Northern Australia is biologically diverse and of national and global conservation signicance. Its ancient landscape contains the world's largest area of savannah ecosystem in good ecological condition and its rivers are largely free-flowing. Agriculture, previously confined largely to open range-land grazing, is set to expand in extent and to focus much more on irrigated cropping and horticulture. Demands on the water resources of the region are thus, inevitably increasing. Reliable information is required to guide and inform development and help plan for a sustainable future for the region which includes healthy rivers that contain diverse fish assemblages. Based on a range of information sources, including the outcomes of recent and extensive new field surveys, this study maps the distribution of the 111 freshwater fishes (excluding elasmobranches) and 42 estuarine vagrants recorded from freshwater habitats of the region. We classify the habitat use and migratory biology of each species. This study provides a comprehensive assessment of the diversity and distribution of fishes of the region within a standardised nomenclatural framework. In addition, we summarise the outcomes of recent phylogeographic and phylogenetic research using molecular technologies to identify where issues of taxonomy may need further scrutiny. The study provides an informed basis for further research on the spatial arrangement of biodiversity and its relationship to environmental factors (e.g. hydrology), conservation planning and phylogentic variation within individual taxa.

Migration is a fundamental aspect of the life history of many fish and must be well understood for targeted conservation and management. We used acoustic telemetry and otolith 87Sr/86Sr analysis, in conjunction with annual ageing, to study intraspecific variation in barramundi Lates calcarifer migration in the Northern Territory, Australia. Acoustic transmitters were implanted into 25 barramundi (420–1010-mm total length (TL); median 510mm TL) from freshwater reaches of the South Alligator River and their movements tracked over >2 years. 87Sr/86Sr transect analysis was also conducted on otoliths of 67 barramundi from the Daly, Mary, South Alligator and Roper rivers. Acoustic telemetry showed that most fish remained in fresh water across wet and dry seasons. Higher rates of movement occurred during the wet season and a minority of fish moved into the estuary during high flows. Otolith chemistry analyses revealed high diversity in salinity histories among individuals. We integrated the telemetry and otolith chemistry data to examine migration as a function of the stage of sexual development, and have proposed a revised life history model that identifies three migratory contingents. We conclude that anthropogenic disturbance, including modified river hydrology, has the potential to alter the frequency of life history contingents in barramundi populations.

The parasite fauna of the blue threadfin Eleutheronema tetradactylum, collected from 14 sites across northern Australia, was examined to evaluate the degree of movement and subsequent stock structure of the fish. Univariate and multivariate analysis of nine 'permanent' parasite species [the nematodes Anisakis (type I) and Terranova (type II), the cestodes Otobothrium australe, Pterobothrium pearsoni, Pterobothrium sp. A, Callitetrarhynchus gracilis, Parotobothrium balli and Nybelinia sp., and the acanthocephalan Pomphorhynchus sp.] demonstrated little similarity between sites, indicating limited mixing and therefore long-term separation of post-juvenile fish. As such, the effects of fishing are likely to be localized within the current administrative boundaries, implying little need for interstate co-operative management. Within each jurisdiction, management of E. tetradactylum populations, including the establishment of harvest strategies and fishery regulations, should be conducted in a way that recognizes the resident nature of the fish.

Natural river floodplains are among the Earth's most biologically diverse and productive ecosystems but face a range of critical threats due to human disturbance. Understanding the ecological processes that support biodiversity and productivity in floodplain rivers is essential for their future protection and rehabilitation. Fish assemblage structure on tropical river floodplains is widely considered to be driven by dispersal limitation during the wet season and by environmental filtering and interspecific interactions during the dry season. However, the individual‐level movement behaviours (e.g. site attachment, nomadism, homing) that regulate dispersal of fish on floodplains are poorly understood. We combined radiotelemetry and remote sensing to examine the movement behaviour of two large‐bodied fishes (barramundi Lates calcarifer, forktail catfish Neoarius leptaspis) over the flood cycle in a tropical river‐floodplain system in northern Australia to: (a) quantify movement responses in relation to dynamic habitat resources at a landscape scale; and (b) determine the extent of spatial ‘reshuffling’ of individual fish following the wet season. Both species altered their behaviour rapidly in response to changes in the availability and distribution of aquatic habitat, with most individuals undertaking extensive movements (up to ~27 km from the tagging location) on the inundated floodplain during the wet season. Although there was considerable individual variation in movement patterns, overall barramundi distributions closely tracked the extent of floodplain primary productivity, whereas catfish distributions were most closely associated with the extent of flooded area. Most individuals of both species exhibited homing back to previously occupied dry season refugia during the wet‐to‐dry transition, even though other potential refugia were available in closer proximity to wet season activity areas. We postulate that homing behaviour modulates temporal variation in fish assemblage composition and abundance and limits the transfer of aquatic‐derived energy and nutrients into terrestrial food webs by reducing fish mortality on drying floodplains. Our study demonstrates the importance of quantifying individual‐level behaviour across the three stages of dispersal (emigration, inter‐patch movement, immigration) for our understanding of how animal movement influences energetic subsidies and other large‐scale ecosystem processes.

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