CT is repeatable and doesn't reduce growth in zebrafish.

Critical thermal maximum (CT) is a commonly and increasingly used measure of an animal's upper thermal tolerance limit. However, it is unknown how consistent CT is within an individual, and how physiologically taxing such experiments are. We addressed this by estimating the repeatability of CT in zebrafish, and measured how growth and survival were affected by multiple trials. The repeatability of CT over four trials was 0.22 (0.07-0.43). However, CT increased from the first to the second trial, likely because of thermal acclimation triggered by the heat shock. After this initial acclimation response individuals became more consistent in their CT, reflected in a higher repeatability measure of 0.45 (0.28-0.65) for trials 2-4. We found a high innate thermal tolerance led to a lower acclimation response, whereas a high acclimation response was present in individuals that displayed a low initial CT. This could indicate that different strategies for thermal tolerance (i.e. plasticity vs. high innate tolerance) can co-exist in a population. Additionally, repeated CT trials had no effect on growth, and survival was high (99%). This validates the method and, combined with the relatively high repeatability, highlights the relevance of CT for continued use as a metric for acute thermal tolerance.