FISHERY BULLETIN: VOL. 84, NO. 2 



Once arriving at the rookery, a parturient female 

 gives birth to one pup, initiates lactation, comes into 

 estrus, and copulates with a male, but does not feed. 

 Gentry and Holt (in press) provided data showing 

 that the average adult female is on shore about 1 

 d before and 7.4 d after parturition. Each subse- 

 quent shore visit lasts about 2.2 d (Peterson 1968; 

 Gentry and Holt in press). The duration of the first 

 sea trip is the shortest (4.8 d), with the duration of 

 the subsequent sea trips increasing at a rate of an 

 additional 1.2 d/30 d postpartum (Gentry and Holt 

 in press). 



Recent data collected on the Pribilof Islands by 

 Gentry and Holt (in press) suggests that nonpreg- 

 nant (= nonlactating) adult females arrive later 

 (about 8 d) on the rookeries and that they may show 

 a somewhat different behavioral pattern than preg- 

 nant females. Their first foraging trip at sea is 

 longer (8.9 d), but each of their subsequent shore 

 visits is of constant duration (2.5 d). From these data 

 we derived values for total percent of time spent 

 at sea during July- September (92 d) for the average 

 adult female. Assuming birth of pups on 5 July, this 

 was 69.3 and 75.9% for lactating and nonlactating 

 females, respectively. However, it should be noted 

 that individual females vary from these averages 

 because the period during which adult females first 

 arrive on the rookeries extends over 30 d (Bartholo- 

 mew and Hoel 1953; Peterson 1968; Gentry and Holt 

 in press). 



Feeding Rate Calculations 



Bigg et al. (1978) provided data on feeding rates 

 for three captive adult female northern fur seals. 

 Their data for these seals were 5,977 kcal/d (3.0 

 kg; 6.7% of body mass), 6,118 kcal/d (3.1 kg; 7.6% 

 of body mass), and 5,055 kcal/d (2.5 kg; 8.5% of body 

 mass). These captive northern fur seals were main- 

 tained with a diet of Pacific herring (2.01 kcal/g dur- 



Table 1 .—Body mass (minus stomach contents mass) of lactating 

 (postpartum) and nonlactating female northern fur seals (ages >4 

 yr pooled) taken pelagically in the eastern Bering Sea and western 

 Alaska, 1958-74. 



ing winter), and it was necessary to consider the 

 energetic concentration of the seal's diet in the 

 wild with respect to the data in Bigg et al. (1978). 

 We derived the following relationship from these 

 data: 



Daily energy consumption (kcal/d) = 375.47 M° 75 



by averaging the results given for the three captive 

 seals. We calculated average daily feeding rates 

 using this relationship and data on seal body mass. 



RESULTS 



Body Mass 



Table 1 gives the mean values of body mass of 

 adult female northern fur seals (age >A yr) taken 

 during June-September in the eastern Bering Sea 

 and western Alaska. During July-September, the 

 average lactating female (mean 35.3 kg, median age 

 10 yr) had a body mass 1.13 times that of the 

 average adult nonlactating female (mean 31.1 kg, 

 median age 5 yr; seals age ^4 yr only). However, 

 as Figure 2 shows, lactating and nonlactating 

 females of the same age were similar in body mass. 

 The differences shown in Table 1 are primarily due 

 to the higher proportions of lactating females at 

 older ages (Lander 1981). 



Lactating females exhibited a significant (P < 

 0.001) loss of 7.1 kg of body mass between June and 

 July following parturition (Table 1). This is based 



60 r- 



50 - 



_ 40 - 

 en 



■o, 



CD 



E 30 



20 



10 



10 

 Age 



15 



> 18 



'Pregnant (prepartum) females. Body mass does not include fetal mass. 



Figure 2.— Mean body mass (minus stomach contents mass) of lac- 

 tating and nonlactating female northern fur seals by age taken 

 pelagically in the eastern Bering Sea and western Alaska during 

 July-September 1958-74. 



374 



