Background
The hot, dry landscape of Australia’s wet-dry tropics is punctuated by the annual wet season. Three months of rainfall ameliorates dry season environmental stress on all biota, from flow-dependent riverine, estuarine and coastal species to the ecosystem scale. Desert landscapes encroach from the south as the latitudinal intensity of monsoon rains subsides. Past studies of banana prawns, barramundi, mud crabs and sawfish have confirmed that the intensity of the wet season and subsequent river flood volumes determine the behaviour, biomass and landings of species important to commercial, recreational and indigenous fishers, as well as of species of conservation importance.
Methods
Within a largely-unregulated Roper River catchment, we used MICE (Models of Intermediate Complexity for Ecosystem assessment) to determine the impact on these species of the modification of natural flow regimes due to the diversion of critical seasonal river flows to support irrigated agriculture. Fishery catch - flow relationships were developed using the modelled historical natural flow series and fitting the model to catch data. Using this model, future flow scenarios modified by water resource development and climate were used to project future change in species biomass. Key estuarine-dependent species including commercial, recreational and threatened fish and crustaceans, as well as habitat-forming flora were modelled.
Results and Conclusion
Anthropogenic disruption to the historical volume and seasonality of flow reduced the biomass of fish and crustaceans, as well their mangrove and seagrass habitats. A future drier climate had a greater negative impact on their modelled abundance. Mitigation scenarios such as guaranteed end-of-system flows and trigger-levels below which water harvesting cannot occur reduced the impact of water diversion. Mitigation strategies suggest management options that facilitate agricultural production while minimising catchment-to-coast loss of water connectivity and refugia for key biota.