Floodplains have long been recognised as critical to the maintenance of food-web productivity in lowland rivers, but the associated mechanisms remain only partially understood. Large-bodied fish comprise a major component of the faunal biomass in healthy lowland rivers, and their movements are responsible for the transport of large amounts of assimilated energy within aquatic ecosystems and across ecotones. The movement of energy by fish often functions as a critical energetic “subsidy” that supports food webs in receiving habitats or ecosystems. For example, recent stable isotope analyses demonstrate that the productivity of fish populations in Northern Australian rivers is highly reliant on relatively short periods of floodplain inundation during the wet season that facilitate the transfer of energy from the floodplain to the main channel. To determine the role of fish movement as a driver of this energetic subsidy, we conducted a radio telemetry study of the wet season movements of 40 barramundi (Lates calcarifer) and 30 forktail catfish (Neoarius leptaspis) in the South Alligator River in Kakadu National Park. Individual fish were tracked by boat or helicopter every two weeks from October 2013 to May 2014 and their use of main channel and floodplain habitats examined throughout the wet season. The findings of the study are used to draw conclusions regarding the importance of inundated floodplains as fish habitat during the wet season and to obtain a better understanding of the nature of the energetic subsidies provided by floodplains in tropical lowland rivers

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