Transferability of models to predict selection of cover by coastal cutthroat trout in small streams in western Oregon, USA Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8w32r8933

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  • We assessed use and selection of cover by coastal cutthroat trout (Oncorhynchus clarkii clarkii) in six headwater streams in three watersheds in western Oregon, USA during the summer low flow period from 1 August and September 30, 2007. We tagged 1037 coastal cutthroat trout (>100 mm) with passive integrated transponder (PIT) tags across all streams. Selection of cover was analyzed by comparing characteristics of locations used for concealment by relocated fish relative to characteristics of randomly available habitat that could be used for concealment. We measured habitat characteristics for 190 relocated individual fish using cover and 797 randomly points potentially available as cover. Of the latter points, only 235 of 797 were potential cover, based on characteristics of cover actually used by fish. In other words, 562 of the 797 randomly sampled points were unlikely to be used as cover by fish. Coastal cutthroat trout used substrate as cover (78%) more often than all other cover types combined (22%). Availability of different cover types was variable, but overall substrate made up 92% of available cover and the remaining 8% represented all other cover types combined. Habitat characteristics measured for both used and available cover included depth at fish location (cm), surface area of cover (m²), proximity to depth of 20 cm for fish located in < 20 cm in depth, b-axis (mm) for substrate >2 mm, and distance under substrate. Each of these habitat characteristics was different for used and available cover (Wilcoxon rank-sum p-values all < 0.0001). Analysis of selection using logistic regression models indicated that cover use was more likely with increasing depth and surface area of cover. A negative interaction effect between the influences of depth and surface area suggested fish were more likely to use cover with smaller surface areas in deeper water. We found good transferability (i.e. predictive capabilities) of the logistic regression models across streams using three different methods: "leave-one-stream-out" cross validation, Cohen's kappa statistic, and receiver operator characteristic curves. Our results suggested that characteristics of used cover were similar across six streams for coastal cutthroat trout in headwater streams. The strong and consistent influence of both depth and surface area of cover on selection of habitat by individual coastal cutthroat trout suggests these features of habitat may be critical to this species during summer low flows.
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