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

CHAPTER 7. WOODY DEBRIS AND OVERHANGING VEGETATION EXPERIMENT
Introduction
In 2001 and 2002, habitat manipulation experiments were conducted in Gene Coulon
Park to test the use of small woody debris (SWD) by juvenile Chinook salmon. In all
experimental tests, no preference for SWD was found (Tabor and Piaskowski 2002, Tabor et al.
2004b). However during snorkel surveys, juvenile Chinook salmon were found to extensively
use natural small woody debris when associated with overhanging vegetation (OHV) in south
Lake Washington and Lake Sammamish. Since no preference was shown for SWD by itself
during experimental tests, then OHV may be an important element of preferred habitat for
juvenile Chinook salmon. In 2003, we conducted the final phase of our habitat manipulation
experiments by examining the use of OHV in combination with SWD.
Methods
We used the same site in Gene Coulon Park that we used in 2001 and 2002 (Figure 14).
The shoreline was divided into six 15-m shoreline sections: two with SWD, two with OHV/SWD
and two with no structure of any kind. The structures within the SWD only sections and
OHV/SWD sections were 8 m long and located in the middle of the 15-m shoreline section. In
the sections with OHV, we placed four fence posts in the water at a 0.3 m depth and then a rope
was tied between them, approximately 0.4 meter above the water. Scotch broom (Cytisus
scoparius) cuttings (1.5 to 2 m long) were then laid down such that the base of each cutting was
close to the edge of the shore and the top part of the cutting rested on the rope (Figure 41). The
cuttings were anchored with sand bags on shore and cable ties along the rope. The small woody
debris consisted of tree branches placed in two rows parallel to shore. Each row was
approximately 1 to 2 m wide. The rows were approximately 1.5 m apart, which allowed room
for a snorkeler to swim between the rows. Small woody debris was placed along 0.4 and 0.7 m
depth contours and was tied together and anchored with sand bags. Snorkel surveys were
conducted within each shoreline section. Surveys were done during both day and night. Surveys
were done along the 0.4 m depth contour. At the beginning of each snorkel survey, the
temperature ( o C) and light intensity (lumens/ft 2 ) was measured. Light intensity measurements
were taken at the water surface with an International Light Inc., model IL1400A
radiometer/photometer.
During the day, Chinook salmon were active and often moved away from snorkelers. To
get a more accurate count and insure that snorkelers did not push fish into an adjoining section,
two snorkelers slowly swam toward each other from the outer edges of each shoreline section.
After surveying each section, snorkelers compared notes on fish observed and adjusted fish
counts to reduce the likelihood that fish were double counted. At night, shoreline sections could
be surveyed by one snorkeler. Fish were inactive and usually did not react to the snorkeler.
Occasionally, a Chinook salmon was startled but usually only swam away a short distance in any
direction. Therefore, it was possible for a fish to have moved into an adjoining section, but we
considered this number to be insignificant in comparison to the total number of fish observed.
Within each shoreline section with structure, we also estimated the number of Chinook salmon
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