Diadromous species migrate between fresh and salt water on a regular, well-defined basis. These species often hold high economic and cultural value and many are at risk from flow regulation and other human activities that diminish connectivity between habitats. With the exception of barramundi (Lates calcarifer), the migratory behaviours of diadromous species in Northern Australia are poorly understood at present, partly due to the difficulties inherent in tracking the movements of fish throughout the entire life history. This study examined the movements of three species of forktail catfish; blue salmon catfish (Neoarius graeffei), shovel-nose catfish (Neoarius midgleyi) and lessor-salmon catfish (Neoarius leptaspis) in the Daly River, Northern Territory, using otolith strontium isotope ratio (⁸⁷Sr/⁸⁶Sr) transect-analysis.

By comparing otolith ⁸⁷Sr/⁸⁶Sr transects with water ⁸⁷Sr/⁸⁶Sr mixing models, we found that the species displayed distinct life history strategies. N. leptaspis exhibited residency in estuarine or marine water during the early life stages, with a subsequent transition into freshwater. N. midgleyi exhibited residency in freshwater only and did not migrate into estuarine or marine water at any point throughout their life history. Although N. graeffei commonly occurred in both estuarine and freshwater, they did not appear to undertake consistent, well defined migrations between fresh and saline water during their life history. The study also revealed strong variation in water and otolith ⁸⁷Sr/⁸⁶Sr corresponding to wet-dry seasonality in water chemistry. We conclude that analysis of otolith and water ⁸⁷Sr/⁸⁶Sr  is an effective technique for tracing whole-of-life salinity histories of riverine fish.