The role of temperature in mediating the behaviour and physiology of fishes is becoming more apparent, as climate change exacerbates the frequency and severity of extreme weather events. Among the many methods used to estimate upper thermal limits, the Critical Thermal Maxima method has been the dominant approach partially due to its relative simplicity in experimental design. However, several concerns have been raised about the ecological relevance of this method, particularly due to the use of rapid rates of thermal ramping. In this thesis, I begin by reviewing the ecological relevance of CTmax. I discuss methodological concerns and limitations, while outlining opportunities to address these concerns and apply CTmax in an ecologically-relevant way. I then provide an example of a field-based study that evaluates the role of CTmax estimates in accounting for variation in life-history traits and fitness in a semi-anadromous population of juvenile brown trout (Salmo trutta).