28 
Fishery Bulletin 1 1 1 (1) 
Figure 1 
Map of the study area, the San Juan Islands and eastern bays, where seal 
scat collections were made for a bioenergetics model to examine the quan- 
tity of fish consumption by the harbor seal ( Phoca uitulina ) population dur- 
ing 2007-08. Black circles indicate harbor seal scat collection sites. 
tat with usually positive results (Shears 
and Babcock, 2002; Shears et ah, 2006); 
however, predators also can cause declines 
in the size distributions and abundance of 
prey species inside marine reserves (Sala 
and Zabala, 1996; Fanshawe et ah, 2003). 
Large-bodied and abundant predators can 
contribute significantly to fish mortality, 
especially when prey species are already 
low in abundance, and may theoretically 
influence prey population recovery (Mohn 
and Bowen, 1996; Bundy, 2001; DeMaster 
et ah, 2001; Fu et al., 2001; Trzcinski et ah, 
2006). Therefore, there is a need to under- 
stand the prey requirements of predators 
that consume fish species of conservation 
concern to evaluate if such requirements 
conflict with regional management goals. 
In the Salish Sea, the harbor seal ( Phoca 
vitulina ) is an abundant, generalist marine 
predator whose population has steadily in- 
creased since gaining protected status in the 
1970s. The harbor seal population in Wash- 
ington State experienced logistic growth 
from the 1970s to the 1990s, increased 7- to 
10-fold in size in different regions, and now 
appears to be at carrying capacity (Jeffries 
et ah, 2003). Estimates of the regional popu- 
lation in the San Juan Islands and eastern 
bays in the early 1970s were approximately 
1000 animals; currently, there are approxi- 
mately 8000. 2 The age structure of the har- 
bor seal population in British Columbia was 
documented in Bigg (1969), on the basis of 
seals collected and aged in the 1960s. After 
exponential population increases, this popu- 
lation was heavily weighted toward juvenile age classes 
by the 1980s (Olesiuk, 1993). Given the population in- 
crease in all regions of the Salish Sea, the current age 
structure of the harbor seal population in the San Juan 
Islands is unknown. 
As with other harbor seal populations in the east- 
ern Pacific, harbor seals in the San Juan Islands take 
advantage of the large influx of adult salmonids in 
late summer and fall and increase the diversity of 
their diet at other times of the year when salmonids 
are less available (Hauser et al., 2008; Lance et al., 
2012). Salmonids, Pacific Herring, Pacific Sand Lance 
(Ammodytes hexapterus), Northern Anchovy (Engraulis 
mordax ), Walleye Pollock (Theragra chalcogramma), 
and estuarine species, such as Shiner Perch ( Cymato - 
gaster aggregata), also form significant proportions of 
their diet in the San Juan Islands and nearby estua- 
rine ecosystems (Lance et al., 2012). 
2 Washington Department of Fish & Wildlife. Unpubl. data. 
Washington Department of Fish & Wildlife, 7801 Phillips 
Road SW, Lakewood, WA 98498. 
To calculate population-level consumption of fish 
species of conservation concern and other common har- 
bor seal prey in the San Juan Islands, a bioenergetics 
model was used to determine energetic requirements. 
The model incorporated seasonal changes in seal diet 
and life history parameters during breeding and non- 
breeding seasons. We also used simulated data and 
sensitivity analyses to address uncertainty in the over- 
all model and in 2 specific components that may have 
a strong influence on predicted consumption of prey: 1) 
uncertainty in age structure of the harbor seal popu- 
lation and 2) seasonal changes in energy intake (e.g., 
fasting during breeding season). 
Methods 
Area and timeframe of study 
The region of the San Juan Islands and eastern bays 
is an area where many fish species of conservation 
concern occur and also an area where the majority of 
the harbor seal population resides in the inland waters 
