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

TABLE 9.—Diet composition of juvenile Chinook salmon at three locations (one shoreline site and two
sites at the mouths of tributaries) in south Lake Sammamish, April 16 to 21, 2003. n = the number of
stomach samples analyzed; the range of Chinook salmon lengths is also given; %N = percent number; %O
= percent occurrence (%); %W = percent weight; %IRI = percent index of relative importance.
Lake shoreline Laughing Jacobs Cr. Tibbetts Cr.
n = 10, range = 60-85 mm FL n = 10, range = 52-80 mm FL n = 11, range = 53-74 mm FL
Prey group %N %O %W %IRI %N %O %W %IRI %N %O %W %IRI
Insecta
Diptera
Chironomid pupae and adults 69.7 90 67.1 81.3 65.8 100 76.5 81.6 48.9 100 56.3 70.6
Chironomid larvae 4.1 50 0.9 1.6 4.8 100 3.1 4.5 19.1 63.6 2.7 9.3
Other aquatic diptera 0 0 0 0 0.1 10 0.02 0.01 2.1 18.2 0.2 0.3
Ephemeroptera 0.1 10 0.2 0.02 0 0 0 0 8.5 45.5 8.0 5.0
Collembola 0.4 20 0.1 0.1 0 0 0 0 0 0 0 0
Other aquatic insects 0 0 0 0 0 0 0 0 3.2 18.2 5.4 1.0
Homoptera (Aphididae) 0 0 0 0 0 0 0 0 0 0 0 0
Other terrestrial insects 0 0 0 0 0.2 20 0.1 0.03 4.3 36.4 10.8 3.7
Crustacea
Cladocera - Daphnia 16.6 40 19.3 9.5 27.2 50 8.2 10.1 1.1 9.1 0.04 0.1
Other crustaceans 5.5 40 0.7 1.6 0.4 20 0.6 0.1 0 0 0 0
Hydrachnida 2.5 50 0.1 0.9 1.1 30 0.2 0.2 3.2 18.2 0.1 0.4
Oligochaeta 0.3 10 0.01 0.02 0 0 0 0 0 0 0 0
Other 0.8 60 11.7 4.9 0.4 50 11.3 3.4 9.6 54.5 16.6 9.6
In 2004, two tributaries, May Creek and Taylor Creek, were surveyed under high
streamflow conditions as well as base streamflow conditions. During high streamflow
conditions at May Creek, the percent of the diet of chironomids pupae and adults
decreased from base flow conditions, while the percent of chironomid larvae,
oligochaetes, and mayflies increased (Table 10). The diet at May Creek during high flow
conditions also included some prey items that are usually only found in flowing waters.
These prey items included the immature stages of rhyacophilid caddisflies, black flies
(Simuliidae), and heptagenid mayflies. Diet breadth was approximately 60% higher than
at the lakeshore and base flow condition (Table 8); however, the diet overlap index was
not significantly different (lakeshore, 0.69; base flow, 0.67). Cutthroat trout (n = 9) at the
mouth of May Creek during the high flow event were foraging primarily on terrestrial
prey items, which included terrestrial isopods or sow bugs (36% by weight), oligochaetes
(28%) and insects (4%).
Several larval longfin smelt (Spirinchus thaleichthys) were consumed by Chinook
salmon at the mouth of May Creek on April 1 (baseflow conditions), which represented
8% of the diet by weight. Much of the consumption of larval smelt was observed in one
individual (64 mm FL), which had consumed 29 smelt. Adult longfin smelt have been
documented to spawn in May Creek (Moulton 1974; Martz et al. 1996).
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