Acclimation dynamics and upper thermal tolerance in three pumpkinseed sunfish populations varying in parasite prevalence

Climate-driven increases in mean water temperature and the frequency of heatwaves affect the thermal tolerance of ectotherms, including fishes. Fishes can increase their thermal tolerance through acclimation, optimizing survival during extreme weather events. Few studies have investigated the exposure duration necessary for individuals to acclimate to warmer conditions or differences in acclimation dynamics among populations, limiting our understanding of how species deal with acute heat stress. Warmer waters can also increase parasite transmission. Although infections can reduce host thermal tolerance, the impact of parasite prevalence and abundance on fish acclimation capacity has not been explored, and may help explain population-level differences in thermal tolerance. We assessed thermal tolerance and acclimation dynamics across three populations of pumpkinseed sunfish ( Lepomis gibbosus ) from lakes in Quebec, Canada, differing in trematode and cestode infection prevalence. Pumpkinseed were acclimated to 22 °C or 27 °C from 3 hours to 60 days before measuring critical thermal maximum (CT max ). CT max increased with acclimation duration, with detectable increases after only 3 hours, indicating rapid induction of acclimatory mechanisms. Fish from all populations appeared to reach full acclimation after 10 days at 27 °C. However, thermal tolerance was consistently highest in the control lake (Lake Triton, no cestodes or trematodes) compared to populations with intermediate (Lake Croche) and high (Lake Cromwell) infection prevalence, despite no relationship with parasite abundance. Although our design does not permit causal inference, these results suggest pumpkinseed rapidly acclimate to higher temperatures, but natural exposure to parasites could contribute to population-level differences in thermal tolerance. • We compared thermal acclimation dynamics among three pumpkinseed populations. • Fish reached full acclimation within 10 days, regardless of parasite prevalence. • Thermal tolerance increased after only 3 hours of warm exposure in all populations. • Fish from the parasite-free lake showed the highest upper thermal tolerance. • Parasite exposure may limit population-level heat tolerance under warming.