The worst mass fish kill in living memory, which saw millions of animals floating dead on the Darling-Baaka river near Menindee, may have been contributed to by an alleged failed strategy to release a combination of “blackwater” and clean water by authorities, researchers have claimed.
The researchers, who host a water program on Broken Hill’s local community radio, also allege that a smaller fish kill in the same river in February was the result of a similar water release strategy by WaterNSW and should have set alarm bells ringing.
Dan Schulz, a PhD student, and Megan Williams, a radio journalist, used publicly available satellite images from the Sentinel-2 satellite, statements from the Department of Planning and Environment and WaterNSW, and photos and observations from locals about the fish kills to piece together what they allege happened.
They say the satellite images show darker toxic blackwater held in Wetherell Lake was released directly into the Darling-Baaka River via the Wetherell outlet on 14 March, two days before the massive fish kill began. They claim this action has not been acknowledged by water authorities in any publicly available documents.
This outlet releases water from the bottommost layer of Lake Wetherell, where the worst-quality water would have accumulated. According to on-the-ground photos, this outlet was closed on 18 March, the day after the huge fish kills were reported in the media.
Blackwater is a natural phenomenon that occurs after floods due to accumulated debris washing from the floodplain into rivers and lakes. Organic matter breaks down, leading to sharp drops in oxygen levels which can be toxic to fish.
The researchers also allege that water authorities claimed to be releasing “oxygenated water” into the Darling-Baaka River from Lake Pamamaroo, but instead the satellite images show a secondary source of blackwater plume making its way from Lake Wetherell, around the shore of Pamamaroo lake and into the outlet that feeds the Darling.
The researchers allege the images show that water authorities used this method of mixing blackwater with better quality water in February, leading to the first fish deaths and then repeated it on 14 March, two days before the unprecedented fish kill began on 16 March.
The number of fish that died is unclear, but authorities agree it was hundreds of thousands, if not millions, of fish. Most were bony bream which had spawned during the floods, but there were also deaths of Murray cod, golden perch and other native species.
The NSW government has announced an inquiry into the fish deaths headed by the state’s chief scientist, Prof Hugh Durrant-Whyte. The terms of reference are now being finalised.
“The March 14th releases of blackwater from Lake Wetherell were made despite acknowledgement from the Department of Planning and Environment in its water quality updates, that blackwater being drawn from Lake Wetherell through the Pamamaroo outlet in February had contributed to fish kills,” Schulz and Williams say.
They also say the authorities should have known that a heatwave was coming and that hot weather would probably make the impact of releasing de-oxygenated water worse.
The pair are not hydrologists, but the ANU professor of hydrology from the Crawford school of public policy, John Williams, reviewed their report.
He said authorities were dealing with an enormous blackwater event as a result of the summer flooding and the way it was managed was critical. He supported the researchers’ interpretation of the satellite imagery.
“We need an independent inquiry. You can’t mark your own homework,” he said.
Water NSW and the Department of Planning and Environment declined to provide detail about what releases had been made from Lake Wetherell prior to the fish kills in February and March.
“WaterNSW operates the Menindee lakes system under a strict government regulatory framework,” a spokesperson said.
“Since the likelihood of a flood-related blackwater event started to increase in October 2022, WaterNSW has worked with government authorities to develop a strategy of operating the lakes to mitigate the worst of the impacts, including the use of environmental water releases.”
“WaterNSW anticipates that these operations will feature in the NSW government’s inquiry into the circumstances surrounding the fish kill and will support that inquiry as the most appropriate forum to explore these matters.”
Since the December floods, the NSW authorities had been attempting to manage an unprecedented blackwater event in the Darling around Menindee by storing the blackwater in Lake Wetherill at the top of the lake system.
The hydrology of the Darling-Baaka system has been dramatically changed by agriculture. During the 2018-19 drought, the river experienced record periods with no flow at all, while large-scale water harvesting on the floodplain has reduced inflows by an estimated 20% to 30%.
This, environmental scientists say, has prevented flushing – the ability of water streams to wash away accumulated organic matter in small- or middle-level floods.
The Menindee lakes themselves, once an ephemeral lake system, are also no longer in their natural state. They are controlled by a series of outlets, weirs and regulators, which were installed in the 1960s to store water for agriculture. The lakes are generally managed by WaterNSW.
Environmental scientists have said these human interventions appear to be making events like the fish kills in 2019 (due to lack of flow) and more recently on 16 March (due to blackwater) more severe.
Schulz and Williams allege the satellite images show the 14 March releases of blackwater from Lake Wetherell were made despite acknowledgment from the Department of Planning and Environment in its water quality updates, that blackwater being drawn from Lake Wetherell through the Pamamaroo outlet in February had contributed to fish kills.
“It was identified that poorer-quality water was entering Lake Pamamaroo from Lake Wetherell and was being drawn through the Pamamarroo outlet and released into the Darling river upstream from the town of Menindee. This increased the risk of stress to fish,” the Department of Plannning and Environment said on 8 March.
But after the fish kills, the public explanation from authorities was that the fish kills were caused by a natural event exacerbated by the weather.
“The deaths are believed to be attributed to hypoxic blackwater, a naturally occurring phenomenon which causes extremely low dissolved oxygen levels. The scale of this event has been exacerbated by recent hot weather and significantly increased numbers of fish in the system as floodwaters recede,” the NSW Department of Primary Industry said in a press release on 19 March immediately after the fish kill.
The Guardian has learned that as fish began dying in February, there were warnings from the department of fisheries and from NSW Environment and Heritage that releases of good quality water were urgently needed to avoid mass fish deaths.
During a series of fiery interdepartmental meetings, at which fisheries and the environment department argued for more water releases and WaterNSW insisted that “someone has to pay for the water”, a decision was made to release 30GL of environmental water held in Pamamaroo.
But this success was short-lived. As the flows decreased, dangerously low oxygen levels returned. Meanwhile, huge numbers of bony bream continued to make their way upstream only to be blocked by the main weir.
This led to a huge biomass of fish in the 3km of river between the weir and the town.
According to Schulz and Williams, sometime about 14 March, the strategy of mixing blackwater from Wetherell and Parmamaroo was repeated.
Millions of fish died on 16 March.
The researchers allege there was inadequate monitoring by WaterNSW of oxygen levels in the river and the emergency response was too slow, even though it was foreseen by authorities. An emergency operations centre (EOC) was not activated at Menindee until 19 March. NSW police coordinated a multi-agency response.
Tests immediately after the fish kill show it was not caused by pesticides. Nitrogen and phosphorus were at high levels but this was also consistent with large quantities of rotting fish.
Sources said there was a real risk that the fish kills could continue in the Darling, especially if there are sharp fluctuations in temperature.
