| Summary: | Redband trout density was examined in three forested streams in central
Oregon at two scales, pool channel unit and microhabitat. Two streams were in
roadless areas and one was in a "highly managed" watershed. At the larger
spatial scale, trout displayed a seasonal shift in habitat use from early to late
summer. There was a positive correlation between trout density and pool
structural complexity during summer base flow. The association was intensified
throughout the summer as stream flow continued to drop. The structural
complexity of each pool was quantified using an index integrating structural
variability and depth. Twenty-two pools were divided into microhabitats, or pool
subunits with similar characteristics using a qualitative classification scheme
describing different structural elements comprising the pool habitat. Microhabitat
was partitioned between fry and older trout throughout the summer: Fry generally
used stream margins, backwaters, and shallow areas; whereas, trout one year
and older used deep areas (depth greater that 0.5 m) and cover associated with
substrate and wood. The use of cover by trout one year and older doubled from
June to August. This change in use was coupled with the increased association
with structural complexity at the channel unit scale. The large substrate and
wood that provided cover also increased the structural complexity of the pool.
As trout increased their use of cover, their densities increased in more complex
pools.
Man-made log weir pools in a simplified stream were evaluated for their
structural complexity and compared to the shallow natural pools in the same
reach. Log weir pools had greater average depth, but were less complex than
natural pools, and could maintain a similar density of trout. The structural
complexity in natural pools appears to compensate for their shallow depth. Log
weir pools enhanced with placement of a rootwad supported higher densities of
trout. Addition of a rootwad provided microhabitats associated with cover that
were lacking in weir pools without other structures added. When using instream
construction to create pool habitat, complex structure that provides microhabitats
associated with cover is more effective at holding higher densities of fish under
late summer low flow conditions. === Graduation date: 1996
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