Graduate Thesis Or Dissertation
 

Effects of controlled flow reductions on the insect community of an Oregon coastal stream

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  • Controlled flow reductions of 75% apportioned over four two-week intervals were studied during the summers of 1975 and 1976 on the North Fork of Elk Creek, an Oregon coastal stream. The effects of reduced flows on invertebrate drift, insect benthos, and the available food for juvenile salmonids were measured. Invertebrate drift showed a two-phased pattern from reduced flows. This was: 1) a decrease in drift immediately following flow reductions, and 2) an increase in drift after a two-week interval. Chironomid larvae and Baetis nymphs were the dominant taxa in the drift. The riffle benthos was sampled by artificial substrate and Hess samplers, while the pool benthos was sampled with a core sampler. Benthes density and biomass were not significantly affected by reduced flows. Differences between the artificial substrates and Hess samples were observed, with artificial substrates generally collecting more but smaller invertebrates than Hess samples. Greater numbers and biomass of invertebrates were collected in the pools than in the riffles. Stomach samples from 99 age 0+ coho (Oncorhynchus kisutch) showed they fed primarily on subsurface drift until the end of the lowest experimental flow, when their diet changed to surface foods. This change in diet coincided with a decrease in subsurface drift. Fifteen age 1+ cutthroat (Salmo clarki) also fed primarily on surface foods during the lowest flow. Only age 0+ cutthroat fed mainly on subsurface food during the lowest flow, possibly because they were restricted to riffle areas while 0+ coho and 1+ cutthroat inhabited pool areas. Overall, the effects of reduced flows were minor. Flooding during the study and variability of the data precluded definitive results.
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