An Independent Review of the February 2004 Lower Darling River Fish Deaths: Guidelines for Future Release Effects on Lower Darling River Fish Populations

"September 2004".

Project Number: Investigation of flow regulation effects on 2004 Darling River Fish Deaths GZ040 M/BUS/58.

MDFRC item.

In February 2004 an extensive kill of Murray cod (Maccullochella peelii peelii) occurred on the Lower Darling River between Menindee and Pooncarie. The deaths, which stretched for over 160 km of river, involved predominantly large Murray cod. Landowner and newspaper estimates of numbers of dead Murray cod ranged from 2 dead fish per kilometre of river, to 30 dead per kilometre. An adjusted estimate over the whole affected stretch of river produced a total of 3,000 Murray cod deaths (with minimum and maximum estimates of 1,000 and 5,000 deaths respectively). Immediately prior to the deaths the Lower Darling had ceased flowing and was reduced to a series of remnant pools. During the week of the fish kill maximum air temperature exceeded 40 °C, with water temperatures over 30 °C. Initial reports indicated large Murray cod died several days after a front of water released from the Menindee Lakes passed down the Lower Darling River. Examination of water quality information, observations made by landholders, and expert opinion from a workshop held in July 2004, suggests the deaths were a consequence of oxygen depletion (due most likely to a combination of extreme temperature, high algal respiration and organic loading), with the possible added stress of sulfide and/or ammonia toxicity released from anoxic sediments in hypolimnetic pools. The ecological significance of this fish kill is discussed in this report, and management guidelines aimed at avoiding similar events are presented.