36 
Fishery Bulletin 1 1 1 (1) 
4000 - 
g 3000 
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2000 
1000 - 
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Nonbreeding 
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Figure 4 
Effect of prey variables on herring consumption of harbor seals (Phoca vi- 
tulina ) relative to season (breeding or nonbreeding), in the San Juan Islands 
and eastern bays during 2007-08. Distribution of model outputs after run- 
ning 1000 simulations with all (“Full”) or single variables selected randomly. 
Proportion=percent of total biomass in seal diet composed of herring (%). En- 
ergy density=energy contained in prey items (J g -1 ). Efficiency=percent of gross 
energy available in prey item that is metabolizable (%). HIF=heat increment 
of feeding (%). Solid circles indicate medians, boxes enclose the interquartiles, 
vertical dashed lines represent 1.5* the interquartile range, and open circles in- 
dicate outliers. All simulations allowed variance in seal energetic requirements. 
al., 1998 ). Pinnipeds increase feeding rates either im- 
mediately after the breeding season or before the next 
breeding season to regain fat stores (Beck et al., 2003). 
In addition, there are seasonal changes in energy in- 
take that occur in harbor seals and other pinnipeds 
(Schusterman and Gentry, 1971; Rosen and Renouf, 
1998). We addressed this discrepancy in timing of pre- 
dicted energetic requirements and feeding through as- 
sessment of how much prey may be consumed by adult 
seals in the winter and spring and later used as fat 
stores. We found the amount to be a minor proportion 
of annual consumption but a more significant portion 
of the breeding season estimates. Therefore, the effect 
of consumption in the breeding season may be reduced, 
and consumption during the winter may be higher than 
we predicted. 
Bioenergetic variables (especially body mass) con- 
tributed most to sensitivity in calculations of energy 
requirements in this study. Other 
pinniped consumption models 
similarly have identified body 
mass and body-mass predicted 
energetic requirements as a sig- 
nificant source of model variation 
(Mecenero et al., 2006; Chassot 
et al., 2009). When the full con- 
sumption model was examined, 
the assumed proportion of each 
prey species in the diet had 
the largest effect on consump- 
tion outputs — a result that was 
also similar to other pinniped 
consumption models (Mohn and 
Bowen, 1996; Shelton et al., 1997; 
Mecenero et al., 2006; Overholtz 
and Link, 2007), suggesting that 
future effort should be focused on 
refining the contribution of differ- 
ent prey to harbor seal diet. Ge- 
netic and molecular techniques 
increasingly are used to identify 
diet composition (Casper et al., 
2007; Deagle and Tollit, 2007). 
It is likely necessary to evalu- 
ate the diet of generalist marine 
predators with a combination 
of techniques, given that these 
techniques often yield different 
results and can answer different 
questions (Tollit et al., 2006). The 
model described here can be used 
to test assumptions about the 
relative importance of salmonids 
and herring compared with other 
species in harbor seal diet as oth- 
er data become available. 
Estimates indicate that rock- 
fish species constituted a rela- 
tively minor proportion of total 
consumption by harbor seals. There are more than 26 
species of rockfish that occur in the inland waters of 
Washington State, and many species are listed as endan- 
gered by the state. Under the federal Endangered Spe- 
cies Act, 2 species are listed as threatened and 1 species 
is listed as endangered. The 2 most dominant species, 
Copper ( Sebastes caurinus ) and Quillback (S. maliger) 
Rockfish, for which abundance data are well document- 
ed, have both undergone serious declines and are consid- 
ered vulnerable to extinction (Mills and Rawson, 2004). 
For depressed species such as these, even small amounts 
of predation may be significant. If we assume an average 
size of 1 kg for a rockfish in harbor seal diet (ignoring 
age- or species-size differences), harbor seals hypotheti- 
cally consumed 84,000 rockfish individuals in 2007-08 
in the San Juan Islands and eastern bays. However, to 
illustrate the importance of age or species preference by 
harbor seals, if we assume that harbor seals eat only 
