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Fishery Bulletin 91(3), 1993 



the thermoneutral zone. However, the seals employed 

 in their tests were restrained and probably post-ab- 

 sorptive. Even if it were assumed that basal require- 

 ments, which account for an average of 43% of the 

 total energy requirements, were elevated to this ex- 

 tent for the 60% of the time seals spent in the water, 

 thermoregulatory costs would only be on the order of 

 18-36 W. However, for a free-ranging seal, 17% of the 

 GE ingested would be liberated as the heat increment 

 associated with feeding and, since musculature is only 

 about 25% efficient (Luecke et al., 1975), the remain- 

 ing 75% of the 23%' of GE expended for swimming 

 would be liberated as heat. Since this "wasted" heat 

 amounts to about 59 W for an average seal, it would 

 appear that the thermoregulatory needs of free-rang- 

 ing seals would be met indirectly through other en- 

 ergy expenditures. 



One important assumption underlying the model was 

 that daily ingestion rates were constant both with sea- 

 son and between regions. In contrast to other phocids 

 such as harp and ringed seals (McLaren, 1958; Ser- 

 geant, 1973), Boulva and McLaren (1979) found no 

 discernible seasonal pattern in the percentage of empty 

 harbor seal stomachs, which implies that harbor seal 

 feed throughout the year. As have previous models, 

 the bioenergetics model assumed that feeding rates 

 were constant in terms of biomass. Alternatively, it is 

 possible that seals alter their foraging patterns in re- 

 lation to the energetic density of prey. For example, 

 greater quantities of poor-quality prey such as hake 

 may be consumed when they are readily available com- 

 pared to high-quality prey such as herring. Because 

 any differences between the amount of energy ingested 

 and required would be reflected by changes in energy 

 reserves, the magnitude of potential biases introduced 

 by these effects can be assessed from the seasonal 

 changes in the condition of animals. Pitcher (1986) 

 reported that the blubber composition of individual 

 harbor seals ranged between extremes of 21% and 55% 

 of body mass. Even if it assumed that the mean blub- 

 ber content declined from 55% to 21% during a 6-month 

 period of reduced food intake (or consumption of poorer 

 quality prey) and increased from 21% to 55% during a 

 second 6-month period of increased food intake (or con- 

 sumption of higher quality prey), daily food require- 

 ments during the first and second 6-month periods 

 would be only 125% and 75% of the annual mean re- 

 spectively. Thus, gradual seasonal changes in food in- 

 gestion rates would not have a major effect on the 

 prey consumption estimates. 



Boulva and McLaren (1979) noted that the condi- 

 tion (i.e., girth:length ratio) of harbor seals of all ages 

 and sexes combined was highest in winter and early 

 spring, decreased during late spring, and was lowest 



in summer and late autumn. Similarly, Pitcher (1986) 

 found that the blubber layer of adult males and fe- 

 males harbor seals were thickest in winter, thinned 

 during the summer, and that females had even thin- 

 ner blubber layers by the autumn moult. These obser- 

 vations are consistent, at least qualitatively, with my 

 bioenergetics model which predicts that animals would 

 accumulate blubber during the winter while feeding 

 mainly on energy-rich herring and deplete blubber dur- 

 ing the summer while feeding mainly on energy-poor 

 hake. The model also predicts that nursing females 

 would utilize an additional 16.8 kg of blubber, or 22% 

 of their total body mass, during the late July to early 

 September nursing period (Bigg, 1969), and would 

 therefore be in poorer condition than males by the 

 autumn moult. 



In an earlier assessment for harbor seals in the 

 Bering Sea, Ashwell-Erickson and Eisner (1981) esti- 

 mated the mean daily per capita gross energy require- 

 ments to be 216-238 watts, which is 26-38% greater 

 than my estimate of 172 watts. The difference appears 

 to be almost entirely attributable to geographic differ- 

 ences in body size. The mean body mass of harbor 

 seals (both sexes were combined) in the Bering Sea 

 was 67.7 kg, which is about 50% greater than the mean 

 body mass of 44.7 kg in British Columbia. This differ- 

 ence translates into about a 37% difference in meta- 

 bolic mass (M 075 ), to which energy expenditures were 

 scaled in both our models. 



After my model had been completed, Harkonen and 

 Heide-Jorgensen ( 1991) published a very similar model 

 for harbor seals in the Skagerrak. There is good agree- 

 ment between our models on how the energy budget is 

 partitioned among various components. Harkonen and 

 Heide-Jorgensen (1991) estimated that, for an increas- 

 ing population, 73.1% of metabolizable energy (NE and 

 the heat increment) is expended on maintenance, 19.0% 

 on activity, 2.0% on growth, 4.5% on reproduction, and 

 1.5% on the annual moult. According to my model, 

 68.1%- is expended on maintenance (including the heat 

 increment), 26.9% on activity, 1.4% on growth, 3.7% 

 on reproduction, and I made no allowance for any costs 

 associated with the annual moult. However, there is a 

 considerable discrepancy between our estimates of the 

 mean daily per capita gross energy requirements, even 

 though the mean body mass of seals in the Skagerrak 

 and British Columbia are very similar (41.8 and 44.7 kg 

 respectively). Harkonen and Heide-Jorgensen's (1991) 

 estimate of 227 watts is 32% greater than my estimate 

 of 172 watts. This discrepancy is due primarily to dif- 

 ferences in the way energetic requirements were scaled 

 to body mass. Harkonen and Heide-Jorgensen (1991) 

 first estimated the requirements of juveniles and then 

 extrapolated these estimates to adults by assuming 



