Migrating anadromous fishes (e.g. Pacific salmon Oncorhynchus spp.) in the Northern Hemisphere play a defining role in temperate ecosystems by delivering marine derived energy and nutrients to oligotrophic environments (Naiman et al. 2002). Assimilation of this energy into food webs occurs via predation and consumption by other organisms and post-spawning decomposition of carcasses. The “material subsidy” (sensu Flecker et al. 2010) provided by these fish is crucial to the maintenance of populations of a wide range of terrestrial flora and fauna (Naiman et al. 2002). In tropical rivers, the importance of energy transported by migrating fish is poorly understood at present, although recent stable isotope studies suggest that fish movement is a potentially critical ecological process . We used otolith ageing and chemical analyses to examine the movements of Diamond-scale mullet Liza ordensis and five species of Forktail catfish Neoarius sp. between fresh water and the estuary/sea in a tropical river (Daly River) in the Northern Territory. Analysis of strontium isotope ratios (87Sr/86Sr) and otolith: body size relationships were used to quantify the marine-derived biomass transported to the freshwater ecosystem. Otolith chemistry analysis revealed evidence of marine residence in the early life history of Diamond-scale mullet examined followed by a return migration of juveniles into freshwater (i.e. catadromy). Forktail catfish exhibited highly variable patterns among species and among individuals within species. Similar to temperate ecosystems, our analyses suggest that migratory fishes play an important role in transporting marine derived energy into tropical rivers.