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The effects of streamside forest harvesting on aquatic macroinvertebrates and rainbow trout (Oncorhynchus mykiss): Diet in the central interior of British Columbia, Canada Mackenzie, Kirsten Dawn

Abstract

Timber harvesting has the potential to alter stream ecosystems in a variety of ways, impacting stream habitat, food resources, and ambient conditions for a variety of stream biota. While ample attention has been focused on salmonid-bearing streams in the coastal areas of British Columbia, comparatively little research has been applied to the interior regions of the province where climate, topography, and forest practices differ wildly from the wet, mountainous west coast. This thesis investigates the effect of streamside forest harvesting on the benthic and drift macroinvertebrate communities, and on the diets of stream-resident rainbow trout in three small, lake-headed streams in the central interior of B.C. Benthic invertebrate biomass and abundance, community composition, and functional feeding guild composition did not exhibit post-logging changes in the two treatment streams relative to the control. Similarly, the biomass, abundance, community composition, and functional feeding guild composition of the drift invertebrate community did not show marked differences between logged and unlogged streams. Rather, patterns within both of these communities tended to reflect natural temperature differences evident among the streams. Rainbow trout in the two logged streams exhibited significantly lower condition (weight for a given length) than trout in the unlogged control stream. Trout in the control stream ate a greater biomass and abundance of invertebrates than trout in the logged streams. Trout in the control stream also tended to eat larger invertebrates than fish in the logged streams; however, this did not reflect differences in the size of prey items available in the drift. Two families of invertebrates tended to dominate the biomass and abundance of the stomach contents of all fish: Ephemeroptera (mayflies) and Diptera (true flies). Neither the size, abundance, nor biomass of these primary prey organisms were affected by the logging treatment. The unique temperature regimes coupled with density dependence associated with these small lake-headed streams were likely the most important factors in regulating invertebrate communities and fish growth in these small streams.

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