52 
Fishery Bulletin 120(1) 
is no evidence that the dietary niche overlap between 
Steller and California sea lions is affecting the popula- 
tion growth of either species, but it bears monitoring if 
prey resources become limiting. Many of the prey spe- 
cies consumed by California and Steller sea lions during 
this study are culturally, economically, and ecologically 
important species (Kaplan and Leonard, 2012; Surma 
et al., 2018; Atlas et al., 2021). The estimates of prey con- 
sumption by the 2 sea lion species highlight the potential 
for increasing abundances of Steller and California sea 
lions in northwest Washington to result in reductions 
in the number of fish available to future recreational, 
commercial, and subsistence fisheries (Weise and Har- 
vey, 2005, 2008; Chasco et al., 2017; Walters et al., 2020). 
Future diet studies conducted at a decadal scale would 
be useful for evaluating the effects of changing pinniped 
populations and ocean conditions. 
Acknowledgments 
We would like to thank our field and lab crews, especially 
P. Gearin, M. Gosho, and M. Murner. S. Riemer performed 
all prey identifications and reviewed draft manuscripts. A. 
Orr, G. Johnson, J. Brandon, and E. Allyn provided advice 
on statistical analysis and coding. P. Mahoney reviewed 
the method of the prey consumption model. R. Jones 
reviewed the paper. Two anonymous reviewers suggested 
improvements to the manuscript. All research activities 
were conducted under Marine Mammal Protection Act 
research permit no. 14326. Our research was supported by 
the National Marine Fisheries Service through a Species 
Recovery Grant to Tribes and by the Bureau of Indian 
Affairs through a grant from the Tribal Climate Resilience 
Program. 
Literature cited 
Akmajian, A. M., J. J. Scordino, and A. Acevedo-Gutiérrez. 
2017. Year-round algal toxin exposure in free-ranging sea 
lions. Mar. Ecol. Prog. Ser. 583:243-258. 
Allyn, E. M., and J. J. Scordino. 
2020. Entanglement rates and haulout abundance trends 
of Steller (Eumetopias jubatus) and California (Zalophus 
californianus) sea lions on the north coast of Washington 
state. PLoS ONE 15(8):e0237178. 
Antonelis, G. A., C. H. Fiscus, and R. L. DeLong. 
1984. Spring and summer prey of California sea lions, 
Zalophus californianus, at San Miguel Island, California, 
1978-79. Fish. Bull. 82:67—76. 
Atlas, W. I., N. C. Ban, J. W. Moore, A. M. Tuohy, S. Greening, 
A. J. Reid, N. Morven, E. White, W. G. Housty, J. A. Housty, et al. 
2021. Indigenous systems of management for culturally and 
ecologically resilient Pacific salmon (Oncorhynchus spp.) 
fisheries. BioScience 71:186—204. 
Aurioles-Gamboa, D., and F. J. Camacho-Rios. 
2007. Diet and feeding overlap of two otariids, Zalophus 
californianus and Arctocephalus townsendi: implications 
to survive environmental uncertainty. Aquat. Mamm. 
33:315-326. 
Bailey, K. M., and D. G. Ainley. 
1981. The dynamics of California sea lion predation on Pacific 
hake. Fish. Res. 1:163-176. 
Berrow, S. D., R. I. Taylor, and A. W. A. Murray. 
1999. Influence of sampling protocol on diet determination of 
gentoo penguins Pygoscelis papua and Antarctic fur seals 
Arctocephalus gazella. Polar Biol. 22:156—163. 
Bowen, W. D., and S. J. Iverson. 
2013. Methods of estimating marine mammal diets: a review 
of validation experiments and sources of bias and uncer- 
tainty. Mar. Mamm. Sci. 29:719-754. 
Brown, R. F., B. E. Wright, M. J. Tennis, and S. Jeffries. 
2020. California sea lion (Zalophus californianus) monitoring 
in the lower Columbia River, 1997-2018. Northwest. Nat. 
101:92-103. 
Browne, P., J. L. Laake, and R. L. Delong. 
2002. Improving pinniped diet analyses through identifica- 
tion of multiple skeletal structures in fecal samples. Fish. 
Bull. 100:423-433. 
Canty, A., and B. Ripley. 
2017. boot: bootstrap R (S-Plus) functions. R package, vers. 
1.3-20. [Available from website, accessed July 2017.] 
Chasco, B. E., I. C. Kaplan, A. C. Thomas, A. Acevedo-Gutiérrez, 
D. P. Noren, M. J. Ford, M. B. Hanson, J. J. Scordino, S. J. Jeffries, 
K.N. Marshall, et al. 
2017. Competing tradeoffs between increasing marine mam- 
mal predation and fisheries harvest of Chinook salmon. 
Sci. Rep. 7:15439. 
Davison, A. C., and D. V. Hinkley. 
1997. Bootstrap methods and their application, 582 p. Camb. 
Univ. Press, New York. 
DeLong, R. L., S. R. Melin, J. L. Laake, P. Morris, A. J. Orr, and 
J. D. Harris. 
2017. Age- and sex-specific survival of California sea lions 
(Zalophus californianus) at San Miguel Island, California. 
Mar. Mamm. Sci. 33:1097-1125. 
Demer, D. A., J. P. Zwolinski, K. A. Byers, G. R. Cutter, J. S. Renfree, 
T. S. Sessions, and B. J. Macewicz. 
2012. Prediction and confirmation of seasonal migration of 
Pacific sardine (Sardinops sagax) in the California Current 
Ecosystem. Fish. Bull. 110:52—70. 
DiCiccio, T. J., and B. Efron. 
1996. Bootstrap confidence intervals. Stat. Sci. 11:189-228. 
Eschmeyer, W. N., E. S. Herald, and H. Hammann. 
1983. A field guide to Pacific coast fishes of North America, 
336 p. Houghton Mifflin Co., Boston, MA. 
Everitt, R. D., P. J. Gearin, J.S. Skidmore, and R. L. DeLong. 
1981. Prey items of harbor seals and California sea lions in 
Puget Sound, Washington. Murrelet 62:83-86. 
Gearin, P. J.,S. R. Melin, R. L. Delong, M. E. Gosho, and S. J. Jeffries. 
2017. Migration patterns of adult male California sea lions 
(Zalophus californianus). NOAA Tech. Memo. NMFS- 
AFSC-346, 29 p. 
Greene, H. W., and F. M. Jaksic. 
1983. Food-niche relationships among sympatric predators: 
effects of level of prey identification. Oikos 40:151—-154. 
Horn, H. S. 
1966. Measurement of “overlap” in comparative ecological 
studies. Am. Nat. 100:419—424. 
Kaplan, I. C., and J. Leonard. 
2012. From krill to convenience stores: forecasting the eco- 
nomic and ecological effects of fisheries management on 
the US West Coast. Mar. Policy 36:947-954. 
Kastelein, R. A., N. Vaughan, and P. R. Wiepkema. 
1990. The food consumption of Steller sea lions (Eumetopias 
jubatus). Aquat. Mamm. 15:137-144. 
