Koh et al. trout growth manuscipt data and scripts

Rivers and streams exhibit inherent variability in environmental conditions, particularly in water temperature and stream discharge. These factors can impact fish growth through both direct (i.e. physiological) and indirect (i.e. ecosystem productivity) mechanisms. Additionally, the effects of temperature and stream flow on the growth of an individual fish may depend on the fish’s age and life history stage. Given this complexity, predicting how individual fish respond to environmental variation is challenging.We aimed to explore how spatial and temporal environmental variation in water temperature and stream discharge affects the growth of brown trout (<i>Salmo trutta)</i> in south-eastern Australia, as recorded in their otoliths. Our findings reveal that brown trout growth was influenced by both short-term and longer-term water temperature and streamflow variation with age-dependent responses. Generally, brown trout grew faster across warmer sites and years and with higher stream discharge. Younger fish experienced relatively slower growth while older fish showed increased in growth in years that were warmer than average. Our findings identify summer as a critical period for brown trout growth, driven by both direct temperature effects and indirect influences of antecedent stream conditions on stream productivity.This study suggests that brown trout may demonstrate higher tolerance to stressful conditions than previously thought, modulated by a combination of thermal tolerance and behaviour. This resilience appears to be age-dependent, with older fish showing increased growth in warmer years, possibly due to their ability to monopolise optimal habitats and resources.Our work has important implications for understanding how fish populations may respond to increasingly variable conditions. Our results highlight the need to understand the drivers of growth variation while considering age-dependent response, which is crucial for assessing the persistence of fish populations under changing environmental conditions.