A million dead fish, floating on putrid green water. Images of this ecological catastrophe on the Darling River over summer shocked the nation. Was it the result of drought? Blue-green algae poisoning? Excessive summer temperatures and climate change? Too much water taken upstream for irrigation? Mismanagement by the authorities?
After several months and at least four published reports, we know the answers. It's time to state plainly what has been going on, and who is to blame.
For the immediate crisis of the fish deaths, we can look at the Interim Report to the Australian Government, released 20 February. There were three large-scale fish kills, all in the Menindee region of the Darling River. This is the point in south-west New South Wales where the river reaches the Menindee Lakes. It's hot and dry, averaging about 250mm of rain a year.
During two periods in 2012 and 2016, it rained enough to fill the lakes. When full, these lakes become important nurseries for native fish. If a lot of water is flowing through the river, the fish can travel both upstream and downstream. Flowing water also circulates oxygen within the water column and helps to reduce algal blooms.
After 2016, there were no further inflows of water to the lakes. Some water was released downstream for the environment, but the largest flows (peaking at 6200 megalitres a day) were for the River Murray, to meet commitments at the South Australian border and for the irrigation season. To get there, the water passed over a nearby pool called 'Weir 32', which is much smaller than the lake. Fish and algae went with the flow, but as the water level dropped they became trapped in the weir pool.
The dry conditions continued. Rainfall for the northern Basin in 2018 was about half the long-term average. Most of the Murray-Darling had its hottest December-January period on record. The highest temperature in Menindee was 48.8 degrees, three days before the third mass fish death.
When it gets this hot and the river isn't flowing much, the water column splits. There's a layer of hotter water at the top, and cooler water at the bottom. The temperature difference means the layers can't mix. This keeps most of the oxygen in the shallow and warmer surface, while the much larger and deeper bottom layer becomes deprived of oxygen. The fish would have been pushed into the shallow surface waters, which were also