Baumgartner, L., Reynoldson, N., Cameron, L. and Stanger, J., 2007. Effects of selected irrigation practices on fish of the Murray-Darling Basin. Final report to the Murray Darling Basin Commission for Project No. R5006. NSW Department of Primary Industries – Fisheries Final Report Series No. 92. 90pp.

Summary

The Murray-Darling Basin is Australia’s largest catchment covering over one million square kilometres and draining water from five separate states and territories. Despite receiving little water, the Murray-Darling Basin supports at least 40% of Australia’s agricultural production, a population of over 2 million people and is one of Australia’s most important natural resources. Such a degree of development has led irrigation to be the largest user of water in the Murray-Darling Basin.

Irrigation systems in Southern rivers occur via regulatory weirs that gravity feed water into canals where irrigation water is required. In contrast, practices in northern reaches of the Basin largely focus on direct pumping from main river channels into large off-river storages. This contrast in extractive water use is likely to have different ecological impacts on the aquatic fauna of Northern and Southern regions. However, there have been no previous Australian studies that have sought to investigate the impacts of irrigation systems on fish, which precludes the effective management of potential impacts. This study was undertaken to identify impacts of irrigation systems on fish, and provide subsequent recommendations to mitigate these impacts for selected irrigation practices.

The study focused on determining the potential effects of three common irrigation practices: (i) diversions of water into irrigation canal systems; (ii) direct extraction via pumping systems; and (iii) seasonal draw-down of canals. Results suggested that fish were substantially impacted by these processes. In southern systems in particular, large numbers of fish were extracted at diversion points near dams and weirs. In some cases up to 200 fish per day were extracted, and up to 900,000 fish can be stranded in pools following draw-down. In northern parts of the basin, fish extracted into pump systems were either killed or injured during passage. Any fish which survived received few opportunities to return to the source river because most irrigation systems were terminal.

Several management actions could help to reduce these impacts on fish. Firstly, operations of irrigation systems could be limited or ceased during periods of expected fish entrainment. Secondly, screening mechanisms could be installed to prevent or reduce the potential for extraction. Finally, structures could be provided to enable extracted fish to be returned to main river systems, such as fishways or transport systems. If these mitigation methods are workshopped and implemented in a logical and systematic manner, the long-term effects of irrigation systems on fish could be effectively reduced or eliminated.