Olesiuk: Prey consumption of Phoca vitulma 



501 



remainder of the first year of life. The average body 

 mass of 12 yearlings (7 females and 5 males) aged 1.0 

 years (±2 months) was 29.3kg (28.4kg for females; 

 30.7kg for males). Growth in subsequent years was 

 asymptotic for both sexes and could be described by 

 von Bertalanffy curves (Fig. 5). The equations indi- 

 cated that females attained an asymptotic mass of 

 66.0 kg, 90% of which was attained by 7 years, and 

 males an asymptotic mass of 89.2 kg, 90% of which 

 was attained by 9 years. In accord with these predic- 

 tions, the mean body mass of the 10 female specimens 

 aged >7 years was 60.0 kg and the mean of the 8 male 

 specimens aged >9 years was 86.6kg (Fig. 5). The mean 

 body mass of all ages, calculated at the end of the 

 pupping season and weighted according to the stable 

 age-structure, was 44.2 kg for females, 45.3 kg for 

 males, and 44.7 kg overall. 



Energetics 



Energetic life tables are shown in Tables 1 and 2. The 

 main advantage of the energetics approach over stom- 

 ach volume analyses was that total energy require- 

 ments could be partitioned into energy required for 

 basal metabolism, activity, growth and reproduction. 

 This enabled an assessment of the relative magnitude 

 and uncertainty associated with each component of 

 the energy budget. 



The estimated gross maintenance requirements of 

 adult harbor seals, based on the rate of energy inges- 

 tion by captive phocids, MRl^, ranged from 1.85- 

 1.94X (X=1.90x) the predicted adult BMR (Eq. 11). 



AGE (years) 



Figure 5 



Mean body mass (±SE) as a function of age. Data are from 

 Bigg (1969) supplemented with data from Bishop (1967). 

 Growth curves represent specialized von Bertalanffy equa- 

 tions fitted to the data by least squares criteria. 



Multipliers to account for the elevated metabolic rates 

 of juveniles, JCF slxl ranged from 1.8 at weaning to 1.0 

 at the onset of maturity. The corrected juvenile main- 

 tenance requirements, MRl s ,,„ thus declined from 3.5 x 

 the predicted adult BMR at weaning, to 3.1x the pre- 

 dicted adult BMR at age 1.0 years, to 2.6 x the pre- 

 dicted adult BMR at age 2.0 years, and ultimately to 

 1.9x the predicted adult BMR at the onset of matu- 

 rity. These predictions appear to be consistent with 

 the metabolic rates of juveniles reported in the litera- 

 ture. Keiver et al. ( 1984) reported that the gross main- 

 tenance requirements of harp seals aged 5-24 months 

 were 2. 1-3.0 x predicted adult BMR levels and data in 

 Innes (1984) indicate that the gross maintenance re- 

 quirements (assuming 70% net efficiency) of grey and 

 harp seals aged 1-28 months were 2.9 x predicted adult 

 BMR levels. The unweighted mean correction for juve- 

 niles of all ages was 1.34, which was similar to the 1.4 

 derived by Innes et al. (1987) for juveniles of various 

 ages pooled. Similarly, the average MR1 SIXI for harbor 

 seals aged 0-3 years was 2.9 x the predicted adult 

 BMR, which was similar to the 2.8 x adult BMR esti- 

 mated for captive harbor seals aged 0-3 years by 

 Markussen et al. (1990) who, interestingly, found no 

 evidence of age-specific changes in maintenance re- 

 quirements between and 3 years of age. 



A second estimate of maintenance requirements, 

 MR2, was obtained by weighting swimming and rest- 

 ing metabolic rates, SMR and RMR, according to an 

 activity budget for free-ranging harbor seals. Davis et 

 al. ( 1985) reported that the SMR of a 63-kg adult har- 

 bor seal swimming at 1.4 msec 1 , its preferred swim- 

 ming speed, was 2.3x its expected BMR (i.e., 2.2X its 

 resting metabolic rate which was reported to be l.lx 

 BMR). Similarly, the net SMR of an 85-kg adult har- 

 bor seal swimming at the same speed was 210 watts 

 (Williams, 1987), or 2.4x its predicted BMR. In con- 

 trast, a 33-kg yearling swimming at 1.4 msec _1 exhib- 

 ited a relatively higher net swimming metabolic rate 

 of 170 watts (Davis et al., 1985). Although this was 

 3.6X the predicted BMR for an adult of equivalent 

 mass, it was only 2.3 x the corrected BMR for a year- 

 ling (i.e., the juvenile SMR appeared to be elevated to 

 the same extent as its total MR1 ). Age-specific SMfl ( „'s 

 were therefore assumed to be 2.3 x BMR SIX , (appropri- 

 ately elevated for juveniles ) for all ages and both sexes. 

 Assuming that seals spent 40% of their time resting 

 on land and 60% swimming and that the RMR, x 's were 

 equivalent to BMR slxl (again appropriately elevated for 

 juveniles), the net MR2 SIX , was estimated to be 1.8x 

 the corrected BMR ilxl , and the gross MR2 s!x , to be 2.5 X 

 the corrected BMR, IXI . 



The second estimate of gross maintenance require- 

 ments, MR2, was approximately 34% (range 31-37%) 

 greater than the first estimate, MR1, for all age-classes. 



