Olesiuk Prey consumption of Phoca vitulma 



495 



were not constant with age (Bigg, 1969; Olesiuk 4 ). The 

 best fitting series of log-linear segments was therefore 

 obtained iteratively by applying piecewise regressions 

 with varying inflection points. Because pups aged <6 

 months were likely over-represented in Bigg's (1969) 

 sample, their abundance, N m , relative to older age- 

 classes, N s!x „ was calculated as 



MA f 



N Sl0) = 0.5-l(N flx :FEC,x 



[7] 



which assumes that the sex ratio at birth was equal. 

 The number of seals in each sex- and age-class at the 

 end of the 1988 pupping was estimated by normaliz- 

 ing the relative N s , x , series to sum to N p . Assuming 

 that births occurred as a pulse at the beginning of the 

 annual cycle and deaths throughout the year, the finite 

 annual birth rate, (3, was estimated as 



P = (N fi0l +N m , 0l 



MA f 



MA„, 

 A"=l X=l 



[8] 



and the mean annual finite death rate, d, as 



d = Ho/tl+pD, 19] 



where a is the finite annual population multiplication 

 rate (1.125; Olesiuk et al., 1990a) and MA s denotes the 

 maximum ages attained by each sex (see Results). 



Rates of growth in body mass were calculated sepa- 

 rately for each sex by fitting specialized von Bertalanffy 

 curves (Zullinger et al., 1984) to the body mass at age 

 data summarized in Bigg ( 1969). Ages were estimated 

 to the nearest month by assuming that all animals 

 were born in June. Owing to a small number of adult 

 males in Bigg's (1969) sample, his data were supple- 

 mented with data for 10 males aged 10-25 years col- 

 lected in the Gulf of Alaska (Bishop, 1967). Param- 

 eters of the specialized von Bertalanffy growth curves 



M., 



:A-(l-0.33-e ,A ' IA '- II, P, 



[10] 



where M slx , represents the body mass of sex S at age X, 

 and A, K, and / are the growth parameters, were esti- 

 mated iteratively by a Quasi-Newton method (Fletcher 7 ) 

 using least squares criteria. 



Energetics 



Daily food requirements were estimated from two 



Tletcher, R. 1972. FORTRAN subroutines for minimization by quasi- 

 Newton methods. AERER 7125. 



sources of data: 1) energetic parameters reported in 

 the literature for harbor seals and related phocids based 

 on captive studies; and 2) volumes of undigested prey 

 in the stomachs of harbor seals collected on the east 

 coast of Canada (Boulva and McLaren, 1979). In both 

 cases, daily food requirements were estimated sepa- 

 rately for each sex- and age-class in the population 

 based on their mean body masses, and mean per capita 

 requirements calculated from energetic life tables by 

 weighting the individual estimates according to the 

 sex- and age-structure of the study population. Mean 

 body masses of age-classes were obtained by taking 

 the geometric mean of their estimated masses at the 

 beginning and end of the age interval (Eqn. 10), which 

 assumes that growth was uniform throughout the year. 

 Energetic parameters were calculated according to the 

 International System of Units (ASTM, 1982). Where 

 necessary, non-conforming values in the literature were 

 converted by assuming that 1 calorie = 4.184 joule (J), 

 such that lkcal-day- 1 = 0.0484 J-sec 1 or Watts (W). 

 Where efficiency was not stated, net energy (NE) ex- 

 penditures were transformed to gross energy (GE) re- 

 quirements by assuming that overall efficiency was 

 70%; i.e. 13% of the GE in the diet, which usually 

 consisted of herring, was not metabolizable (6% lost in 

 feces and 7% in urine; Keiver et al, 1984) and 17% of 

 GE was expended as the heat increment associated 

 with feeding (Gallivan and Ronald, 1981). 



The basal metabolic rates of adult pinnipeds (Lavigne 

 et al., 1986), like those of other adult mammals, con- 

 form with Kleiber's (1975) relationship. The net basal 

 metabolic rates of adults of aged X, BMR XX , in watts, 

 was therefore estimated as 



BMR, 



3.4-Af, 



(from Kleiber 1975) [11] 



where M s(xl denotes the mean body mass (kg) of sex S at 

 age X. Since other major components of the energetic 

 budget also scale to M" 75 ( Lavigne, 1982 ), it is convenient 

 to consider total energy requirements relative to BMR. 



A large portion of the overall energy budget of seals 

 is expended on maintenance, which encompasses the 

 energy required for basal metabolism, activity and ther- 

 moregulation (Lavigne et al, 1982). Innes et al. (1987) 

 provided one estimate of the gross maintenance re- 

 quirements of non-growing, adult phocids based on the 

 rates of energy ingestion in captivity, MR1„ X , in watts: 



MRL 



7.5-M , " 71 (Eqn. 7 in Innes et. al. 1987) [12J 



The metabolic rates of juveniles, however, are usu- 

 ally elevated relative to those of adults of equivalent 

 mass (Innes et al., 1987). For mammals, the magni- 

 tude by which juvenile metabolic rates are elevated 

 generally declines from a peak at the onset of feeding 



