Individual fish growth is a fundamental aspect of a broader demographic process linking environmental conditions to freshwater fish population dynamics, yet the spatial scale at which somatic growth is regulated remains poorly resolved. In impoundments (reservoirs) subject to both active hydrological management and broad-scale climate variability, understanding whether individual fish growth is driven by local or regional-scale processes has direct implications for population forecasting and adaptive management. We examined annual growth of three native freshwater fish species (Murray cod (Maccullochella peelii), golden perch (Macquaria ambigua) and silver perch (Bidyanus bidyanus)) across four impoundments in south-eastern Australia. We used otolith microstructure to reconstruct growth histories spanning three decades. Growth differed among species and the nature of these differences varied by impoundment, indicating that species-level growth patterns are not consistent across sites. Generalised additive mixed effects models explained 63% to 70% of variation in individual growth, with environmental responses, across hydrology, temperature, rainfall, and broad-scale climate indices, consistently stronger and more informative when modelled at the local (site-specific) scale. The direction and magnitude of environmental effects varied markedly among the three species and impoundments, indicating that individual growth in these systems was regulated primarily by local rather than regional processes. Broad-scale climate indices were nevertheless informative, suggesting utility as predictive tools where local-scale data are unavailable. The remaining unexplained variation highlights the likely contribution of unmeasured biotic processes operating across extended timescales. Local-scale environmental data are therefore the most informative basis for forecasting individual fish growth in these systems, with broad-scale climate indices offering practical value where local monitoring capacity is limited. Our findings resolve this uncertainty by demonstrating that long-term somatic growth in impounded freshwater fish populations is fundamentally regulated by local environmental processes rather than broad-scale climatic variability, providing a stronger foundation for adaptive fisheries management under increasing climatic variability.