Environmental variables as predictors of fish assemblages in the Tillamook Basin, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9k41zh68m

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  • The Tillamook and Kilchis subbasins of the Tillamook Bay watershed (Oregon Coast Range) have differences in geology and land use history and, therefore, varied environmental conditions that could affect fish assemblages. Fifty-two randomly selected wadeable stream reaches in these two subbasins were surveyed for stream habitat and fish assemblage composition during two summer field seasons. Data collected were used to identify environmental variables most associated with fish assemblages, determine major predictors of fish species relative abundance, and test for any differences in fish assemblages between the Tillamook and Kilchis subbasins and among three stream orders. Eleven fish species were encountered, including four salmonid species (Oncorhynchus spp.) and five species of sculpin (Cottus spp.). For three sites visited twice in both field seasons, variability in fish assemblages and environmental measurements between sample years and between early and late-summer visits to sites was low. Fish assemblages differed between the two subbasins. The assemblages in the Kilchis subbasin contained higher proportions of trout species, while the Tillamook subbasin assemblages contained higher proportions of sculpin species. A breakdown of fish assemblages by stream order showed that different fish species dominated the assemblages in different sized streams. With data from both subbasins combined, trout species composed a smaller proportion of the fish assemblages in larger streams, and sculpin species composition also varied with stream order. Results from non-metric multidimensional scaling ordinations indicated that substrate type, stream gradient, and variables related to stream size were the most highly correlated to fish species relative abundances. Multiple linear regression was used to assess the usefulness of an uncorrelated subset of the environmental variables as predictors of relative abundance of the six most common fish species. The environmental variables were good predictors of relative abundance for some species, but were less successful in predicting abundances of entirely anadromous species. These results provide valuable insight into fish-habitat relationships using a fish assemblage perspective and incorporating species, such as sculpins, for which there is little existing information. There has been recent interest in quantifying the effects of human land use on stream fish, and these data can be used in future studies to explore potential links between important environmental variables and land use practices.
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