Howard et al.: Fish consumption by harbor seals ( Phoca vitulma ) in the San Juan Islands, Washington 
35 
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Figure 3 
Effect of bioenergetics and population variables, relative to season (breeding or 
nonbreeding), on net population energy use (in megawatts) of harbor seals ( Phoca 
vitulina) in the San Juan Islands and eastern bays during 2007-08. Distribution 
of model outputs after running 1000 simulations with all variables (“Full”), single 
(individual variables), or “groups of variables” (“Bioenergetics” [mass, activity, 
and growth rates]” or “Population” [fertility and abundance!) selected randomly. 
Solid circles indicate medians, boxes enclose the interquartiles, vertical dashed 
lines represent 1.5* the interquartile range, and open circles indicate outliers. 
Discussion 
The prey consumption model 
was quite sensitive to body 
mass: when body mass was 
varied +10% around the aver- 
age, there was a corresponding 
+ 10% change in the energy use 
outcome. Body mass controls 
many physiological functions in 
organisms, and because mass- 
based predictive relationships 
were used for metabolic rate, 
the sensitivity of the model to 
body mass was not entirely un- 
expected. By simply account- 
ing for body size and number 
of harbor seals, the model cap- 
tured the bulk of energy use in 
the population. In fact, omission 
of reproduction costs (lactation 
and gestation costs) did not af- 
fect estimates of nonbreeding 
season energy use and lowered 
breeding season estimates by 
approximately 10%. 
Predicted per capita fish con- 
sumption of 2.1 kg day -1 seaP 1 
fell within the range estimated 
for the harbor seal populations 
in British Columbia, Canada, 
and Norway: 1.9 kg and 4 kg, 
respectively (Harkbnen and 
Heide-Jprgensen, 1991; Ole- 
siuk, 1993; Bjprge et al., 2002). 
Despite their large body size, 
adult males were the least nu- 
merous sex-and-age class in the 
population — information that 
explained their low proportion of total population con- 
sumption when the population was considered as a 
unit. Consumption was for the most part proportional 
to the biomass of the total seal population; therefore, 
any change in total population size would correspond 
to a roughly equal percent change in estimated con- 
sumption. With this prediction, all other model vari- 
ables were assumed to be similar among years, and 
this assumption seems reasonable given that the total 
population size has stabilized during the last decade 2 
(Jeffries et al., 2003). Nevertheless, at dramatically 
different population sizes, there may be different be- 
havioral or population changes that would need to be 
taken into account (e.g., individual prey preferences, 
intraspecific competition, fertility rates, and mortality 
rates) to predict population consumption. 
In contrast to the other population variables, only 
point estimates were used for mortality rates. The age 
structure of the harbor seal population used in the ba- 
sic consumption model was heavily dominated by sub- 
adults, and the population structure was based on data 
from a time period when the harbor seal population 
was depressed. However, changing the age structure in 
our alternative model (see Appendix) caused relatively 
minor changes in the energy budget, especially com- 
pared with the sensitivity of the model to body mass. 
If the increase in population size since the 1970s has 
led to decreased juvenile survival rates, as is predicted 
to be the case for marine mammals (Fowler, 1981; Hiby 
and Harwood, 1985), and adult seals are now more 
dominant in the population, overall consumption rates 
still should be similar to those that we predicted, at 
least at the adult to subadult ratios that were tested 
in alternate model versions. 
For species, such as harbor seals, that use fat stores 
during fasting periods, inferring consumption directly 
from energetic requirements may be somewhat mis- 
leading. Harbor seals fast or reduce feeding rates for 
2-6 weeks and can lose up to 33% of body mass during 
the breeding season (Bowen et al., 1992; Coltman et 
