National Marine 
Fisheries Service 
NOAA 
Fishery Bulletin 
@& established in 1881 «< 
Spencer F. Baird 
First U.S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Abstract—The diets of Steller (Eume- 
topias jubatus) and California (Zalophus 
californianus) sea lions in northwest 
Washington are poorly documented. We 
hypothesized that these species exploit 
the same prey in Washington because 
they are both generalist predators that 
utilize the same haul-out sites and 
are similar in behavior and body size. 
We analyzed 776 samples of scat from 
Steller sea lions and 263 samples of 
scat from California sea lions collected 
throughout each year during 2010-2013. 
The aim of this analysis was to charac- 
terize seasonal and annual diets, esti- 
mate biomass of prey consumed, and 
evaluate dietary niche overlap. Steller 
and California sea lions ate diverse 
diets that varied seasonally and annu- 
ally. Primary prey groups for both sea 
lion species were Clupeidae, Salmoni- 
dae, Sebastidae, Rajidae, Pleuronecti- 
formes, Squalidae, and Merlucciidae. 
We estimated that Steller sea lions ate 
11,327 metric tons (t) (standard devia- 
tion [SD] 1600) and that California sea 
lions ate 9063 t (SD 4098) of prey per 
year during our study. We found sig- 
nificant dietary niche overlap between 
California and Steller sea lions that 
feed in northwest Washington. 
Manuscript submitted 7 April 2021. 
Manuscript accepted 7 January 2022. 
Fish. Bull. 120:39-54 (2022). 
Online publication date: 26 January 2022. 
doi: 10.7755/FB.120.1.4 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Dietary niche overlap and prey consumption 
for the Steller sea lion (Eumetopias jubatus) and 
California sea lion (Zalophus californianus) in 
northwest Washington during 2010-2013 
Jonathan J. Scordino (contact author)! 
Adrianne M. Akmajian” 
Stacy L. Edmondson? 
Email address for contact author: jonathan.scordino@makah.com 
" Marine Mammal Program 
Makah Fisheries Management, Makah Tribe 
150 Resort Drive 
Neah Bay, Washington 98357 
? Marine Ecology Program 
Makah Fisheries Management, Makah Tribe 
150 Resort Drive 
Neah Bay, Washington 98357 
3 Mathematics and Statistics Department 
Whitman College 
345 Boyer Avenue 
Walla Walla, Washington 99362 
The diets of California (Zalophus califor- 
nianus) and Steller (Eumetopias juba- 
tus) sea lions have been studied in much 
of the California Current Ecosystem, yet 
notable knowledge gaps exist. Diets of 
populations of California sea lions have 
been studied at the Channel Islands 
(Antonelis et al., 1984; Lowry et al., 
1991; Lowry and Carretta, 1999; Orr 
et al.,2011), the Farallon Islands (Bailey 
and Ainley, 1981), and Monterey Bay 
(Weise and Harvey, 2008) in California. 
However, California sea lions, predom- 
inately males (Gearin et al., 2017), are 
distributed north of California as far 
as Alaska (Maniscalco et al., 2004). 
Diets of California sea lions north of 
California are poorly documented with 
only 2 peer-reviewed published studies 
(Everitt et al., 1981; Roffe and Mate, 
1984) and 3 reports (Reimer and Brown!; 
1 Riemer, S. D., and R. F. Brown. 1997. Prey 
of pinnipeds at selected sites in Oregon 
Scordino?; Trites and Rosen). Like- 
wise, the diet of Steller sea lions has 
been studied in Oregon and Northern 
California (Riemer et al., 2011), but in 
Washington it has been reported only in 
gray literature (Scordino”; Wiles, 2015). 
identified by scat (fecal) analysis, 1983— 
1996. Oregon Dep. Fish Wildl., Tech. Rep. 
97-6-02, 34 p. [Available from Oregon 
Dep. Fish Wildl. Mar. Reg., 4034 Fairview 
Industrial Dr. SE, Salem, OR 97302.] 
Scordino, J. 2010. West Coast pinniped 
program investigations on California sea 
lion and Pacific harbor seal impacts on 
salmonids and other fishery resources, 97 
p. Pac. States Mar. Fish. Comm., Portland, 
OR. [Available from website.] 
Trites, A. W., and D. A. S. Rosen (eds.). 
2019. Synthesis of scientific knowledge 
and uncertainty about population dynam- 
ics and diet preferences of harbour seals, 
Steller sea lions and California sea lions, 
and their impacts on salmon in the Salish 
Sea. Technical workshop proceedings, May 
29-30, 2019, 67 p. Mar. Mamm. Res. Unit, 
Univ. B. C., Vancouver, Canada. [Available 
from website.] 
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