Short-term acclimation dynamics in a coldwater fish

Critical thermal maximum (CTmax) is widely used for measuring thermal tolerance but the strong effect of acclimation on CTmax is a likely source of variation within and among studies/species that makes comparisons more difficult. There have been surprisingly few studies focused on quantifying how quickly acclimation occurs or that combine temperature and duration effects. We studied the effects of absolute temperature difference and duration of acclimation on CTmax of brook trout (Salvelinus fontinalis), a well-studied species in the thermal biology literature, under laboratory conditions to determine how each of the two factors and their combined effects influence critical thermal maximum. Using an ecologically-relevant range of temperatures and testing CTmax multiple times between one and 30 days, we found that both temperature and duration of acclimation had strong effects on CTmax. As predicted, fish that were exposed to warmer temperatures longer had increased CTmax, but full acclimation (i.e., a plateau in CTmax) did not occur by day 30. Therefore, our study provides useful context for thermal biologists by demonstrating that the CTmax of fish can continue to acclimate to a new temperature for at least 30 days. We recommend that this be considered in future studies measuring thermal tolerance that intend to have their organisms fully acclimated to a given temperature. Our results also support using detailed thermal acclimation information to reduce uncertainty caused by local or seasonal acclimation effects and to improve the use of CTmax data for fundamental research and conservation planning.