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Published: 09 December 2010   •   Media release

The Murray-Darling Basin Authority today advised of the continuing impacts of the 'blackwater' in the waters of the River Murray downstream of Barmah and in the Edward and Wakool River System.

The 'blackwater' has very low dissolved oxygen levels and has resulted in fish deaths in some locations and has forced exceptionally high numbers of Murray crays to leave the water and seek refuge on the river banks.

River users are reminded by the NSW Department of Industry and Investment that it is currently a 'closed season' for Murray crays and that people should not have them in their possession. The Murray crays will be very vulnerable over the coming weeks until they return to the river and river users are asked to minimise any disturbance to Murray Crays they see on the river banks. It is also fish spawning season and anglers may wish to voluntarily limit their catch to minimise further impacts on the fish population into the future.

Blackwater events occur naturally due to the rapid breakdown of leaf litter on the forest floor causing water discolouration. The breakdown of leaf litter plays an important ecological role as it provides nutrients back into the river system thereby promoting the growth of many aquatic organisms. However, at times such as now, this process can result in very low dissolved oxygen levels.

The 'blackwater' is arising from flooding of the Barmah-Millewa, Gunbower-Koondrook and Werai Forests and other floodplain areas that have not been adequately flooded for many years. The flooding has occurred following several months of heavy rainfall across the upper Murray catchments resulting in high inflows from Kiewa, Ovens and Goulburn Rivers, Billabong Creek and Hume Dam spilling.

With more rain forecast, it is expected that flooding in the Mid-Murray and Edward-Wakool System will continue for at least another month. The low oxygenated water in the Mid-Murray will mix with fresh inflows reducing its impact downstream.

The Authority and New South Wales and Victorian agencies have implemented several measures to lessen the impact of the blackwater event. This includes releasing fresh water from irrigation systems to the river system at a number of locations and releasing environmental water from storages to provide dilution. Whilst such measures may reduce the severity of blackwater impacts at some locations, they are unlikely to eliminate them.

In the Edward-Wakool System, an emergency fish rescue plan is being implemented by NSW Agencies and the community, which has already relocated some native fish to areas of better water quality. In the River Murray, monitoring is indicating that many of the large bodied native fish may have migrated upstream and downstream of the worst affected areas.

On a positive note, the floods have watered over 90% of the Barmah-Millewa Forest, which has not happened for over 10 years. The flooding has also attracted many thousands of waterbirds such as Ibis, Spoonbills, Great Egrets, Cormorants and Nankeen Night Herons to begin nesting within the forest. It is expected these birds will continue to breed over the next few months. Waterbird species such as the Blue-billed Duck and Australasian Bittern are listed as threatened in NSW and have been observed in the forests.

Further advice will be issued if circumstances change significantly.

For more information contact the MDBA Media office at media@mdba.gov.au

For more information on recreational fishing regulations refer to the NSW Department of Industry and Investment at http://www.dpi.nsw.gov.au/fisheries/recreational/info/guide

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