FISHERY BULLETIN: VOL. 84, NO. 2 



Table 2.— Body mass and arcsine transformed mass of stomach contents (expressed as a percent- 

 age of body mass) for lactating (LACT) and nonlactating (NON) female northern fur seals (age >4 

 yr) by hour of collection after sunrise during July-September 1958-74. The relative consumption index 

 (percentage expression of the ratio of the proportion of body mass which was stomach contents for 

 lactating females relative to that for nonlactating females) is also given. 



'Significance levels for comparisons between the mean proportions of body mass which was stomach contents 

 (arcsine units) for lactating and nonlactating females were determined by t tests. 



To derive a single index value for relative food 

 consumption between lactating and nonlactating 

 females, we performed alternative calculations. In 

 this case we did not simply pool data because that 

 would not adequately account for digestion trends. 

 Northern fur seals feed primarily at night or in the 

 early morning hours (Fiscus et al. 1964; Gentry et 

 al. in press); therefore, we considered the value at 

 0-3 h after sunrise (0.96; Table 2) as the relative daily 

 quantity of stomach contents for nonlactating seals. 

 Feeding more than once a day to satisfy only energy 

 needs of maintenance and routine activity should be 

 done by all fur seals, and would already be included 

 in these results (Table 2) when the inherent relative 

 rate of digestion is examined. However, lactating 

 females require additional food intake for milk pro- 

 duction, and we added an increment (0.12) to the 

 value observed at 0-3 h after sunrise (1.56; Table 

 2) to calculate an adjusted index of 1.68% of body 

 mass. This incremental value was derived first by 

 calculating the rate of decrease between data values 

 for partially digested stomach contents at the dif- 

 ferent hourly time intervals. We assumed the rate 

 of digestion throughout the day was the same for 

 lactating females as that observed for nonlactating 

 females. Next, keeping the value for lactating 

 females at 0-3 h after sunrise (1.56) as constant, we 

 summed the absolute value of the differences be- 

 tween the expected values for remaining stomach 

 contents and the observed values in Table 2 to ob- 

 tain a value of 0.12. We then calculated a value of 

 174% as our index of relative food intake (i.e., the 

 ratio of 1.68 for lactating females relative to 0.96 

 for nonlactating females) for a typical foraging day. 



However, because females do not feed every day 

 during the breeding season (Bartholomew and Hoel 

 1953; Peterson 1968; Gentry and Holt in press), the 

 average daily feeding rate (adjusted for percent- 

 age of time feeding at sea) for lactating seals is 1.6 

 times that for nonlactating seals during July- 

 September, i.e, the increased cost of lactation is 

 + 59.8%. 



Estimated Energy and Food Requirements 



Lactating and nonlactating female northern fur 

 seals consumed the same species of prey in relatively 

 similar proportions within their diet, when feeding 

 in the same general area at the same time during 

 1958-74. Ranks of importance of prey to the diet 

 were significantly correlated (P < 0.05); the percent 

 similarity of relative prey importance by percent 

 modified volume was 80%; and there was no signifi- 

 cant difference in the frequency of food stomachs 

 containing fish or squid. Being culled from the same 

 region and for the same season, data for all adult 

 females were pooled. 



We derived a gross energy estimate of 1.40 kcal/g 

 as the average energetic density of northern fur seal 

 prey during July-September based on their relative 

 dietary importance and information in the literature 

 on their energy content (Table 3). Using the data 

 on seal body mass (Table 1) and increased cost of 

 lactation ( + 59.8%), we calculated average daily 

 feeding rates of 18.2% (6.42 kg) and 11.4% (3.55 kg) 

 of total body mass, respectively, for the average lac- 

 tating and nonlactating adult female. This repre- 

 sents daily energy consumption requirements of 



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