Howard et ai.: Fish consumption by harbor seals ( Phoca vitulina ) in the San Juan Islands, Washington 
29 
of Washington State. The San Juan Islands (48°35'N, 
122°55'W) are characterized by tidally influenced rocky 
reefs and isolated rocks surrounded by deep water 
where harbor seals often congregate at haul-outs (loca- 
tions where seals come ashore). The adjacent eastern 
bays, in contrast, consist of large, soft-bottomed, shal- 
low bays (48°33'N, 122°30'W) (Fig. 1). 
The consumption model was constructed for a sin- 
gle annual cycle for the harbor seal population dur- 
ing 2007-08. The model included 2 seasons: breeding 
(15 June-15 September) and nonbreeding (16 Septem- 
ber-14 June) determined on the basis of seal pupping 
phenology in the San Juan Islands (Huber et al., 2001; 
Patterson and Acevedo-Gutierrez, 2008). The 2 sea- 
sons were delineated to reflect known behavioral shifts 
(more time spent ashore to nurse pups, shallow-water 
breeding displays by males) related to pupping and 
breeding activities and subsequent changes in ener- 
getic expenditures (Coltman et al., 1998; Bowen et al., 
1999). 
The model was programmed in R software, vers. 
2.7.1 (R Development Core Team, 2008) and used re- 
gional activity, abundance, and diet data, as well as 
physiological data from the literature. Model param- 
eters were grouped into 3 categories: bioenergetics, 
population, and diet (Lavigne et al., 1982; Winship et 
al., 2002) (Table 1). 
Model structure 
Bioenergetics Energetic requirements were calculated 
with a bioenergetics approach that described the en- 
ergy budget of an individual seal, which is a function 
of body size, activity budgets, growth, and reproductive 
costs. Sex- and age-specific gross energy requirements 
were calculated with Equation 5 in Boyd (2002): 
EG, = [l(""‘-(r f 9 Ai )86400] + ft 
I“‘ ’ U) 
where EG t = energy requirements in a particular stage 
i of the annual cycle; 
y, = the power (watts) generated under activity 
/"within stage i of the annual cycle; 
= proportion of time spent in activity f\ 
g : - the cost of growth in stage i of the annual 
cycle; and 
d l - the digestive efficiencies of food being 
eaten. 
The model had 6 sex-and-age classes: 1) adult fe- 
males (>6 years), 2) adult males (>8 years), 3) subadult 
females (1-6 years), 4) subadult males (1-8 years), 5) 
female pups (<1 year), and 6) male pups (<1 year). The 
subadult to adult division was made at the age(s) har- 
bor seals reach their predicted maximum weight (ap- 
proximately 66 kg and 89 kg for females and males, 
respectively) on the basis of the growth curve in Ole- 
siuk (1993). Daily growth increments for each sex-and- 
age class were calculated from the same growth curve. 
Activity budgets were estimated from free-living har- 
bor seals tagged with data recorders that recorded 3 
behavioral periods: haul-out, diving, and shallow-water 
activity (Table 1). 
Population abundance and age structure Aerial popula- 
tion surveys of harbor seals have been conducted an- 
nually by the Washington Department of Fish & Wild- 
life with fixed-wing aircraft to estimate the number of 
animals hauled-out during the lowest tide of the day 
since 1978 (Jeffries et al., 2003). Results from these 
surveys were used to estimate the abundance of harbor 
seals in the study area in 2007-08. The breeding sea- 
son (July) correction factor of 1.53 (to account for seals 
not hauled-out at the time of the survey) was used to 
estimate the size of the breeding season population 
(Huber et al., 2001). Age-dependent mortality rates in 
Olesiuk (1993) were used to estimate the age structure 
(number of seals in each sex and age class) of the har- 
bor seal population: 
^s(x+l)=N s(x ,e~ rt ’ ( 2 ) 
where N S(x) = number of seals in sex class S and age 
class x; 
-r = the age-dependent mortality rate; and 
t = time interval between age classes. 
The breeding season population vector was adjusted 
by iteration to sum to the total population estimate 
from aerial surveys. Seal abundance in the nonbreed- 
ing season was calculated by estimating the numbers 
still alive in each sex and age class, by using the same 
age-dependent mortality rates calculated per day (in- 
stead of annually) and by multiplying the number of 
days in the breeding cycle. 
Population energetic requirements were calculated by 
multiplying individual requirements by the population 
abundance vectors to estimate energetic requirements 
for each sex and age class. Reproductive costs were then 
calculated for the entire population on the basis of val- 
ues from the literature for gestation and lactation costs 
and fertility rates (Bigg, 1969; Olesiuk, 1993). 
Digestive efficiency Data from the literature were used 
to translate net energy requirements of the harbor 
seal population into gross energy requirements and 
prey consumption by first taking into account assimi- 
lation efficiency and the heat increment of feeding (the 
increase in metabolism or heat produced during di- 
gestion) for harbor seals. We used the minimum and 
maximum values reported in the literature to account 
for differences in digestive efficiencies related to pro- 
tein and fat content of prey (Markussen et al., 1994; 
Trumble et al., 2003). 
