Little variation in field-based upper thermal tolerance among populations of a freshwater fish exhibiting strong life history differences

Abstract

As aquatic ecosystems warm, intraspecific variation may contribute to population- and species-level resilience. Thermal tolerance is typically studied in the laboratory, which can control for acclimation but might mask population differences. We measured critical thermal maximum (CT_max) of pumpkinseed (Lepomis gibbosus) in lakeside assays for nine populations in central and eastern Ontario, Canada. These populations had previously been shown to exhibit profound differences in life history. Trials were conducted in the late summer and early fall and replicated over two years (2023 and 2024). Temperature loggers provided acclimation temperatures; CT_max was best predicted using shorter acclimation windows (<10 days). CT_max ranged from 33.64 to 37.34 °C, with no differences between years. Acclimation temperature had a positive effect on thermal tolerance. Larger fish had higher CT_max, contrary to much of the literature. Overall, while we found that pumpkinseed thermal tolerance was affected by thermal history and body size, we found no evidence for population differences. Therefore, inter-individual variation is more likely to contribute to resilience to warming in this species than inter-population variation.