Temperate fishes thrive in dynamic thermal environments, where water temperature can vary dramatically across days, seasons, and years. To survive in these variable environments, they must acquire essential energy and nutrients to support their physiological responses to temperature change. However, climate change is expected to impact the nutritional landscape (prey options, quality, and quantity) for many marine animals, which may limit their ability to respond effectively to co-occurring environmental changes, like temperature rise. In my PhD thesis, I studied how an abundant kelp forest fish, opaleye (Girella nigricans), is affected by simultaneous changes in their diet and environmental temperature.

One way opaleye may respond to temperature change is by adjusting their diet selection or ingestion rate to modify their nutritional status. However, my results demonstrate that diet and temperature have inconsistent effects on the thermal sensitivity of ecologically important traits and that opaleye are prone to making diet choices that have negative impacts on their performance in warm water. I also explored the untested hypothesis that nutritional status can affect the rate of thermal plasticity but found that opaleye’s rate of plasticity was insensitive to diet. However, their capacity for plasticity was diet-sensitive and could be partially explained by their fatty acid assimilation. Finally, I found that opaleye energetics and thermal tolerance are impacted by the combined effects of food restriction and dietary fat composition, where the quality of the fish’s diet can rescue them from the harmful effects of food restriction. Overall, this dissertation demonstrates that diet mediates thermal plasticity in ectotherms, like opaleye.