Human-driven climate change and habitat modification are negatively impacting coastal ecosystems and the species that reside within them. Uncovering how individuals of key species respond to environmental influences is crucial for effective and responsive coastal resource and fisheries management. Here, using an otolith based analysis, we recreated the growth history of rock flathead (Platycephalus laevigatus) over a 32 year timeframe and related growth variation to changes in key environmental variables. Growth increased with higher temperatures during the fish growing season (December – May) and also increased with higher freshwater flow during the period important for seagrass growth (July-February). We hypothesise that fish are responding to enhanced productivity in the seagrass food web, driven by increased nutrient input from freshwater flows. Fish appear to also be responding to higher temperatures via a direct physiological pathway. We then predicted fish growth under three plausible climate change scenarios. Growth is predicted to increase across all our projections, because any predicted decrease river flow will likely be offset by rapid predicted increases in temperature. Our results highlight the value of understanding the drivers of long-term growth variation in harvested fishes as this allows for the prediction of future productivity under a range of environmental and management scenarios.