Date of Award

Spring 5-3-2024

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Wildlife Ecology and Wildlife Conservation

Advisor

Christina Murphy

Second Committee Member

Allyson Jackson

Third Committee Member

Michael Kinnison

Additional Committee Members

Amanda Klemmer

Abstract

Fish are affected through a variety of mechanisms, including community composition shifts and changes in temperature and river flow. We used case studies in two different locations: Acadia National Park (Maine, USA) and the Willamette Valley (Oregon, USA) to investigate fish responses in a variety of conditions.

The diverse fish stocking history of the Acadia National Park (ACAD) lakes provided us the opportunity to study food webs with varying fish community compositions in a small geographic area. We used carbon and nitrogen stable isotopes to understand trophic relationships in ACAD lakes, focusing efforts on the fish community, their prey, and basal resources. We then used this data to create biplots to map out food webs and mixing models using SIMMR to estimate the proportion of each fish predator species’ diet. We also quantified the niche opportunity each fish species was utilizing, and competition in the form of standard ellipses areas corrected for small sample sizes (SEAc). Through our sampling and analyses we documented that fish compositions may have changed between 1998-2022 and that certain species may be unable to coexist without competition at our study sites. We then used our mixing model estimates to present evidence that fish at our study sites consumed a greater proportion of resources within an aquatic system than outside. Finally, SEAc overlap demonstrated isotopic niche crowding amongst invertivorous fish and less so between common piscivores. In aggregate, we add to the knowledge of trophic ecology in Maine.

We furthered our understanding of fish communities, species interactions, and how environment influences a species by analyzing a long-term passage dataset. Climate change is expected to stress freshwater systems, potentially altering fish movement and survival. Water temperature increases influence fish physiology and more frequent extreme weather events result in changes to the hydrologic regimes. Contemporary fish movements may provide insight into environmental drivers and could highlight species of particular concern. We examined 15 years of water temperature, river flow, and fish passage data to assess their relative influence on fish moving upstream in the McKenzie River, Oregon USA and patterns over time. Comparisons of realized timing and conditions while each species passed upstream through the Leaburg Dam fish ladders revealed that some species were more consistent seasonally, experiencing a more restricted range of conditions compared to others. For both groups, calendar date appeared to be a primary driver of movement timing, even considering environmental factors of temperature and flow. This suggests that fish may continue to move during consistent time periods, even under shifting thermal regimes, potentially increasing their exposure risk to sub-optimal environmental conditions. We also note surprising declines of Mountain Whitefish [Prosopium williamsoni] and Largescale Sucker [Catostomus macrocheilus] during years of extreme weather events. Our results demonstrate the utility of long-term passage data in detecting patterns in timing, co-occurring environmental conditions in fish movement, and the potential sensitivity of fishes to future change.

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Available for download on Thursday, July 24, 2025

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