity not directly at- 

 tributable to lactation were estimated to be 5,440 

 kcal/d (gross energy). This estimate is about 5.4 

 times the amount predicted (1,010 kcal/d metaboliz- 

 able energy or 49.0 WO for basal metabolism by the 

 relationship between metabolic rate (MR) in watts 



3 Perez, M. A. 1981. Unpubl. data. Northwest and Alaska 

 Fish. Cent. Natl. Mar. Mammal Lab., Natl. Mar. Fish. Serv., 

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



(W) and body mass (M) shown by Kleiber (1961) (MR 

 (W) = 3.39 M° 75 ). 



These estimates are not typical of energy expen- 

 diture during the first week (7.4 d average) post- 

 partum, a period during which the parturient female 

 does not feed. Lactating seals must metabolize their 

 energy from fat reserves during this period (in- 

 cluding the day before parturition when they usually 

 do not feed, although we considered only the post- 

 parturition period). The loss in body mass (Table 1) 

 in postpartum females following parturition ac- 

 counts for some of this metabolism of energy from 

 fat reserves. This loss includes about 0.6 kg (12% 

 of pup mass as in harp seals, Lavigne and Stewart 

 1979) of placental matter and 3.3 kg (7% prepar- 

 turient female mass) of amniotic and other fluids 

 during parturition (Costa and Gentry in press). 

 There is a calculated net mass loss of 3.2 kg. Loss 

 of body water, as has been reported for some mam- 

 mals, e.g., cattle (Degen and Young 1980) is also 

 probably part of this loss. In addition, this loss in- 

 cludes the utilization of fat reserves to satisfy energy 

 requirements for lactation (Sadleir 1969) during the 

 first few days of the pup's life, a period when par- 

 turient females remain on shore and do not feed 

 (Bartholomew and Hoel 1953; Peterson 1968; Gen- 

 try and Holt in press). 



Our estimate of net mass loss, presumably through 

 fat metabolism, is an underestimate because it was 

 derived from mean body mass data from seals taken 

 at sea, and, therefore, includes lactating animals 

 which probably regained some body mass after their 

 first foraging trip at sea. Costa and Gentry (in press) 

 measured an average of 8.75 kg of mass loss, 

 presumably by tissue metabolism and water loss, 

 prior to the female's initial departure to sea, after 

 which they gained additional body mass. This situa- 

 tion is analogous to that in gray seals. The gray seal 

 does not feed during its entire 18-d lactation period 

 from parturition to weaning (Amoroso and Mat- 



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