Olesiuk: Prey consumption of Phoca vitulina 



503 



production. Daily growth requirements accounted for, 

 on average, only 1.2% of the total gross DER (1.7% of 

 NE), and only 2.1% (3.0% of NE) of the gross DER for 

 seals aged 1-2 years which exhibited the highest 

 growth costs. However, the estimates reflect only the 

 direct costs associated with growth. A portion of the 

 elevated metabolic rates of juveniles, which account 

 for 8.4% of the overall population energy budget (12.0% 

 of NE) but were incorporated into maintenance costs, 

 may be indirectly associated with growth. Although 

 the DER of lactating females were 2.8 x those of non- 

 lactating females of equivalent mass, and lactation ac- 

 counted for 89% of the total costs of reproduction, lac- 

 tation accounted for only 14.6% of the overall energy 

 requirements of reproductive females when amortized 

 over the entire year. Overall, net reproductive costs 

 accounted for only 3.2% of the total population energy 

 budget (4.5% of NE). Thus, growth and reproduction 

 combined accounted for only 4.4% of the total popula- 

 tion energy budget (6.2% of NE). 



With respect to maintenance, basal metabolism ac- 

 counted for 33.9% of the total population energy bud- 

 get (48.4% of NE) when the corrections for elevated 

 juvenile levels are excluded, or 42.3% (60.4% of NE) 

 when the corrections are included. If it is assumed 

 that 13% of gross maintenance requirements is lost in 

 faeces and urine and 17% expended in the heat incre- 

 ment associated with feeding, and thermoregulatory 

 costs were negligible, the remaining 23.4% of the total 

 population energy budget (33.4% of NE) was expended 

 on activity. However, there is considerable uncertainty 

 in this estimate. Had total maintenance costs been 

 directly extrapolated from captive animals using MR1, 

 only 16.1% of the total budget (23.0% of NE) would 

 have been available for activity. On the other hand, 

 had total maintenance been estimated from the activ- 

 ity budget using MR2, 29.0% of the total budget (41.4% 

 of NE) would have been available for activity. Averag- 

 ing MR 1 and MR2 thus introduced a potential error of 

 about ±13% into the overall population energy budget. 



Based on the energetic densities of 10 of the 15 most 

 important prey species, which accounted for 86.1% of 

 the overall diet, the mean weighted energetic density 

 of the diet was estimated to be 6.65 MJ kg -1 (see Table 

 3). The total daily energy requirements, DER^ x: , there- 

 fore translated into daily food requirements, FR1^, X „ 

 ranging from 2.0 kg for yearlings of both sexes to 2.8 kg 

 for full-grown males (Tables 1 and 2), which repre- 

 sented 8.2% and 3.1% of their mean body masses re- 

 spectively. The mean daily per capita food requirement, 

 weighted according to the sex- and age-structure of 

 the population, was estimated to be 2.2 kg, or 5.0% of 

 mean body mass. 



Estimates of daily food requirements based on the 

 volumes of stomach contents, FR2 s!l) , were consistently 



lower than the estimates based on energetic life tables, 

 FR1 S , XI (Fig. 6). The mean per capita food requirement 

 estimate based on stomach volumes, FR2, was 1.6 kg, 

 or 73% of the mean estimated of 2.2 kg based on ener- 

 getic life tables, FR1. It is unlikely that much of the 

 discrepancy between the two estimates can be attrib- 

 uted to differences in the energetic density of diets on 

 the east and west coasts. A crude estimate of the mean 

 energetic density of the east coast diet, obtained by 

 applying energetic densities of similar species on the 

 west coast to 10 of the 15 most important prey on the 

 east coast, which combined accounted for 81.2% of the 

 east coast diet, was 6.31 MJ kg -1 , or 95% of the diet in 

 the study area. 



The magnitude of the discrepancy between FRl,,^, 

 and FR2 xlxl declined with age (Fig. 6). There was fairly 

 good agreement between the two methods for adults: 

 2.1kg from stomachs versus 2.5 kg from energetic life 

 tables. The former was almost within the ±13% poten- 

 tial error introduced in the latter owing to uncertainty 

 in the costs associated with activity (see above). In 

 contrast, juvenile estimates based on stomach volumes 

 were substantially lower than corresponding estimates 

 based on the energetic life tables (Fig. 6). The average 

 per capita requirement for immature seals based on 

 stomachs, 1.3 kg, was only 65% of the estimate based 

 on energetic life tables, 2.1kg. The estimate based on 

 stomach volumes for yearlings, the age-class exhibit- 

 ing the greatest discrepancy, was only 52% of the esti- 

 mate based on energetic life tables. The reason for the 

 larger discrepancy for immature age-classes was that 

 the volumes of stomachs, when all ages were pooled, 

 was scaled to M 076 (Eqn. 15), which was close to the 

 M 075 expected for adults (Eqn. 11). In other words, 

 there was no evidence that juveniles consumed greater 

 quantities of prey than adults of equivalent mass, 

 whereas significant corrections were applied in the en- 

 ergetic life tables to account for the elevated metabolic 

 rates of juveniles. 



One factor that might have contributed to this dis- 

 crepancy may have been post-weaning changes in body 

 composition. At weaning, about 50% of body mass of 

 pups is composed of blubber which, as the animal ages, 

 is used and displaced by proteinaceous tissue. Thus, 

 although pups exhibited some post-weaning growth in 

 body mass during the first year, they may actually 

 have experienced negative growth in energetic terms. 

 However, even if it is assumed that all of the post- 

 weaning blubber reserves (37.8MJ-kg~') were replaced 

 with proteinaceous tissue (6.5 MJ-kg~') during the first 

 year, the daily energy requirement of yearlings would 

 decrease by only 0.15 kg day -1 , or 15% of the observed 

 discrepancy. 



The discrepancy between FR1 ,. and FR2 , for juve- 

 niles suggests that the metabolic rates of free-living 



