34 
Fishery Bulletin 1 1 1 (1) 
During the seal breeding season, the average con- 
sumption for prey species calculated over 1000 simu- 
lations was 783 (±380) metric tons (t) of salmonids, 
646 (±303) t of herring, 50 (±17) t of Walleye Pollock, 
and 22 (±4) t of Shiner Perch (Fig. 2). Subadult seals 
of both sexes consumed the greatest proportion of the 
total biomass (approximately 30-40% each), followed 
by adult females (27%). Adult males consumed a rela- 
tively small proportion of total biomass compared with 
adult females and subadults, and their consumption 
was only slightly higher than the biomass consumed 
by pups of both sexes (each <10%). 
During the nonbreeding season, consumption of 
herring and salmonids had the widest range of val- 
ues; rockfish, Shiner Perch, and Walleye Pollock were 
less variable. The average consumption for prey spe- 
cies calculated over 1000 simulations was 84 (±26) 
t of rockfish, 675 (±388) t of salmonids, 2151 (±706) 
t of herring, 66 (±13) t of Walleye Pollock, and 86 (±22) 
t of Shiner Perch (Fig. 2). 
The per capita fish consumption rate predicted 
by the model was 2.1 kg day -1 
seal -1 (annual average 2.9, 2.8, 
2.0, 2.2, and 1.0 kg for adult 
females, adult males, subadult 
females, subadult males, and 
pups, respectively). As was 
evident during the breeding sea- 
son, subadults (which included 
pups from the previous breed- 
ing season) of both sexes con- 
sumed the greatest proportion of 
the total biomass (approximately 
30-45% each), followed by adult 
females (19%). Adult female 
consumption dropped slightly in 
the nonbreeding season. Adult 
males consumed the smallest 
proportion in the population 
(5%). 
Sensitivity analyses and assessment 
of model uncertainty 
Variation in seal body mass had 
the largest effect on energy use 
of the population and account- 
ed for >80% of model variance 
in both seasons. Taken togeth- 
er, all bioenergetics variables 
(mass, growth rates, and activity) 
accounted for the majority of the 
variance in the simulation mod- 
el. Fertility rates accounted for 
the next-greatest variance 
(7.3%) after body mass during 
the breeding season while pop- 
ulation size contributed least 
(1.3%) to overall model variabil- 
ity (Fig. 3). 
Consumption estimates of salmonids and herring 
were most sensitive to estimates of proportion of prey 
in the diet and energy density of prey. Variation in con- 
sumption estimates was low when the heat increment 
of feeding and assimilation efficiency parameters were 
varied within their estimated ranges. The variance in 
the nonbreeding season seen in the overall simulation 
model for both salmonids and herring was not well ex- 
plained by any single prey variable (Fig. 4). 
We estimated that adult seals used approximately 
1,100,000 MJ of fat stores during the breeding season. 
Assuming an average prey energy density of 4000 J 
g 1 , this use of energy was equivalent to consumption 
of 300 t or approximately 6% and 21% of annual and 
breeding-season energy use, respectively. Increasing 
the number of adult seals in the population led to a 
positive increase in population energy use, although at 
a relatively slow rate of increase: even when we dou- 
bled the number of adults in the population, energy 
use increased only by 7% (Fig. 5). 
