Graduate Thesis Or Dissertation

 

A comparison of nest site selection and reproductive effort by reticulate sculpin (Cottus perplexus) in two streams of different geologies in the central Coast Range of Oregon Public Deposited

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  • Nest sites of reticulate sculpin (Cottus perplexus) were located in two stream reaches, one from a basalt basin and one from a sandstone basin. Stream reaches were similar in gradient, basin area, elevation, climate, and riparian vegetation but differed in biologic community structure and substrate characteristics. An electivity index was used to determine if selection for nest sites occurred and also to compare patterns of selection between habitat types and stream reaches. Eggs from nests were collected so comparisons in reproductive effort could be made between streams and habitat types. Cobble sized substrate was positively elected in all habitat units examined and moderate embeddedness (6-25%) was positively elected in all units but one. No nests were found on bedrock, wood, or fine sediment substrate. A small number of nests were found on both larger gravel and boulder size particles. Only two out of 471 nests located were associated with rocks which were embedded 51% or more. Reproductive effort on a per nest basis was greater in the basalt stream. Nest weights in the basalt stream exceeded nest weights in the sandstone stream by 90% (95% confidence interval 52-152%) on average and the number of eggs per nest was 39% (95% confidence interval 8-79%)greater in the basalt stream. Differences in reproductive effort per nest within an individual stream were not detected in comparisons among scour pools, riffles, high cobble density and low cobble density habitat units. Reproductive effort per nest varied through the sampling period with intermediate effort early, low effort in the mid portion and the highest effort late. This pattern was apparent in both streams but statistically significant in the sandstone stream only. Nest densities were similar at 0.16 nest/m² and 0.17 nest/m² for the basalt and sandstone streams respectively. Differences in nest densities within streams were not detected between scour pools and riffles in either stream or between high and low cobble density units in the basalt stream. A difference was detected between high and low cobble density units in the sandstone stream (p=0.02). Nest densities were greater on average in the tailout portions of pools as opposed to the head and body portions in both streams but statistically significant in the sandstone stream only (p=0.04). We speculate that because patterns of electivity were similar between streams and between different habitat types within a stream, all with different levels of habitat availability, that male fitness is tightly linked with habitat selection for nest sites. Our results would suggest that reticulate sculpin nest site selection and reproductive effort are not strongly influenced by habitat on a geomorphic channel unit scale. It may be more appropriate to delineate sculpin habitat by substrate patches within geomorphic habitat units.
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