Climate change is associated with an increase in global air temperatures, and this trend is predicted to intensify over the next century, threatening a vast number of species worldwide. Migratory fish are disproportionately affected, because successive life stages depend on favourable environmental conditions within the wide range of habitats they traverse on their reproductive and/or feeding migrations. Salmonids have been extensively studied in this context, establishing that suboptimal river water temperatures reduce salmonid migration success and increase pre-spawning mortality. To date, however, little is known about the sub-lethal effects of increased water temperature on the reproductive success of migration survivors. Here, a comprehensive methodological approach including behavioural observations, geometric and analogue morphometrics, as well as sperm quality and morphology analysis, revealed complex trade offs between different facets of brown trout, Salmo trutta L. 1758, reproduction. The results show that warm acclimation exacerbates energetic trade-offs for reproductive facets in brown trout males, who consequently forfeit investment in secondary sexual characteristics and instead increase their courtship effort towards females. Warm acclimation furthermore reduced the quality of brown trout sperm, possibly indicating a delay in maturation. As a potential mechanism for differences in quality, morphological and hydrodynamic changes in warm acclimated male spermatozoa were identified suggesting that their energy supply for thrust production was limited. These results elucidate previously unknown impacts of temperature on multiple stages in the brown trout reproductive cycle and therefore increase our understanding of the impacts of climate change on their reproductive success. Due to their continued decline, salmonid populations are increasingly augmented with hatchery-reared stocks, but not always successfully. Genetic, behavioural and morphological deficiencies often negatively affect hatchery fish survival after release. Previous efforts to implement forced prolonged swimming (exercise training) in captivity to improve the survivability of a number of fish species have yielded conflicting results. Here, growth trajectories and body robustness were increased in exercised, domesticated S. trutta parr, but overall, their adaptive potential appeared limited. These results contribute to our understanding of the role of domestication and rearing habitat conditions on the success of future stock enhancement projects, and ultimately to the continued abundance of salmonids in the wild.