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

Water column depth (m) .
0.80
0.60
0.40
0.20
a
b
c
0.00
March 10 April 7 May 12
mean length (mm) 42.7 50.0 91.8
FIGURE 26.—Nighttime water column depth (mean ± 2SE) of juvenile Chinook salmon in Gene Coulon
Park, Lake Washington, 2004. Bars with diferent leters are significantly diferent (ANOVA and Fisher’s
LSD; P < 0.001). The mean length of Chinook salmon on each date is also given. Data were collected
along perpendicular snorkeling/scuba diving transects between 0- and 3-m deep.
Discussion
Observations of both day and night depth distribution clearly showed that juvenile
Chinook salmon progressively shift to deeper waters as they grow. Juvenile Chinook
salmon in riverine environments have also been shown to inhabit deeper waters as they
increase in size (Lister and Genoe 1970; Allen 2000; R. Peters, USFWS, unpublished
data). The same general pattern has been shown in several other fish species including
salmonids as well as non-salmonids. McIvor and Odum (1988) and Ruiz et al. (1993)
demonstrated that predation risk for juvenile fish decreases in shallow water. Power
(1987) suggested small juvenile fish inhabit very shallow water because they are
especially vulnerable to piscivorous fishes and less vulnerable to wading birds because
juvenile fish are very small. As juvenile fish grow they shift to deeper waters because
they are less vulnerable to fish and more vulnerable to wading birds. In Lake
Washington, small juvenile Chinook salmon may be in shallow water to avoid cutthroat
trout (O. clarki) and prickly sculpin which are important predators in the littoral zone
(Nowak et al. 2004; Tabor et al. 2004a). When they increase in size they may become
more attractive to wading birds such as great blue herons (Ardea herodias) but less
vulnerable to piscivorous fishes.
The last survey (June 2) indicated some juvenile Chinook salmon had moved into
water that was 4-5 m deep but no feeding activity was observed in deeper waters. Recent
results of ultrasonic tracking at Gene Coulon swim beach (May 24 to June 5, 2004)
indicated some Chinook salmon may be in water > 7 m deep (M. Celedonia, USFWS,
unpublished data). However, only fish greater than 100 mm FL were tagged. Fresh
(2000) also found that Chinook salmon are further offshore in the upper pelagic area after
mid-May. Thus our results may reflect the water column depth for the portion of the
population that still inhabits the nearshore area and could be a gross underestimate for the
36