Summary

The Murray-Darling Basin is Australia’s largest catchment covering over one million square kilometres. Much of the basin is located in semi-arid to arid climatic zones and receives low annual rainfall with high evaporation. Ninety-eight percent of the catchment contributes little or no run-off, and the system has a relatively small annual discharge compared to other Australian rivers. Despite this generally low flow, the Murray-Darling Basin supports at least 40% of Australia’s agricultural production, is home to over 2 million people and is one of Australia’s most important natural resources.

The overall health of the Murray-Darling system has declined over the past 100 years largely due to things like overfishing, water extraction, land clearing, alteration of natural flow regimes, riparian degradation and reduced connectivity. The degradation of the Murray River has had detrimental effects on many native plans and animals. Impacts on the abundance and diversity of native fish have been particularly severe, and numbers of native fish within the Murray-Darling Basin may have fallen by as much as 90% since European settlement. Furthermore, the Murray Darling Basin is one of the most regulated rivers in the world. Each year, approximately 87% of annual runoff is extracted for irrigation purposes. Such a high level of water loss from the Basin’s rivers could have potentially damaging effects on native aquatic fauna. Previous studies and expert workshops have concluded that irrigation systems could result in the removal of significant numbers of eggs, larvae and juvenile fish from such systems. In some cases, mortality of adult fish might also occur, especially when fish are drawn into areas such as power turbines or irrigation canals.

The literature review has gathered previously published information on the extraction of fish eggs, larvae and juveniles into irrigation offtake systems and examined the possible impacts of this on aquatic communities of the Murray-Darling Basin and other Australian freshwater systems.

A project completed by the NSW Department of Primary Industries (NSW DPI) in 2003 examined native fish movements in the Murray River and estimated that millions of eggs and larvae of Murray cod, golden perch and silver perch could be extracted from the main channel of this large river on an annual basis. Although similar estimates had been made for fish overseas, no study had been done to determine how many species of native fish were being diverted or extracted from rivers in the Murray-Darling Basin. NSW DPI has since commenced such a study, which will finish in late 2006. The pamphlet has been produced to present the background to the issue, describe the objectives of this project and outline how the project will build on previous information. It provides a description of the background and the methods being used to determine which native fish species are most susceptible to the effects of water extractions and diversions.