Graduate Thesis Or Dissertation
 

New insights on an old topic : understanding the effects of forest harvest on trout in the context of climate

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1c18dk25f

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  • Studies of the effects of forest harvest on streams and fish have a long history in the Pacific Northwest. Results of this work have prompted development of new forest harvest practices that are more protective of these resources, but the effectiveness of these new practices has not been fully evaluated. Furthermore, the effects of contemporary forest harvest in the context of climate change are poorly understood. To address these issues, my overall research goal was to understand how water quality, water quantity, and instream habitat influence individuals and corresponding population dynamics of coastal cutthroat trout (Oncorhynchus clarkii clarkii). My work was designed to complement field results from a new generation of watershed studies currently underway in western Oregon. I combined semi-natural experiments with modeling based on field observations to understand fundamental causes and processes influencing trout. This multifaceted approach provided a novel process-based perspective on issues related to forest harvest and climate. In Chapter 2, I evaluated individual- and population-level responses of coastal cutthroat trout to instream cover. Although the influences of forest harvest on stream flow, temperature, and turbidity are often the focus, instream cover may be strongly influenced by forest harvest. In addition, restoration of instream cover is now a common practice, but the importance of cover itself to stream-living fishes is still a major question. To address this issue, I conducted large-scale manipulative experiments in outdoor semi-natural stream units to approximate conditions experienced by trout in headwater streams in western Oregon. I determined that infrequent cover use by trout leads to emigration. Next, I built upon key ideas within stream ecology related to the importance of location within a landscape to aquatic biota, which have been explored and debated extensively. The variability in population responses across similar locations within a landscape is less understood. My objective in Chapter 3 was to understand the variability in population biomass of coastal cutthroat trout across headwater streams by understanding of the relative roles of two general classes of variables that occur in headwater streams: dynamic environmental regimes and relatively fixed habitat structure. I provided evidence that environmental regimes contribute to biomass variability while also being constrained by the habitat structure, given the range of conditions that I was able to simulate. Although the effects of contemporary forest harvest and climate change occur simultaneously, they are not typically considered together, as they are in Chapter 4. Here, I tracked population responses of trout, including biomass, survival, growth, and timing of emergence during six decades across four modeled headwater streams using the same individual-based trout model as in Chapter 3. I modeled four scenarios: 1) baseline conditions (simulation of existing conditions); 2) effects of contemporary forest harvest; 3) effects of climate change; and 4) the combined effects of forest harvest and climate change. Differences among scenarios were tied to changes in flow and temperature regimes. Here, I found that there was a high degree of local variability in the responses that I simulated. Whereas localized responses to forest harvest have been observed, my findings contrast with the vast majority of work on species responses to climate change, most of which reports relatively synchronous or uniform responses. I highlighted the role of individual variability of trout and local variability of streams, which ultimately suggest that some individuals and populations of trout may be more vulnerable than others to the effects of forest harvest, climate change, or both processes together.
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