Water temperature and river discharge drive age-dependent changes in brown trout (Salmo trutta) growth.

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 characteristics of the fish itself. Given this complexity, predicting how individual fish, and by extension fish populations, 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 (Salmo trutta) in south-eastern Australia and if this was modulated by inter- or intraspecific interactions. We used otoliths from fish sampled from self-sustaining populations to estimate somatic growth. Our findings reveal that S. trutta growth was influenced by both short-term and longer-term water temperature and streamflow variation, with these responses being dependent on fish age. Overall, S. trutta grew faster at warmer sites and in years with higher annual mean stream discharge. Younger fish did, however, grow more slowly while older fish grew faster in years that were warmer than the site's average. We found that summer was a critical period for S. trutta growth, with evidence suggesting that water temperature in the warmest part of the year directly affects growth. Furthermore, antecedent stream conditions indirectly affected growth, likely through their impact on stream productivity and thus the provision of trout prey. Our study suggests that Australian S. trutta may have a higher tolerance to warmer conditions than previously thought, potentially facilitated by a combination of local adaptation 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 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.