Graduate Thesis Or Dissertation
 

Seasonal shifts in redband trout use of pools and their microhabitats in three central Oregon streams

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1z40kw337

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  • 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.
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