Graduate Thesis Or Dissertation
 

Distribution of fishes and their relationships to environments in selected coastal streams, Douglas and Coos Counties, Oregon

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  • This study was an investigation to determine the distribution, abundance and general food habits of salmonids and non-game fishes residing in several selected coastal streams, Douglas and Coos Counties, Oregon. The physical conditions that characterize the various stream habitat types were monitored and the influence of physical conditions on the distribution and abundance of fish was investigated. Study of fish distribution began fall 1979 and continued through summer 1980. Biological and ecological monitoring was undertaken at eleven permanent study sites on four occasions during the summer period of decreasing flow. Population size of each species and several environmental characteristics were measured during these occasions. The study streams were characterized by good populations of juvenile coho salmon (Oncorhynchus kisutch) and steelhead (rainbow) trout (Salmo gairdneri), demonstrating the importance of these streams as rearing areas for anadromous species. Reticulate sculpin (Cottus perplexus) were the most abundant and widespread non-game fish collected during this study. Insects were the principal prey organisms in the diets of all species studied, with Ephemeroptera and Diptera larvae the major components. There was also considerable overlap in the diets. Coho salmon and steelhead trout were most versatile in acceptance of foods, taking 19 and 15 of the separate categories tabulated. The benthic fishes, speckled dace (Rhinichthys osculus), reticulate and riffle sculpin (Cottus gulosus) were more restricted in diet, taking 12, 7 and 14 food types respectively. Diversity of habitat within the various study streams was attributable not only to geological and climatic factors, but disturbances by fire, erosion, activity of beaver and logging. Mostly such disturbances are deleterious, though in at least one instance effects of disturbances were beneficial. Beaver ponds created excellent rearing pools for coho salmon in one stream. Instream distribution of fishes appears to be mainly a function of habitat selection. Coho salmon were found in loose schools throughout pools in areas where short pools alternated with riffles. Steelhead and cutthroat trout (Salmo clarki) were found most often associated with instream cover. The reticulate and riffle sculpin, both of which are benthic and cryptic and select the same kind of cover, were usually associated in gravelly riffles. Speckled dace were common throughout large streams and almost always in the vicinity of adequate cover. Redside shiners (Richardsonius balteatus) were associated with pool like environments and were excluded from upstream areas with increasing gradient and shallow pools. Limitations on populations of salmonid fishes in the study streams are probably associated with lack of adequate summer flow, lack of instream cover, siltation of gravel and rubble bottoms and departure from an approximate 1:1 pool to riffle ratio.
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