Howard et al Fish consumption by harbor seals ( Phoca vituiina ) in the San Juan Islands, Washington 
33 
ring, salmonids, rockfish, Walleye Pollock, and Shiner 
Perch. Gross energy requirements were translated to 
consumption rates by applying the energetic density of 
prey to the proportion by wet weight of prey items in 
seal diet (Perez, 1994; Van Pelt et ah, 1997; Paul et ah, 
1998; Payne et ah, 1999; Anthony et al., 2000; Roby 
et al., 2003). After biomass reconstruction, all species 
of adult and juvenile salmonids were combined into a 
“salmonid” complex. A “herring” complex represented 
Clupea pallasii and unidentified clupeid species. There 
are 2 other clupeid species in the study area, but, be- 
cause of their rareness, we assumed most species were 
C. pallasii (M.M. Lance, personal commun.). When prey 
were placed into broader taxonomic groups, we used 
the minimum and maximum values for energetic densi- 
ty reported for all prey sizes and ages in the literature 
to represent the prey group. 
Model uncertainty and parameter estimation 
Model variables described in Table 1 were randomly 
chosen during 1000 simulations from probability dis- 
tributions to estimate uncertainty in all model outputs. 
Where estimation of distribution parameters was not 
straightforward (e.g., lognormal), a maximum likeli- 
hood technique with the MASS package in R was used; 
this technique estimates the joint likelihood for dis- 
tribution parameter values, given the seal body mass 
values for each sex-and-age class (Venables and Ripley, 
2002). We also made the following changes to diet re- 
sults to adjust the uniform distribution parameters for 
percentage by wet weight of prey in diet. If we had 
set the minimum and maximum values for a uniform 
distribution for proportion in diet exactly as found in 
diet samples, it would have been uninformative (i.e., 
a range of 0-100 often occurred but would imply no 
prior knowledge of diet composition; Table 2). There- 
fore, zero values from diet samples were discarded and 
minimum values for herring and salmonids were set as 
calculated from the remaining diet samples. For Shiner 
Perch and Walleye Pollock, zero values also were dis- 
carded. The minimum possible value was assumed to 
be 1%, and the maximum value was set near the aver- 
age calculated from diet samples. Harbor seal diet is 
diverse; therefore at least 20-30% of harbor seal diet 
was assumed to be made up of other species, and the 
maximum value possible for any prey species was set 
at 70-80% (the maximum value for nonbreeding season 
was set slightly lower because of increased diversity of 
diet). All model outputs are reported as means ^stan- 
dard deviation). 
Sensitivity analyses also were used to identify pa- 
rameters with the most influence on model outputs by 
systematically allowing one parameter at a time to be 
chosen randomly while other variables were fixed at 
their mean value(s). In this manner, any variation in 
the model outputs should be the direct result of varia- 
tion in the parameter of interest (Shelton et al., 1997; 
Stenson et al., 1997; Winship et al., 2002). The percent- 
age of variance explained by a single variable was cal- 
culated as the variance of model outputs when single 
random variables were used and divided by the total 
variance when all variables were randomly chosen. 
To estimate the effect of age structure on total prey 
consumption, we used different ratios of adults to sub- 
adults in 3 alternate model scenarios. We increased the 
number of adults in the population by 25%, 50%, and 
100% and kept the total population size stable. 
During the breeding season, adult harbor seals fast 
or reduce consumption (Bowen et al., 1992; Coltman 
et al., 1998); therefore, there may be a discrepancy 
between predicted energy requirements and timing 
of consumption during an annual cycle. Rather than 
use direct consumption, we addressed the effect of this 
discrepancy with a correction factor that accounted for 
energy obtained from burning body fat stores in the 
breeding season. We estimated the amount of energy 
consumed, stored as body fat, and later metabolized by 
adult seals with the same estimates of digestive effi- 
ciency and energy density of prey that were used in the 
overall consumption model. 
Results 
Fish consumption 
There were 196 and 361 scat samples collected dur- 
ing the breeding and nonbreeding seasons, respective- 
ly. In these samples, 23 and 29 prey taxa were iden- 
tified during the breeding and nonbreeding seasons. 
Ten prey taxa were selected for reconstruction in this 
study; they had a frequency of occurrence >5.0 in the 
broader harbor seal diet study (Lance et al., 2012) or 
were species of conservation concern. Of these 10 taxa, 
3 prey groups (unidentified gadid, skate species, and 
American Shad [Alosa sapidissima ]) could not be used 
because we had insufficient methods (e.g., lack of cor- 
rection factors) to reconstruct their presence in seal 
diet. Of the remaining prey, herring comprised the vast 
majority of reconstructed samples: >80% of wet weight 
in both breeding and nonbreeding seasons. Salmonids 
composed 15% and 9% in the breeding and nonbreed- 
ing seasons, respectively (Table 2). We were not able to 
identify rockfish otoliths to species in either season. In 
the breeding season, rockfish frequency of occurrence 
was 0.5% and therefore was assumed to contribute 
little in energetic terms to diet and was not further 
considered for calculation of consumption rates. Mea- 
surable otoliths were not found for rockfish species in 
the nonbreeding season; therefore, we were unable to 
determine species or size. During the nonbreeding sea- 
son, rockfish frequency of occurrence was 1.4% (Lance 
et al., 2012); we set a hypothetical range for proportion 
of wet weight of rockfish in diet at 1. 0-2.0%. Walleye 
Pollock and Shiner Perch constituted a relatively mi- 
nor portion (averages 0.5-2. 8%) of reconstructed diet 
(Table 2). 
