With increasing temperatures and extreme weather events it is imperative to understand species potential for adaptation in changing environmental conditions. This is particularly relevant for ectothermic species, including fishes, which are largely reliant on environmental conditions for maintenance of homeostasis. Adaptation to temperature changes requires that selection act on existing functional genetic variation within a population but determining the links between candidate genes and putatively functional traits remains challenging. Recently developed genomic techniques, such as gene editing, have shown promise for examining variation in candidate genes and in characterization of adaptive trait variation. In fishes, one such candidate gene, is peroxisome proliferator-activated receptor alpha (PPARa), which encodes a transcription factor that appears to play a role in temperature response across multiple fish species, including threespine stickleback (Gasterosteus aculeatus). In this thesis I aimed to gain a better understanding of PPARa in relation to the thermal response and body size of fishes using stickleback with introduced genetic variation in PPARa and examining the changes in phenotypic expression. Non-homologous end joining (NHEJ) CRISPR was used on four families of laboratory stickleback, which were then crossed to create an F1 generation. Using a combination of DNA sequencing techniques, I determined that 29.5% of the alleles present in the F1 generation were CRISPR modified alleles. All F1 individuals were measured for body size and trialed for thermal tolerance, testing the hypothesis that modified alleles would lead to a reduced thermal tolerance and increased body size. I found no phenotypic differences in thermal tolerance between modified and wildtype individuals, indicating that PPARa and its relationship to thermal tolerance may be associated with other pathways, or that other mechanisms compensate for the modified alleles. I also found that individuals with two modified alleles accumulated significantly more mass over the course of the study, which is consistent with literature on PPARa and mass accumulation in other species. This thesis also highlights the importance of understanding adaptive genetic variation and the potential of gene editing techniques to aid in our understanding of functional genetic variation within populations and subsequently their potential for adaptation under environmental change.