Scordino et al.: Consumption of Oncorhynchus spp. by Zalophus californianus and Eumetopias jubatus in Washington 151 
of evolutionary significant units of salmon listed as threat- 
ened or endangered under the U.S. Endangered Species 
Act (ESA) (Federal Register, 1999). 
In northwest Washington from 2010 through 2013, 
Pacific salmon made up 11.7% of the diet of Steller 
sea lions (Eumetopias jubatus) and 13.5% of the diet 
of California sea lions (Zalophus californianus), and 
estimates of the consumption of Pacific salmon were 
1330 and 1220 metric tons (t) per year for Steller and 
California sea lions, respectively (Scordino et al., 2022). 
_ Counts of Steller and California sea lions in northwest 
Washington increased 7.9% and 7.8% per year during 
2010-2018 (Allyn and Scordino, 2020). The rise in abun- 
dance of these sea lion species likely has led to a doubling 
of the levels of consumption of Pacific salmon estimated 
by Scordino et al. (2022) for Steller and California sea 
lions over the past decade. The method of prey identifi- 
cation used by Scordino et al. (2022) could not be used 
to reliably identify salmon to the species level (Purcell 
et al., 2004; Korzow Richter et al., 2020), preventing eval- 
uation of consumption of salmon by species. 
The objective of this study was to conduct genetic 
analysis of the salmon bones used in the work reported 
in Scordino et al. (2022) to determine the seasonal and 
annual consumption of Pacific salmon by species and 
size class for California and Steller sea lions in north- 
west Washington. Determining the species and size class 
of Pacific salmon consumed by these species will allow 
improvements in modeling of effects of this predation on 
species of Pacific salmon and of the competition between 
fisheries and Steller and California sea lions (Chasco 
et al., 2017a, 2017b). This information will also help 
in evaluation of the hypothesis by Hilborn et al. (2012) 
that predation by sea lions and other predators on large 
Chinook salmon (250 cm in total length [TL]) reduces the 
availability of this resource to killer whales (Orcinus orca) 
of the endangered southern resident distinct population 
segment (Federal Register, 2005) that depend on large 
Chinook salmon as their primary prey (Ford et al., 1998, 
2016; Hanson et al., 2021). A second objective of the study 
described here was to document how California and Steller 
sea lions responded to the large increase in the number of 
Pacific salmon present in 2011 due to the large odd-year 
run of pink salmon (O. gorbuscha) (Losee et al., 2019). 
Materials and methods 
Sample collection 
All scat samples analyzed in this study were collected at 
haul-out sites used by California and Steller sea lions in 
northwest Washington (Fig. 1), as reported by Scordino 
et al. (2022). Site selection for scat collections depended 
on distributions of California and Steller sea lions and on 
the ability of researchers to land safely at sites. Samples 
of Steller sea lion scat were collected primarily from haul- 
out sites in the Tatoosh Island Complex in the winter and 
spring and from Sea Lion Rock in the summer and fall; 
scats were obtained monthly between August 2010 and 
February 2013 as conditions allowed. Samples of Cali- 
fornia sea lion scat were collected primarily from East 
Bodelteh Island in the Bodelteh Island Complex and only 
in the spring, summer, and fall (Table 1). 
Processing of salmon bones 
Scat samples were cleaned either by washing them in 
paint strainer bags in a residential-style washing machine 
or by washing them through nested sieves (Orr et al., 
2003). After cleaning, hard parts from the scat were hand- 
picked from the paint strainer bag or sieves and placed in 
a glass vial with 70% isopropyl alcohol. Hard parts were 
then air-dried in a heated room. S. Riemer of the Oregon 
Department of Fish and Wildlife identified prey hard 
parts to the lowest taxonomic level possible on the basis of 
comparisons with a reference collection of fish and ceph- 
alopod hard parts from the region (Scordino et al., 2022). 
Salmon bones were identified to the family level because 
salmon bones cannot reliably be identified to species by 
using morphological features (Korzow Richter et al., 
2020). Identified salmon bones were separated from the 
rest of the prey hard parts into gelatin capsules for subse- 
quent analysis. 
Salmon bones were sorted by the size of salmon con- 
sumed on the basis of a comparison to a reference col- 
lection of salmon from estuaries and coastal areas of the 
Pacific Northwest. Bones from salmon consumed during 
their first year in the ocean, fish that were roughly less 
than 25 cm TL (Duffy and Beauchamp, 2011), were 
assigned to the small size class of salmon. The large 
size class of salmon corresponded with the size range of 
returning adult salmon, fish that were roughly 50 cm TL 
and greater. Recovered salmon bones that could not be 
definitely sorted to the large or small size class, because 
the bone had eroded during digestion or because the recov- 
ered type of bone (e.g., gill rakers) is known to have a wide 
range of sizes within a fish, were recorded as salmon of 
unclassified size. This unclassified size class also includes 
medium-sized salmon ranging roughly from 25 to 50 cm 
TL. Our reporting of recovered bones from salmon in 
the small and large size classes are minimum estimates 
because an unknown proportion of the unclassified size 
class was composed of small and large salmon. 
We selected 2-3 representative salmon bones from 
each size class in each scat sample for genetic analysis. 
In the genetic analysis, a single bone from each size class 
was used for species determination. If the sample failed 
to amplify sufficiently for species identification, another 
representative bone was analyzed. Our research method 
required the assumption that the sea lions ate only 1 spe- 
cies of salmon per each size class of salmon observed in a 
scat sample because we did not have enough funding to 
analyze all of the salmon bones found in scat samples from 
the Scordino et al. (2022) study. This assumption likely is 
not valid given that a previous study found more than one 
species of salmon present in the gastrointestinal tract of 3 
of 28 sea lions sampled (Roffe and Mate, 1984). 
