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Fishery Bulletin 91(3), 1993 



to adult levels by the onset of maturity (Kleiber, 1975; 

 Brody, 1945). Data given in Worthy (1987a )'s Figure 4, 

 together with his reported efficiency of 68%, indicates 

 that the gross maintenance requirements of neonate 

 harp and grey seals increases to about 1.8 x the pre- 

 dicted adult MR1 (Eqn. 12) at the onset of feeding. In 

 the absence of precise empirical data on the ontogeny 

 of juvenile harbor seal metabolic rates, it was assumed 

 that MR1 converged from a post-weaning maximum of 

 1.8X the predicted adult MR1 at the onset of feeding 

 to adult MR1 levels at the onset of sexual maturity in 

 an exponential fashion. A multiplier to account for el- 

 evated metabolic rates of juveniles at a given age, 

 JCF, lx „ was thus derived by calculating a series of 

 3- 4- 5- and 6-year exponential decays and weighting 

 them according to the proportion of animals of each 

 sex that matured at each of these ages. The correction 

 for an age-class was calculated as the geometric 

 mean of JCF s(x) at age X and age X+l, which assumes 

 that the metabolic rate evolved at a constant rate 

 throughout the year. The correction was used to cor- 

 rect both basal (Eqn. 11) and maintenance (Eqn. 12) 

 requirements. 



One of the potential shortcomings in directly ex- 

 trapolating the maintenance requirements of captive 

 seals, MR1, to free-ranging seals is that normal activ- 

 ity patterns may be disrupted in captivity. For example, 

 Innes et al. (1987) noted that some of the phocids in- 

 cluded in their analysis were quiescent, and would 

 thus be expected to have lower energy requirements 

 than seals in the wild which spend a portion of their 

 time foraging. A second estimate of gross maintenance 

 requirements, MR2 slz) , was therefore derived by weight- 

 ing the metabolic rates of swimming, SMR XIX „ and rest- 

 ing, RMR, IX „ harbor seals according to a crude activity 

 budget for free-ranging harbor seals: 



MR2 slxl = (P H -SMR slxl ) = (P/RMR XIXI ) 



[13] 



where P s and P r denote the proportion of time seals 

 spend swimming and resting. P, and P r were set at 0.6 

 and 0.4 respectively based on the mean estimated per- 

 centage of time free-ranging radio-tagged harbor seals 

 spent hauled out on land: 44% (Sullivan, 1979), 35- 

 60% (Pitcher and McAllister, 1981) and 37% (Yochem 

 et al., 1987). Age-specific swimming metabolic rates, 

 SMR slxl , were inferred (see Results) from the swim- 

 ming metabolic rates of captive harbor seals (Davis et 

 al., 1985; Williams, 1987). Resting metabolic rates, 

 RMR SIX „ were assumed to be equivalent to BMR, (XI (ap- 

 propriately elevated for juveniles). Since the extreme 

 air and sea temperatures in the study area were likely 

 within the thermoneutral zone, thermoregulatory costs 

 were assumed to be negligible (see Results and Gen- 

 eral Discussion I. 



In addition to maintenance requirements, growing 

 animals require energy for body growth. Daily energy 

 requirements for growth for each sex- and age-class, 

 DGR„ U , were calculated as 



DGR M = CGGI S , 



[14] 



where CG is the apparent gross cost of growth, 201 

 W(kgd')- 1 , as given in Innes et al. (1987), and GI slxl 

 the daily growth increment of each sex- and age-class 

 (i.e. [M ste+1) -Af sW ]/365). 



Finally, mature seals may invest additional energy 

 in reproduction. For females, the total costs of repro- 

 duction were partitioned into: 1) foetal development; 

 and 2) nursing. The net energy invested in foetal de- 

 velopment was estimated from the mass and energetic 

 density of term fetuses and the placenta. The net en- 

 ergy invested in lactation was estimated indirectly from 

 the amount of energy transferred to nursing pups as 

 reflected by changes in the total mass and body com- 

 position of pups between birth and weaning and their 

 maintenance requirements, MR, while nursing. The 

 energy content of the placenta and the energetic den- 

 sities of neonate carcasses and of the mass gained dur- 

 ing nursing were extrapolated from those reported for 

 harp seals (Worthy and Lavigne, 1983). The MP-of 

 nursing pups was assumed to be the same as that of 

 adults of equivalent mass (2.0 x BMR) and growth of 

 pups was assumed to be linear while nursing. Gross 

 reproductive costs were estimated from net costs by 

 assuming that the net efficiency of mothers was 70% 

 (see above) and that for lactation, 95% of the energy in 

 milk transferred to pups was metabolizable (Oftedal 

 and Iverson, 1987). Since females deplete blubber re- 

 serves accumulated during the non-breeding season to 

 meet these costs (Pitcher, 1986), age-specific daily re- 

 productive requirements, DRR lxl , were estimated by 

 amortizing the annual cost over the entire year and 

 applying it to female age-classes based on their fecun- 

 dity rates, FEC txl . Since harbor seals are promiscuous 

 and males are not known to fast or fiercely compete 

 for breeding rights (Bigg, 1981), reproductive costs for 

 males were assumed to be negligible and absorbed into 

 their daily maintenance requirements. 



Two estimates of daily food requirements, FR lx , in 

 kg, were derived. The first estimate, FRl slxl , was ob- 

 tained by summing the components of the energetic 

 budget (MR stII , DGR slxl and, for females, DRR lxl ) to de- 

 termine the total daily energy requirements, DER slxl . 

 Estimates for each sex- and age-class were derived by 

 taking the geometric mean of the parameters at age X 

 and X+l, which assumes that the parameters changed 

 at a constant rate throughout the year. DER SIX , was 

 subsequently converted to units of biomass, FRl slxl by 

 dividing it by the mean weighted energetic density of 



