38 
Fishery Bulletin 111(1) 
abundance along the west coast of North America is 
linked to cooler than average ocean water tempera- 
tures. The high salmonid consumption values in our 
study may reflect higher than average salmonid abun- 
dance driven by changes (warm phase through 2005, 
neutral-to-cold phase after 2005) caused by the Pacific 
Decadal Oscillation since approximately 2006 (Mantua 
et ah, 1997). We suggest that the overall high consump- 
tion rates of herring and salmonids (along with great 
uncertainty in these consumption rates) by harbor 
seals found in this study indicate that harbor seal con- 
sumption should be examined on broader spatial and 
historical scales to further explore the potential effect 
of harbor seal consumption on prey groups. 
Conclusions 
Harbor seals are a large-bodied and abundant predator 
whose consumption of depressed fish populations may 
conflict with regional fish recovery goals. This study 
established baseline consumption estimates for major 
prey groups and highlighted the potential range of 
consumption for the most common minor prey groups 
in the San Juan Islands region. Although there was 
great uncertainty in quantitative diet composition of 
harbor seals, salmonids and herring clearly constitut- 
ed the majority of biomass consumed during the study 
period. Rockfish, one of the fish groups for which ma- 
rine reserves are being planned, were among the minor 
prey groups consumed. The relative importance of prey 
items in harbor seal diet can be tested with future diet 
data in a model framework that incorporates estimates 
of uncertainty, similar to the one used in this study. Re- 
lation of consumption rates to mortality rates for any 
of the depressed fish species will require a multidisci- 
plinary approach because of the complexity of harbor 
seal diet. 
In this study, we explored how changes in the age 
structure of the harbor seal population influenced con- 
sumption values and found age structure to have rela- 
tively little influence. However, more work is needed to 
establish the current age structure of the harbor seal 
population because it may have significant implications 
for prediction of harbor seal body size, which strongly 
controlled model predictions. In further modeling exer- 
cises, the variables that most heavily influenced con- 
sumption values (body size of seals and quantitative 
diet composition) should be considered as some of the 
most important factors for prediction of consumption 
and food requirements of harbor seals in the study area. 
Acknowledgments 
We would like to thank N. Schwarck, G. McKeen, 
and members of the Marine Behavior and Ecology 
Laboratory of the Western Washington University for 
logistical support in field work. The lead author was 
supported through National Science Foundation Grant 
No. 0550443 awarded to A. Acevedo-Gutierrez, a re- 
search assistantship from Padilla Bay National Es- 
tuarine Research Reserve, and the Office of Research 
and Sponsored Programs and the Biology Department 
at Western Washington University. Suggestions from 3 
anonymous reviewers substantially improved previous 
versions of this manuscript. 
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