Abstract:
Longitudinal patterns of fish and benthic invertebrate distribution and
habitat use were similar in Thomas Creek, Oregon but clarity of these patterns
differed. I studied fish and aquatic invertebrates simultaneously, at multiple
scales, and used multivariate statistical techniques to compare responses to the
same environmental conditions. Both types of organisms exhibited distinct
longitudinal patterns along a 51 river kilometer (R km) transition from mid-elevation
(ca. 365 m elevation) conifer dominated reaches to Willamette Valley
agriculture dominated reaches (Ca. 73 m elevation). In summer 2000,
preliminary surveys of 30 R kms suggested that longitudinal changes in benthic
invertebrate assemblage structure and rainbow trout diet (Oncorhyncous mykiss)
(n=53), were minor and likely driven by three sites in the upper reaches. This led
me to expand the survey length to 51 R kms and modify the survey design in
2001.
During 2001, I performed repeated, intensive (4 sites) and extensive (218
survey units) snorkel surveys to examine fish distribution and habitat selectivity.
The intensive snorkel surveys of the upstream reaches revealed similar habitat
preferences for pools with riffles directly upstream by juvenile chinook salmon
(Oncorhyncous tshawytscha) and ages of trout (from 0 to >3 years). The
extensive survey identified two distinct fish assemblages: a salmonid-dominated
one in the upper 12 R kms and a second dominated by non-game fish in the lower 20 R kms. The transition between these two zones (between 20 and 39 R
kms) was populated sparsely by members of both assemblages. Fish
assemblages were associated with broad-scale environmental conditions (e.g.
temperature and elevation) and not with local conditions (e.g. water velocity,
substrate size, depth). Differences between assemblages in riffles, glides, and
pools, were only detected by blocking data according to location.
Benthic aquatic invertebrates were collected from each habitat type at 27
sites, in nine reaches, during May 2001. Invertebrate assemblages
demonstrated strong longitudinal (broad-scale) and habitat type patterns. Fish
assemblages changed abruptly but invertebrate assemblages changed gradually
along distinct topographic and vegetation zones. My results demonstrate the
importance of extensive surveys with continuous stream data and numerous
sampling sites. Fish and invertebrates appeared to respond to environmental
conditions at different spatial scales detected only by comparing the two groups
of organisms simultaneously along an extended longitudinal gradient.
