Howard et al Fish consumption by harbor seals ( Phoca vitulina) in the San Juan Islands, Washington 
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Figure 5 
Effect of altering age structure on the net population energy use (in megawatts) 
of the harbor seal ( Phoca vitulina ) population in the San Juan Islands and 
eastern bays during 2007-08. Base=basic model with age structure from 1970s; 
for the other graph lines, 25, 50, and 100 correspond to percent increases in 
numbers of adults in population. Solid circles indicate medians, boxes enclose 
the interquartiles, vertical dashed lines represent 1.5* the interquartile range, 
and open circles indicate outliers. 
Puget Sound Rockfish (S. empha- 
eus ; the smallest of the rockfish at 
~40 g), they could have consumed 
more than 2 million individuals, 
a number that presumably can 
affect the rockfish population. It 
seems clear that prey that consti- 
tute even a minor proportion of 
harbor seal diet may be affected 
by predation, if such predation 
increases their natural mortality 
rates. Therefore, harbor seal inter- 
actions with prey species of man- 
agement concern merit further at- 
tention, and modeling prey vulner- 
ability to predation will require a 
multidisciplinary approach. 
Consumption estimates calcu- 
lated in this study illustrate the 
energetic importance of herring 
and salmonids to harbor seals in 
the San Juan Islands and the im- 
portance of considering predation 
effects on prey groups from mul- 
tiple perspectives. In this study, 
we contrasted high consumption 
rates of prey species (salmonids 
and herring) with less commonly 
consumed prey groups, such as 
rockfish, to illustrate the capacity 
of models to test assumptions in 
situations with high uncertainty 
in input values, such as percent- 
age by wet weight of rockfish in 
seal diet. We provided evidence 
that the apparently minor con- 
tribution of rockfish biomass to 
harbor seal diet may neverthe- 
less indicate that large numbers 
of individuals are being consumed, but the number con- 
sumed is highly dependent on the species and age of 
prey. Harbor seals consumed large amounts of the more 
commonly consumed species, such as herring, even at 
the lower estimated limits of consumption rates calcu- 
lated in this study. Many herring stocks have under- 
gone critical declines, and there is concern that pinni- 
ped predation may have increased the natural mortal- 
ity rate of herring in some areas (Musick et al., 2000), 
although it is acknowledged that there are likely many 
factors that contributed to the decline of herring (Stout 
et al., 2001). Spawner biomass of herring for the north- 
ern Puget Sound, an index of population abundance, 
remained low through the study period, 4 yet herring 
has been identified as one of the top prey species of 
4 Stick, K. C., and A. Lundquist. 2009. 2008 Washington 
State herring stock status report. Stock Status Report FPA 
09-05, 111 p. Washington Department of Fish & Wildlife, 
Fish Program, Fish Management Division. [Available from 
http://wdfw.wa.gov/publications.] 
harbor seals in a San Juan Islands diet study since 
2005 (Lance et al., 2012). 
Like herring populations, salmonid populations have 
undergone serious declines, and there is also concern 
that pinnipeds may affect salmonid recovery (NMFS, 
1997; Wright et al., 2007). Five species of salmonid oc- 
cur in the study area and all have been documented 
in harbor seal diet. Chinook Salmon (Oncorhynchus 
tshawytscha ) was the only salmonid species confirmed 
by the scat samples of our study; however, Pink Salm- 
on are the salmonid species most commonly consumed 
by harbor seals in the San Juan Islands (Lance et al., 
2012). Pink Salmon runs in the northern Puget Sound 
were relatively abundant during the study period, but 
abundance indices indicate Chinook Salmon remained 
at critically depressed levels through 2008. 5 Salmonid 
5 Salmonid stock inventory (SaSi). Washington Department 
of Fish & Wildlife. [Available from http://wdfw.wa.gov/ 
mapping/salmonscape/index.html.] 
