- Visual estimation techniques were used to quantify habitat characteristics,
habitat type (pool, riffle) use and longitudinal distribution of steelhead
(Oncorhynchus mykiss), cutthroat trout (0. clarki), and coho salmon (0. kisutch)
in spring, summer and fall in 8.8 km of Cummins Creek, a basin in the central coast
of Oregon. Fish were distributed significantly different than habitat type
availability in most samples. Pool habitats contained a disproportionate percent of
the salmonid assemblage and 1+ fish in each sample, and the percentage of fish in
pools increased as flow decreased. In spring, coho salmon fry were concentrated
in side channels and valley floor tributary habitats. Large woody debris formed 57-
68% of pool habitats and was significantly correlated with pool volume, maximum
pool depth, slow surface velocity in pools, and pieces of small woody debris.
Longitudinal distribution of the salmonid assemblage did not differ from
habitat distribution seasonally or between years, even though certain species
differed Coho salmon and cutthroat trout were distributed in proportion to
longitudinal habitat availability only when fish abundance was relatively high and
streamflow was low. In most samples, both 0+ and 1+ steelhead were distributed
in proportion to longitudinal habitat availability. Differences in coho salmon
abundance between years appeared to influence longitudinal distribution of each
species and age class. Certain reaches had consistent numbers of fish between
years while the number of fish in other reaches varied widely. In most samples,
reaches with highest abundance for steelhead were in the lower basin, cutthroat
trout in the upper basin and coho salmon between the two other species.
Timing of reduction in number of fish varied among species. Fifty-five
percent of 0+ steelhead and 73% of 1+ steelhead lost between August 1988 and
April 1989 were lost between August and October during low flow conditions.
However, only 18% of the losses, for 0+ coho salmon, occurred between August
and October with the remaining losses occurring after October.
This study illustrates that habitat availability is not a good index of fish
distribution when fish abundance is low, and it highlights the importance of habitat
in the lower portions of basins when fish abundance is high. It also demonstrates
that the basin wide distribution of salmonids varies among species, age classes,
seasons, and years and suggests that our understanding of salmonid distribution
and abundance could be greatly enhanced by adopting a basin-wide, community,
and seasonal perspective. In addition, the methods used in this study offer one
way to assess the seasonal distribution and abundance of salmonids in a relatively
quick, inexpensive, and non-destructive manner.