Regulation of water flow along the Murray River has significantly altered the frequency and extent of natural flooding of Barmah-Millewa Forest. This has caused a decline in ecosystem health, which has impacted on various aspects of fish life. Environmental water (water released for ecological purposes) can be used to imitate natural flooding and help rehabilitate the Forest. However, we need to better understand how fish respond to different types of flows. ARI has recently completed a six year research program investigating the effect of varying flows and the use of environmental water, on aspects of fish breeding in the Forest.

The fish community in Barmah-Millewa Forest was sampled over five consecutive years (2003-2007), which were characterised by different timing and magnitudes of flooding, (including an environmental water release of 513 GL in 2005). Fish were also sampled again after a major flood event (late 2010). This study found that the best breeding response occurred after moderate spring and early summer flooding, which was supplemented by the environmental water release. For Murray and Trout Cod the number of young fish (less than a year old) increased, while for Silver and Golden Perch there were greater numbers of eggs and larvae caught. This is a particularly important outcome as these four species have high conservation and recreational significance.

Unexpectedly, the large-scale protracted spring-summer flooding of 2010/11 reduced native fish spawning and recruitment. This was likely due to the creation of hypoxic blackwater (tea stained water that has little oxygen) which arose from a combination of four years of drought and associated leaf litter build up on the floodplain, and floodplain inundation during the summer months. This finding is a timely reminder that flooding does not always deliver short-term benefits for native fish populations. More information on how different species and different life stages respond to flow will better guide future deliveries of environmental water. This research was funded by the Murray-Darling Basin Commission and will be used to help make decisions on the timing and volume of future allocations that will be most beneficial to native fish populations.

A lake in Barmah-Millewa Forest

Inundated floodplain in the Barmah-Millewa Forest

For more information contact: scott.raymond@delwp.vic.gov.au

The following journal articles are available:

King, A.J., Ward, K.A., O'connor, P., Green, D., Tonkin, Z. and Mahoney, J. (2010) Adaptive management of an environmental watering event to enhance native fish spawning and recruitment. Freshwater Biology 55(1): 17–31

King, A., Tonkin, Z. and Mahoney, J. (2009) Environmental flow enhances native fish spawning and recruitment in the Murray River, Australia. River Research and Applications 25(10): 1205–1218