| Abstract |
- 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.
|
|---|
