Metabolic drivers of salmonid growth and survival in a warming world

Global warming is altering aquatic environments worldwide, with important consequences for ectotherms whose metabolism is directly influenced by temperature. Although metabolic responses to temperature are well studied in laboratory settings, much less is known about how metabolism affects fitness in wild populations. This thesis develops and applies approaches to understand how environmental conditions associated with climate change, particularly increased temperature and thermal variability, shape metabolism and its consequences for fitness in salmonids, a taxonomic group of major ecological, cultural, and economic importance. Chapter 1 introduces salmonid ecology and the role of metabolism in shaping individual performance. Chapter 2 develops and validates a novel method for measuring fish metabolic rates in the field against laboratory-based intermittent-flow respirometry (the current gold standard). Chapter 3 applies this approach to show that exposure to greater thermal variability produces more thermally sensitive metabolic rates and associated fitness traits, suggesting increased phenotypic plasticity. Chapter 4 expands this work across the latitudinal range of Atlantic salmon, showing that thermal regimes influence how metabolic rate affects fitness traits. Chapter 5 addresses management by presenting a conceptual framework for applying emerging chemical tracers to reconstruct fish movements and growth histories. In the final chapter these findings are synthesised to demonstrate that thermal environments shape both metabolic phenotypes and their fitness consequences in wild salmonids, providing new insights into how populations may respond to ongoing climate change and highlighting priorities for future research and conservation.