Author ORCID Identifier

https://orcid.org/0000-0002-0339-0628

Semester

Spring

Date of Graduation

2026

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Wildlife and Fisheries Resources

Committee Chair

Kyle Hartman

Committee Member

Brent Murry

Committee Member

Stuart Welsh

Committee Member

John Sweka

Committee Member

Dave Thorne

Abstract

Climate change has been identified as one of the greatest threats to global biota in recorded human history. Consequences are expected to be more pronounced in ectotherms because most of their physiology is controlled by external temperature conditions. Predicting how ectotherms may respond to increasing global temperatures necessitates information about thermal limits of thermal sensitive species and how increasing temperatures may affect individual and population-level fitness. Therefore, the objectives of this research were aimed at gaining a better understanding of the mechanisms that underpin thermal tolerance limits of fish and how those mechanisms may affect fitness and population persistence under climate warming conditions. Specifically, we sought to (1) develop estimation methodology for and assess the utility of an ecologically relevant thermal tolerance metric compared to a historically utilized metric, (2) describe and apply field methods to estimate in situ thermal tolerance metrics, assessing in situ acclimation effects on thermal tolerance limits of wild fish, (3) evaluate whether differences in consumption and growth of wild-sourced Brook Trout (Salvelinus fontinalis) housed at stressful temperatures can be attributed to differences in thermal tolerance, and (4) predict climate change induced extirpation risks and establish a possible management prioritization for 25 headwater Brook Trout populations in central Appalachia.

The first chapter of this dissertation serves as a brief review of the relevant literature about thermal tolerance in ectotherms and how it relates to ectotherm physiology. Particular focus is given to describing methods used to research thermal tolerance limits in fishes, as well as outlining knowledge gaps in the existing literature. Specifically, I outline the shortcomings of using critical thermal maximum (CTmax) as a thermal tolerance metric and describe how improvements can be made in conducting thermal tolerance assays in fishes.

In the second chapter, I developed a procedure for estimating critical temperature (Tc) as a more ecologically relevant thermal tolerance metric when compared to CTmax, evaluated potential factors that may contribute to differences in these thermal tolerance metrics, and compared the repeatability of these metrics using Brook Trout as a model organism. Most notably, results suggest that Brook Trout thermal tolerance limits decreased with repeated testing (i.e., “heat weakening”), which has implications for climate change management. Additionally. Tc was less repeatable than CTmax, which was expected due to the additional sensitivity of the estimation process for Tc when compared to CTmax. Finally, Tc increased with increasing short-term (1-6 hour) conditioning temperatures, suggesting an acute acclimation effect may be present in Brook Trout.

Streamside thermal tolerance assays were the main focus of the third chapter, used to evaluate and compare in situ acclimation effects on Tc and CTmax across four wild populations of Brook Trout. Results in this chapter suggest that Tc was more related to short-term acclimation effects, whereas CTmax was more related long-term acclimation, suggesting the presence of different acclimation mechanisms across the two thermal tolerance metrics used in this study. Additionally, larger fish tended to have lower thermal tolerance limits, which could be attributed to differences in ontogeny or previous exposure to stressful temperatures for older (thereby larger) fish resulting in in situ heat weakening.

Wild-sourced Brook Trout from four headwater streams were used to evaluate whether differences in bioenergetic performance (i.e., consumption and growth) when housed at stressful temperatures were related to individual thermal tolerance limits in the fourth chapter. I hypothesized that more tolerant individuals would exhibit higher performance given a wider breadth of permissible temperatures for physiological functions. Results suggest that there were population-level differences among the streams, but thermal tolerance did not appear to be a good predictor of performance at stressful temperatures.

Finally, the fifth chapter examines long-term Brook Trout monitoring data in conjunction with contemporary temperature data and climate projections to simulate and predict extirpation risk for Brook Trout populations in 25 headwater streams in West Virginia, United States across a suite of climate scenarios. Results from the extirpation risk simulation were then used to inform management prioritization under a resist-accept-direct decision-making framework. Results from the simulation suggest that a large proportion (19/25) of these streams are at risk of extirpation and that site-level climate management techniques to resist changes could be effective in nearly half of the streams (11/25) included in the present study.

These chapters helps provide more insight into the physiological consequences and population responses of ectotherms in a warming climate. The results presented here improve our understanding of thermal limits in ectotherms and can help to guide management to promote persistence of thermally sensitive and culturally important species like the Brook Trout.

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