Individual and life stage variation in exercise fatigue under acute warming

There has been much interest in understanding the mechanisms that determine the thermal tolerance of fishes. Given the importance of swimming for fish survival, it is critical to understand the mechanisms that determine why fish fatigue from exercise when temperatures increase to improve our ability to predict the impacts of climate change on fish populations. For the same reason, it is also necessary to understand the drivers of inter-individual and life stage variation in warming tolerance. Here, we used the CTswim methodology (i.e., exposing swimming fish to acute temperature increase) to examine how and why individuals and life stages differ in their warming tolerance. Specifically, we tested whether muscle lactate accumulation and enzyme activities indicative of aerobic and anaerobic capacity, predict inter-individual variation in CTswim at two life stages. We used Chinook salmon (Oncorhynchus tshawytscha) fry and parr acclimated to four temperatures for several weeks to further explore the effect of acclimation temperature on these mechanisms. Our findings indicate that the capacity to remain aerobic for as long as possible while swimming (and thus maintain low lactate) during acute warming is likely a factor determining fatigue in young fry, but that other factors may become more important as salmon age.