Nearshore Habitat Use by Juvenile Chinook Salmon in Lentic ...

TABLE 5.—Diet overlap indices (C) and diet breadth indices (B) of the mouth of Kennydale Creek and a
lakeshore reference site, Lake Washington, 2003. Streamflow data were collected close to the mouth of the creek.
ND = no data. Diet overlap index less than 0.6 indicates a significant difference. Diet breadth index values can
range from 1 (no diet breadth) to infinity. Values less than 2 indicate little diet breadth.
Streamflow Diet overlap index (C ) Diet breadth index (B )
Date (cfs) trib. mouth and lake shore tributary mouth lake shore
February 19 0.55 0.78 1.58 1.02
March 12 4.80 0.74 1.97 1.25
April 3 0.51 0.94 1.05 1.14
May 8 0.20 0.98 1.05 1.05
June 3 ND 0.70 2.37 3.17
Three other tributary mouths in Lake Washington were sampled in 2003, which included
Kennydale Beach tributary, May Creek, and Taylor Creek; however, we were only able to survey
each site under base streamflow conditions. Chironomid pupae and adults were the most
important prey item for each tributary mouth as well as the lakeshore reference sites (Table 6).
Chironomid larvae and terrestrial insects were more important in the diet at each tributary mouth
than at the lakeshore reference sites. However, there was no significant difference in the diet
between the tributary mouths and lakeshore sites (Table 7). The diet breadth index was higher at
the tributary mouths than the lakeshore (Table 8).
In Lake Sammamish, the mouths of Tibbetts Creek and Laughing Jacobs Creek were
sampled in April 2003. Chinook salmon were also collected at one lakeshore reference site,
Lake Sammamish State Park boat ramps. Similar to Lake Washington, the diet of Chinook
salmon in all Lake Sammamish sites was dominated by chironomid pupae and adults. In contrast
to Lake Washington, Daphnia made up a substantial portion of the diet of Chinook salmon in
Lake Sammamish sites (Table 9). In Lake Washington, Daphnia usually does not become an
important prey item until June (Koehler 2002). The diet at the mouth of Tibbetts Creek was
somewhat different than the lake shore (overlap index = 0.68 and a higher diet breadth index).
The diet at the creek mouth included several chironomid larvae, mayfly nymphs
(Ephemeroptera), oligochaetes, and terrestrial insects. The diet at the mouth of Laughing Jacobs
Creek was similar to the lakeshore (Tables 7 and 8).
Several water mites (Hydrachnida) were often found in stomach samples, especially in
samples collected in May and June. At the mouth of Kennydale Creek (May and June), they
represented about 20% of the prey by number and %IRI was approximately 10%. Ingested water
mites were quite small and were generally much smaller than any other prey item. They were
probably larval water mites, which are parasites of aquatic insects, especially larval dipterans
such as chironomids (Smith et al. 2001). Therefore, they probably were not a true prey item of
Chinook salmon.
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