Scordino et al.: Consumption of Oncorhynchus spp. by Zalophus californianus and Eumetopias jubatus in Washington 159 
California and Steller sea lions, but the approach does not 
provide size estimates specific enough to allow computation 
of the number of Pacific salmon consumed. In future stud- 
ies of the diets of sea lion species, all salmon bones should 
be measured to allow quantitative estimates of the size of 
salmon consumed. 
Scordino et al. (2022) noted a number of caveats to their 
interpretation of their diet analysis that also apply to the 
results of this study. Most important of those caveats is 
that estimates from this study could overreport the con- 
sumption of small Pacific salmon and underreport the 
consumption of large Pacific salmon because of the use of 
SSFO to evaluate the diets of California and Steller sea 
lions (Laake et al., 2002; Tollit et al., 2007). Compounding 
the underreporting of large Pacific salmon is the possibil- 
ity that bones from large salmon were not available in scat 
samples. Otariids are known to regurgitate and spew out 
bones from large fish (Gudmundson et al., 2006) and to not 
fully consume large fish, potentially reducing the avail- 
ability of such bones to be identified (Tollit et al., 2017). 
A second important caveat is the possibility that some 
of the juvenile Pacific salmon for which bones were exam- 
ined in this study were consumed by another predator 
that was in turn eaten by a sea lion (Pierce and Boyle, 
1991). Sea lions can target small-bodied prey, making it 
difficult to determine what portion of the bones of small 
Pacific salmon were from secondary consumption. Fiscus 
and Baines (1966) found 1280 capelin (Mallotus villo- 
sus) in the stomach of a juvenile Steller sea lion that had 
an average mass of 14.1 g, similar to the mass of juve- 
nile Pacific salmon (Chasco et al., 2017a). For the third 
consideration regarding the results of this study, keep in 
mind that we analyzed only one bone from each size class 
of salmon for which bones were found in scat samples of 
California and Steller sea lions. This approach effectively 
required the assumption that California and Steller sea 
lions were eating only one species of Pacific salmon of 
a given size class during a meal. Roffe and Mate (1984) 
reported finding more than one species of salmon in sea 
lion gastrointestinal tracts, and given that result, we may 
not have detected all of the species of salmon present in 
each scat sample. Lastly, results from both captive feeding 
trials and studies in which genetic analysis was used for 
prey identification indicate that salmon bones are often 
not found in scat samples, even when sea lions are known 
to have consumed salmon (Tollit et al., 2003, 2017). 
Conclusions 
California and Steller sea lions depend on Pacific salmon 
for a portion of their diets. This dependence can create con- 
servation concerns for depleted runs of Pacific salmon and 
competition with fisheries given that populations of both 
species of sea lions in Washington are large and growing. 
It was not possible to quantify the effect of predation by 
California and Steller sea lions on populations of Pacific 
salmon in this study alone. Future studies will benefit from 
ecosystem modeling that incorporates data on the diets of 
California and Steller sea lions from this study and from 
Scordino et al. (2022) to determine the effect of predation 
by both species of sea lions on populations of Pacific salmon. 
Acknowledgments 
Field research was conducted under marine mammal 
research permit no. 14326. Funding for this study was pro- 
vided by the Species Recovery Grant to Tribes through the 
National Marine Fisheries Service. P. Gearin, M. Gosho, 
M. Murner, and A. Chaparro assisted with field and lab- 
oratory work. E. Iwamoto provided samples of salmon for 
our assessment of genotyping error and provided input on 
study design. T. Petersen conducted the analysis with the 
Fishery Regulation Assessment Model. We give a special 
thank you to S. Riemer for identifying and sorting the 
salmon bones by size class. 
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