Bromaghin et a!: Diets of Phoca vitulina in the Salish Sea revealed by analysis of fatty acid signatures 
23 
ficients are applicable to wild seals with a more diverse 
diet is unknown, and use of previously published co- 
efficients is a potential source of bias. To conduct an 
independent feeding trial in association with every 
field investigation obviously is infeasible and therefore 
reliance on published calibration coefficients may be 
unavoidable. However, some investigators have noted 
that diet composition estimates are sensitive to the 
values of calibration coefficients (Meynier et ah, 2010), 
and such sensitivity may also be the case for the suite 
of fatty acid compounds used in mixture modeling. 
Achievement of adequate sample sizes of all potential 
prey species, including representatives of the same spe- 
cies at various life history stages and seasons, such as 
immature and mature species of salmon, is obviously 
an important precursor to implementation of QFASA. 
Although such considerations do not negate the util- 
ity of QFASA as a tool to estimate diet composition, 
researchers need to be cognizant of these issues, and 
therefore the development of analytical procedures to 
assess sensitivity may be helpful. 
Conclusions 
Several fish stocks of historic commercial importance 
within the Salish Sea are considered to be depressed 
and their recovery is a high management priority. 
Whether abundant pinniped populations may be im- 
peding management actions intended to stimulate re- 
covery is an open question in this region. Our findings 
confirmed the importance of salmon species and Pacific 
Herring in harbor seal diets, but they also revealed that 
other species, including rockfish species, may contrib- 
ute more substantially to harbor seal diets than had 
been realized previously. Although estimates of harbor 
seal diet composition varied spatially, demographically, 
and among individual seals, species of rockfish were 
estimated to compose a large proportion of the diets 
of several individual seals. These results, in combina- 
tion with the current high abundance of harbor seals, 
indicate that predation may be an important ecologi- 
cal factor in the regulation of the local and regional 
abundance of rockfish populations — a possibility that 
warrants additional investigation. 
Acknowledgments 
We thank B. Applegate, R. Tee, and S. Ali for their 
assistance in the ASET Laboratory; B. Hagedorn for 
logistical support and direction; D. Lambourn, B. Mur- 
phie, J. Gould, T. Cyra, J. Gaydos, K. Reuland, S. Peter- 
son, P. Olesiuk, and many others for their help captur- 
ing seals; R. Sweeting (Fisheries and Oceans Canada 
and RV Ricker), S. O’Neill (NOAA), and G. Williams 
(NOAA) for providing fish samples; A. Default (NOAA), 
and Western Washington University students for assis- 
tance processing fish samples; and A. Thomas for creat- 
ing Figure 1. We also thank K. Oakley (U.S. Geological 
Survey) for providing helpful comments that greatly 
improved the manuscript. This study was supported 
by National Science Foundation Award No. 0550443 
to A. Acevedo-Gutierrez, the University of Alaska An- 
chorage, Washington Department of Fish & Wildlife, 
Olympia, Washington, U. S. Geological Survey, and the 
Alaska Science Center. Harbor seal research activities 
were conducted under Marine Mammal Protection Act 
Research Permit 782-1702-00. 
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