This report is aimed at fishery and threatened species managers, and industry. Defining management units is critical for the globally Endangered Narrow Sawfish Anoxypristis cuspidate, and genomic approaches were used to analyse the species population structure across northern Australia. Samples were obtained from bycatch of commercial fisheries in collaboration with the Northern Prawn Fishery. Barriers to gene flow were evident between all regions where there were more than three samples, demonstrating female philopatric behaviour at finer spatial scales than previously suspected. In contrast, no evidence of population structure was detected using nuclear markers, suggesting male-biased dispersal. Sampling of neonates in nursery areas would help characterise the species philopatric behaviour. Results indicate that each region should be treated as a discrete management unit, with risk of population depletion if local bycatch mortality was to exceed the species biological productivity.