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

Depth (m) .
0.6
0.5
0.4
0.3
2003
2004
0.2
Feb Mar Apr May Jun
37653 37683 37713 37743 37773
FIGURE 35.—Mean water column depth (m) where juvenile Chinook salmon were located in the index
reach of Johns Creek, 2003 and 2004. Figure only includes dates when at least 10 Chinook salmon were
observed.
A total of only six Chinook salmon were collected in riffles (only sampled in
2004). They were collected in February and early March and were located in small
pocket water behind boulders. Juvenile Chinook salmon density was highest in glides in
February and early March. In both 2003 and 2004, the density in the beginning of March
was about twice as high in glides than scour pools. The density in glides declined
dramatically in late March and after the beginning of April, few Chinook salmon were
present in glides and those that were present were almost always under overhanging
vegetation. In April and May, the density in scour pools was 3 to 65 times higher than in
glides. Juvenile Chinook salmon were present in scour pools throughout the study period
(Figure 36). In February, they were located in shallow areas of the pool such as the edges
and tailouts. After February, they were found in deeper water and by the end of March
they were usually in the deepest part of the pool (Figures 37 and 38).
Similarly to scour pools, Chinook salmon were present in the convergence pool
throughout the study period, albeit at a much lower density (Figure 39). Chinook salmon
in the convergence pool were usually close to the edge and associated with shoreline
vegetation. One notable exception was in February, 2004 when most Chinook salmon in
the convergence pool were located under the footpath bridge. Large numbers of juvenile
Chinook salmon were also observed under the bridge in 2002. The March and April
abundance of Chinook salmon in the convergence pool was higher in 2004 than 2003,
even though the abundance in all habitats combined was higher in 2003. To compare the
use of the convergence pool to the rest of the index reach, we calculated the number per
stream length because the convergence pool is wide and Chinook salmon do not appear to
use the large area in the middle of the stream channel. The number of Chinook salmon
per stream length was 3 to 26 times lower in the convergence pool than the rest of the
stream in 2003; however, in 2004 it was only 2 to 7 times lower (Figure 36). The water
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