Freshwater fish and connectivity in tropical northern Australian rivers

Background/Question/Methods

Connectivity characterizes riverine ecosystems.  Seasonal pulses in discharge create tributary and floodplains habitats and connect them with the main channel and the near shore environment. Hydrologic connectivity creates movement pathways for fish and this has profound influence on aquatic ecosystem dynamics via material and process subsidy.  Fish movement must therefore be an integral component of environmental flow assessment and management.  Northern Australia is currently being targeted as a source of water and land to boost agricultural production. Consequent hydrologic changes and infrastructural development are likely to result in loss of riverine and floodplain connectivity in an area of global biodiversity significance. This presentation explores the importance of migratory fish in the ecology of the region’s rivers with special reference to the issue of natural connectivity.  We use recent outcomes of research conducted by the Tropical Rivers and Coastal Knowledge (TRaCK) consortium concerning the distribution of freshwater fish across the region and the nature and spatial extent of variation in food web structure revealed by stable isotope analysis.

Results/Conclusions

At least 50% of the freshwater fish of northern Australian rivers moves between freshwater and the estuarine and coastal environments.  The proportional contribution of such species to total richness per basin exceeds 35% on average.  Several species are of high conservation significance (e.g. elasomobranchs) and many are of high fisheries and cultural significance and therefore contribute substantial socioeconomic value to the region.  Such species penetrate far upstream in many basins, with the extent of movement being largely determined by channel gradient, the existence of natural barriers and the spatial arrangement of perennial flow in the landscape. They also make extensive use of floodplain environments.  Stable isotopes analyses reveals substantial movement of material between parts of the riverine landscape, with the extent depending on the levels of connectivity between these areas.  Diadromous species are ecologically important in transporting nutrients and influencing ecological process (i.e.  material and processes subsidies).  Many potamodromous species as well as numerous  species that migrate between permanent and intermittent habitats for purposes not related to reproduction occur in the region.  It seems that extensive movement characterizes nearly all of the freshwater fishes of northern Australian rivers, especially in rivers with intermittent flow regimes.  This has profound influence on the nature of food webs.  These results offer an empirical basis for developing a very cautious approach to future water resource development in the region.