Abstract Seasonal fluctuations in temperature, salinity and food availability impose strong physiological demands on fishes inhabiting temperate estuaries. Here, we examine how the resident population of Odontesthes argentinensis from the Mar Chiquita Coastal Lagoon (Argentina) adjusts digestive function, metabolic reserves and body condition across seasons. We quantified reproductive gonadosomatic index (GSI), hepatosomatic index (HSI) and intestinal coefficient (IC) indices, intestinal alkaline phosphatase (AP) activity and major energy stores (glycogen, free glucose, triglycerides and proteins) in key metabolic tissues. Fishes exhibited pronounced seasonal plasticity. Intestinal mass and AP activity declined in winter–spring, indicating reduced digestive capacity during periods of low resource availability, while liver glycogen also decreased, reflecting both reduced feeding and increased energetic demands associated with reproduction. Lipid reserves accumulated in liver and intestinal fat during summer–autumn and were subsequently mobilized in spring, whereas muscle triglycerides peaked in winter. Muscle protein declined during winter–spring, suggesting mobilization to support gametogenesis. Together, these co‐ordinated adjustments reveal a flexible digestive–metabolic strategy that enables O. argentinensis to maintain body condition and sustain reproductive activity under the oligohaline and highly variable conditions of this coastal lagoon. By linking digestive function with energy storage and reproductive investment, this study provides an integrative framework for understanding physiological responses to environmental variability in estuarine fishes and establishes a baseline for future ecological monitoring and management of this regionally important fishery resource.