Government inaction risks another Darling River fish kill this summer

Tonnes of native fish are pouring into the Darling River out of Lake Menindee, but are trapped unable to head upstream.

Neither Menindee main weir nor the Lake Wetherell outlets have fishways, leaving golden perch and bony bream trapped, unable to travel upstream.

The Darling River is headed for its fifth fish kill since 2018 this summer, after repeated government failures to build a fishway at Menindee that allows Murray cod, golden perch and bony bream to swim upstream.

Locals say tonnes of two and three-year-old fish are pouring into the Darling River out of Lake Menindee, which has acted as a nursery for at least a million Golden Perch, plus even more bony bream and carp since March 2021.

Australian Floodplain Association vice chair Graeme McCrabb said the fish, which were cued to swim upstream, were now trapped in the Menindee weir pool, unable to travel into Lake Wetherell, the upper Darling and its tributaries.

Neither Menindee main weir nor the Lake Wetherell outlets have fishways, leaving golden perch and bony bream trapped, unable to travel upstream.

Mr McCrabb said the risk of another major fish kill this summer was high, as the weir pool fish density rose, oxygen levels declined and summer temperatures rose.

He said the lack of a fishway was a massive blockage in the system, which fisheries scientists had been calling for since the 1990s.

But rather than taking action, successive governments have simply called for scientific reviews into each fish kill, from the Vertessy and Australian Academy of Science reports into three 2018 and 2019 events to the NSW Chief Scientist’s report into the March 2023 event that killed 20 million bony bream – all of which called for fishways.

Federal Water Minister Tanya Plibersek responded to last year’s fish kill by allocating $2.3m to fund a business case into building a fishway, with the NSW Government chipping in $6.5m.

But estimates on the cost of building a permanent fishway that allows perch and other species to swim from the Menindee weir into Lake Wetherell and up the Darling River range from $90m to $110m.

Mr McCrabb said the repeated failure of governments to take action was “a disgrace” and “business cases don’t save fish”.

A NSW Government spokesman said it “takes fish deaths very seriously” and it would be trialling a “cost-effective” short-term, temporary tube fish passage on the Lake Wetherell outlet regulator at Menindee this summer.

“This will be the first time that tube fishway technology will be trialled under Australian conditions on native inland freshwater fish.”

HOW A TUBE FISHWAY WORKS

However Mr McCrabb said a contract was yet to be awarded for the temporary tube fish passage, with no guarantees it would work, given it was developed overseas, was untested in Australia and had limited capacity.

“It’s good to be trialling that stuff, but this area is prone to fish kills and it doesn’t offer us any certainty,” he said.

The NSW Government has promised to install a yet untested temporary tube fishway on the Lake Wetherell outlet, to pump fish upstream.

The Scandinavian technology relies on attracting fish into a tube and then using valves to shut off sections and then pump them over dams.

At Menindee the tube technology would have to pump fish into Lake Wetherell from the Darling River seven metres below.

Without a permanent solution ecologists warn native fish numbers in the upper Darling River and its tributaries are at risk of extinction.

A recent Centre for Freshwater Ecosystems report found that not only was the Menindee weir pool “a significant choke point” for fish movement, it stopped repopulation of the upper Darling catchments, risking localised extinction of some native species.

“Our results show that two large bodied long-lived species, Golden perch and Murray cod, are at risk of local extinction under multiple future scenarios.”

“Given the largely impassable structures (until significant flows cause the drowning of these weirs) of the lower Darling (Baaka) system, particularly Weir 32 and the Main weir at Menindee isolating the “Nursery habitat” of the Menindee Lakes from areas upstream, there is limited ability for abundant remnant populations to colonise into areas effected by prolonged cease to flow events and waterhole turnover.”