Distribution and juvenile ecology of bull trout (Salvelinus confluentus) in the Cascade Mountains Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/z890rw84n

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  • Distribution and juvenile habitat use of bull trout (Salvelinus confluentus) were surveyed in selected areas of the Upper Willamette, Deschutes, upper Yakima, and upper Cedar River basins in Oregon and Washington from 1989 to 1991, using day snorkeling, night snorkeling, and electrofishing. These methods were selected after a preliminary diel streamside study of juvenile bull trout showed fry (age 0) counts were significantly higher (P<0.00l) during the day, while counts of juvenile fish (age 1 and 2) were significantly higher (P<0.00l) at night. The highest counts of juveniles occurred during a "quiet period" immediately after dusk, during which time fish were inactive, out of cover, and easily counted with underwater flashlights. In a comparison of four sampling methods on Jack Creek, electrofishing was significantly correlated (P<0.05) with day (r=0.8l) and night (r=0.89) snorkeling counts, but not with streambank counts. In a comparison of day and night snorkeling in 10 streams, total density estimates were significantly greater (P<0.0l) for night snorkeling than day. The diel study and sampling methods comparison suggested surveys of distribution and habitat use of bull trout should include night surveys as well as day. Distribution surveys found that, except for one stream, juvenile bull trout were found only in, or near, spring-fed areas created by recent lava flows. Presence of bull trout in a stream was related to cold groundwater temperatures, as they were not found in streams with temperatures above 14°C. Distribution of bull trout in Oregon and Washington followed a pattern of decreasing elevation with increasing latitude and longitude(Ra=-0.9l6) Presence at lower than expected elevations were explained when groundwater temperatures were predicted from mean annual air temperatures. Actual water temperatures for these spring-fed streams were significantly lower (p<0.00l) than predicted based on elevation, latitude, and longitude. Comparison of historical distribution of bull trout showed extant bull trout were found in the Willamette and Deschutes River Basins of Oregon, respectively, in only 26.2 and 56.2% of their former ranges. Factors associated with bull trout demise in Oregon Cascade streams were (1) isolation and inundation of spring-fed stream habitat by water control structures, (2) introduction of brook trout and brown trout, and (3) large flood events and habitat degradation. Juvenile habitat use was analyzed at the macrohabitat (habitat unit) level in five river basins. Diel and seasonal microhabitat use was also documented for one spring-fed stream, Jack Creek. There was a clear difference in habitat use day and night, associated with the low water temperature of the spring-fed streams (mean=8.0°C). All habitat unit types (pool, riffle, glide, side channel) were used day and night, but bull trout only elected to use side channels in both time periods. Bull trout were found at night in (1) a higher percentage of habitat units, (2) in higher densities by unit, and (3) in increasing numbers with increasing habitat unit area. In Jack Creek, bull trout elected to use shallow water depths, low mean velocities, instream woody debris, and small substrates.
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