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

To describe the diet of juvenile Chinook salmon, we followed the procedures of
Cortés (1997) and Liao et al. (2001). For each prey group in each sample, we determined
the percent weight (%W), percent number (%N), and percent occurrence (%O). A
percent index of relative importance (%IRI) was then developed for each prey group:
IRI
% Oi
(% Wi
%
N
i
)
and,
% IRI
IRI
i
100n
IRI
i1
i
To help compare the diet between samples, we also calculated Schoener’s diet overlap
index (Schoener 1971):
Cxy 1
0.
5
p xi
p
yi
where C xy is the index value, p xi is the proportion of food type i used by Chinook salmon
at site x and p yi is the proportion of food type i used by Chinook salmon at site y.
Researchers commonly use an overlap index level of 0.6 or less to indicate a significant
difference in diet (Zaret and Rand 1971; Johnson 1981). Comparisons were made
between tributary mouths and lakeshore reference sites, as well as between high and base
streamflow conditions at each tributary mouth.
A diet breadth index (B; Levins 1968) was also calculated to determine if
Chinook salmon utilize a wider variety of prey types at the tributary mouths in
comparison to the lake shore:
B
1
2
p i
where p i is the proportion of the diet represented by food type i. Diet breadth index
values range from 1 (no diet breadth: only one prey type in the diet) to infinity. Values
less than 2 indicate little diet breadth.
Results
Catch. —In 2003, beach seine catch rates of juvenile Chinook salmon at tributary
mouths and lakeshore sites were extremely variable between sites and between day and
night. During the day, we were able to catch Chinook salmon at some tributary mouths
but not at lakeshore reference sites. At some lakeshore sites, we could visually observe
Chinook salmon but they could easily avoid the beach seine. At tributary mouths, they
could be collected more easily, likely because the water was turbid or they retreated to
the tributary mouth where they could be easily encircled with the seine. Because of the
difficulty of collecting Chinook salmon at most lakeshore sites during the day, we
collected Chinook salmon in 2004 at one site in Gene Coulon Park where they were
known to be abundant.
Nighttime sampling was conducted at a few tributary mouths. Although night
sampling was logistically more difficult, it appeared to be less variable than daytime
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