502 



Fishery Bulletin 91(3), 1993 



As noted earlier, MR1 probably tended to be an under- 

 estimate as it was derived from captive seals that were 

 sometimes quiescent. On the other hand, MR2 may be 

 an overestimate as it is unlikely that seals swim con- 

 tinuously while in the water, and also because the 

 RMR of sleeping seals may actually fall below BMR 

 (Ashwell-Erickson and Eisner, 1981; Worthy, 1987a). 

 The mean of MRl Klxl and MR2 XIX „ denoted as MR S(X) , 

 was therefore adopted for subsequent calculations. 



Ambient sea and air temperatures in the study area 

 were probably within the thermoneutral zone of free- 

 ranging harbor seals. Oritsland and Ronald (1975) de- 

 tected no change in the metabolic rate of an adult 

 harp seal swimming in water temperatures ranging 

 from 8.5-26°C and Gallivan (1977) no change in the 

 metabolic rates of 3 adult harp seals in water tem- 

 peratures ranging from 1.8 to 28.2°C. Hart and Irving 

 (1959) reported that the lower critical temperature of 

 harbor seals in air was 2°C and Matsuura and Whittow 

 (1973) found that the metabolic rate of a resting har- 

 bor seal was constant in air temperatures up to 35°C. 

 Mean monthly sea surface temperatures in the study 

 area typically range between 6.2 and 17.0°C/' and mini- 

 mum and maximum monthly air temperatures between 

 -0.4 (January minimum) and 23.5°C (July maximum) 

 (Canadian Hydrographic Service, 1990). Thermo- 

 regulatory costs were therefore assumed to be negli- 

 gible (see also General Discussion). 



The costs associated with growth were calculated 

 based on actual growth rates rather than applying 

 Innes' et al. (1987) empirical equations for growing 

 phocids because there may be substantial differences 

 between growth rates of captive and free-ranging seals. 

 For instance, growth rates reported for recently weaned 

 grey and harp seals in captivity (Worthy, 1987a) were 

 about an order of magnitude greater than those esti- 

 mated for harbor seals in the wild. The apparent gross 

 cost of growth, 201 WfkgdT 1 (Innes et al., 1987), rep- 

 resents, assuming net efficiency was 70%, a net cost of 

 141W(kg-d-')-' or 12.2 MJ-kg- 1 . Given the wet-weight 

 energetic density of tissues (37.8 MJkg 1 for blubber 

 and 6.5MJkg~' for proteinaceous tissue; Olesiuk and 

 Bigg, unpubl. data), this implies that post-weaning body 

 growth was composed of about 20% fat and 80% pro- 

 tein. Using these values to extrapolate the estimated 

 body composition at weaning (see below), the adult 

 body would be composed of approximately 30% blub- 

 ber, which is consistent with the 27-30% reported for 

 free-ranging harbor seals (Pitcher, 1986). Daily growth 

 requirements, DGi? s ,,„ were low for all age-classes, 

 ranging from about 2.5-5.5 watts for juveniles to neg- 

 ligible values for adults. 



l| H. Freeland, Institute of Ocean Sciences, Pat Bay. B.C.. pers. 

 commun. 1989. 



In calculating the energy invested in foetal develop- 

 ment, the energetic density of the fetus was assumed 

 to be the same as the 7.2MJ-kg~' reported for harp 

 seal neonates (Worthy and Lavigne, 1983). This im- 

 plies that neonates are essentially devoid of fat, which 

 is probably true as neonate and near-term harbor seals 

 have very thin blubber layers (Pitcher, 1986). In addi- 

 tion, neonates can tolerate little mass loss before dy- 

 ing (Boulva and McLaren, 1979). Applying this value 

 to the mean mass at birth of 10.2kg (Bigg, 1969), the 

 total energy content of the term foetus is estimated to 

 be about 73.4 MJ. If it is assumed that, as in harp 

 seals (Worthy and Lavigne, 1983), the placenta con- 

 tains an additional 5.9 MJ, the net foetal investment 

 is estimated at 79.3 MJ, and the gross investment at 

 113 MJ. Additional energy would be required for the 

 metabolism of the fetus. However, the foetal mass 

 would represent only a negligible portion of a female's 

 total mass through most of the pregnancy. Moreover, 

 since the foetal masses were not subtracted from the 

 total masses of pregnant females incorporated into the 

 growth curves, it was assumed that the costs were 

 absorbed into the maintenance requirements of adult 

 females. 



In calculating the costs invested in lactation, the 

 energetic density of the mass gained by pups was as- 

 sumed to be the same as the 33.1 MJkg- 1 reported for 

 harp seal pups (Worthy and Lavigne, 1983), which im- 

 plies that the mass gained was approximately 85% fat 

 and 15% protein. In view of their rapid rate of growth, 

 this value is probably also applicable to nursing har- 

 bor seal pups. When applied to the 13.8 kg increase 

 between the mean birth and weaning mass (10.2 and 

 24.0 respectively; Bigg, 1969), the energy assimilated 

 by nursing pups is estimated to be 456.8 MJpup '. In 

 addition, each nursing pup would require about 

 172.4 MJ for maintenance during the 5-1/2 week nurs- 

 ing period (Bigg, 1969). Thus, the total net nursing 

 investment was estimated to be 629.2 MJ, which rep- 

 resents a gross investment of 946 MJ. The total an- 

 nual cost of foetal development and lactation was thus 

 estimated to be about 1060 MJ for each reproductive 

 female. 



Estimates of the total daily energy requirements, 

 DER X , were surprisingly constant with age, ranging 

 from 150 W for yearlings of both sexes to 215 W for 

 full-grown males (Tables 1 and 2). The range in DER,,,, 

 was much narrower (1.4x) than the range in body 

 mass (3.7x) because the major energy expenditures 

 scaled to M" 7 \ and also because juvenile metabolic 

 rates were elevated relative to adults of equivalent 

 mass. The mean per capita DER was estimated to be 

 172 W. 



Most of the daily energy requirement, DER, was 

 expended for maintenance and comparatively little for 



