Graduate Program

Biological Sciences

Degree Name

Master of Science (MS)

Semester of Degree Completion

Fall 2025

Thesis Director

Eden L. Effert-Fanta

Thesis Committee Member

Eloy Martinez

Thesis Committee Member

Thomas Canam

Abstract

As global temperatures continue to rise, understanding the physiological responses of aquatic organisms becomes increasingly important. Elevated temperatures can significantly impact behavior, metabolism, and other essential survival processes. One species of particular interest is the Green Sunfish (Lepomis cyanellus), a member of the Centrarchidae family known for its relative thermal resilience compared to other congeners across the Midwest, a region already experiencing rapid warming due to climate change. L. cyanellus are not invulnerable to thermal stress, and recent evidence suggests that both performance and thermal tolerance decline under sustained temperature increases. In this context, evaluating their response to fluctuating thermal regimes is essential. The primary goal of this study was to assess how L. cyanellus respond to daily thermal fluctuations and extreme temperatures. To investigate this, we exposed individuals to control (18°C), fluctuating (18-28°C) and avoidance (28°C) temperature regimes to assess how the thermal tolerance of L. cyanellus changes under three regimes. Thermal tolerance was assessed as the Critical Thermal Maximum (CTMax), which was determined by exposing individuals to a continuous temperature ramp at a rate of 0.3 °C per minute. This approach allowed us to identify the species' upper thermal limits and evaluate performance across varying conditions. Our findings showed that thermal tolerance in L. cyanellus increased in the avoidance temperature treatment. However, fluctuating thermal regime elicited a decrease CTMax over time, compared to the avoidance temperature treatment. Specimens in the control treatment also decreased CTMax over the sampling period. As most studies assess CTMax under constant thermal regimes, our results suggest these approaches should be revised since environmental temperature is a highly dynamic variable. Daily temperature variation significantly alters thermal performance, highlighting the need for future evaluations to consider dynamic thermal regimes rather than static conditions alone

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