Diadromy is a remarkable form of migration where individuals undergo regular movements between fresh and marine waters for the purposes of feeding and reproduction. Diadromy occurs in many taxa and can take various forms (catadromy, anadromy and amphimodry). Despite having arisen in many independent lineages of fish, molluscs and crustaceans, the evolutionary drivers of diadromous migration remain debated. Here, I will present a novel test of the often-cited productivity hypothesis. This theory predicts that diadromy will occur at an evolutionary mid-point as species transition between entirely marine and freshwater lifecycles, due to a primary productivity differential between marine and freshwater habitats. Otolith chemistry and biochronology data are analysed in a partially catadromous fish (barramundi Lates calcarifer) to determine the effect of diadromy on growth. On average, individuals that accessed fresh water grew ~25% faster than estuarine residents in the year following migration, suggesting that catadromy elicits a fitness advantage over non-catadromous life-histories (i.e. barramundi that remain in the estuary), as predicted by the productivity hypothesis. We argue, however, that temporal and ontogenetic fluctuations in relative reproductive success stabilise selection on differing migratory phenotypes.