Given that reproductive behaviours involve high-energy activities, scientists have hypothesized that reproductive fitness is related to an organism's physiological performance. The present thesis explored this hypothesis by relating heart rate performance to parental care behaviour in smallmouth bass (Micropterus dolomieu), and dominance behaviour in sockeye salmon (Oncorhynchus nerka). The results did not detect a relationship between overall heart rate performance and reproductive behaviour in either species. Instead, heart rate was found to follow a diel pattern irrespective of behaviour. It is therefore proposed that reproductive behaviour is mediated by the combined effects of intrinsic (e.g. hormones, cardiac performance) and extrinsic (e.g. photoperiod, predator burden) factors. Hence, this thesis demonstrates the complex relationship between cardiac performance and reproductive behaviours in wild fish. Future studies are required to explore the relationship between physiological performance and reproductive fitness to better understand mechanisms driving animal life history and evolutionary ecology in changing environments.