How repeatable is CT within individual brook trout over short- and long-time intervals?

As stream temperatures increase due to factors such as heated runoff from impervious surfaces, deforestation, and climate change, fish species adapted to cold water streams are forced to move to more suitable habitat, acclimate or adapt to increased thermal regimes, or die. To estimate the potential for adaptation, a (within individual) repeatable metric of thermal tolerance is imperative. Critical thermal maximum (CT) is a dynamic test that is widely used to measure thermal tolerance across many taxa and has been used in fishes for decades, but its repeatability in most species is unknown. CT tests increase water temperature steadily over time until loss of equilibrium (LOE) is achieved. To determine if CT is a consistent metric within individual fish, we measured CT on the same lab-held individually-marked adult brook trout Salvelinus fontinalis at three different times (August & September 2016, September 2017). We found that CT is a repeatable trait (Repeatability ± S.E.: 0.48 ± 0.14). CT of individuals males was consistent over time, but the CT of females increased slightly over time. This result indicates that CT is a robust, repeatable estimate of thermal tolerance in a cold-water adapted fish.