Menindee mass fish deaths symptomatic of degradation of river ecosystem, NSW chief scientist says | Murray-Darling Basin

The mass fish deaths at Menindee in March this year are “symptomatic of degradation of the broader river ecosystem over many years”, the New South Wales chief scientist has concluded.

Prof Hugh Durrant-Whyte’s report released on Thursday concludes that 20-30m fish died in March after floods in the region. This eclipsed the other major fish kill in January 2019, which occurred after months of drought and a wave of high temperatures.

Durrant-Whyte has warned that without “substantive change to our regulatory approach … there will be further environmental degradation and recurrence of such events”.

“Difficult decisions will need to be made. These are essentially social and not scientific in nature,” he said.

A NSW Fisheries survey in May found no mature native Murray cod in about 300km of the lower Darling-Baaka. While most of the fish that died in the March fish kills were bony herring, which breed rapidly in flood conditions, the impact on native species was devastating.

The chief scientist’s report, which investigated the causes of the March fish kills, points to changes to flows in the river, the lack of fish passage ladders, which would have allowed fish to move upstream into the lakes and “altered water use in the Northern Basin” as likely key factors in decreasing water quality and the decline of native species.

This is a reference to irrigators and the rules around when they can extract water from rivers, as well as the increasing practice of harvesting rainfall and flood events under what is known as flood plain harvesting.

Over the last three decades there has been a transition from grazing to irrigated crops, which involves flattening the landscape using lasers and then building channels and levies to trap any slowly moving water during rain events.

This is estimated to have reduced flows in the Darling-Baaka by as much as 30% over the last 20 years.

The immediate cause of the March catastrophe was hypoxia resulting from low dissolved oxygen in the water column.

“Low dissolved oxygen in the water column was driven by a confluence of factors, including high biomass (particularly carp and algae), poor water quality, reduced inflows and high temperature,” Durrant-Whyte said.

“Explicit environmental protections in existing water management legislation are neither enforced nor reflected in current policy and operations. Water policy and operations focus largely on water volume, not water quality.

“This failure in policy implementation is the root cause of the decline in the river ecosystem and the consequent fish deaths.”

Durrant-Whyte found that authorities were aware in March of the potential for fish deaths because of the deteriorating water quality, but failed to take action.

He said agencies responsible for the river – the water branch of Primary Industries, NSW Fisheries, WaterNSW, and environmental agencies – were uncoordinated in their response.

“Trusted voices within specific communities and Aboriginal groups, were not engaged. Local and Traditional knowledge and experience was rarely used by agencies to inform management actions,” he said.

He said that further mass fish deaths were likely.

Durrant-Whyte called for an urgent overhaul of the regulatory framework to include legally enforceable obligations and powers to give effect to environmental protections and whole of catchment ecosystem health.

The Nature Conservation Council of NSW (NCC) echoed the call.

“The Baaka River has lost its heartbeat. Now there’s only extreme dry and extreme flood – as a result we are seeing the river in its death throes,” Mel Gray, the NCC water campaigner, said.

“Since the summer of 2019, the community has been continually rocked by waves of fish kills, both in times of drought and flood.”

The NSW Irrigators’ Council welcomed the acknowledgment that complementary measures, such as fish ladders, were needed to address the issues of fish deaths in our waterways.

But it warned: “More water recovery from farmers will not address what are now the primary environmental degradation drivers, such as invasive species like carp, habitat deterioration, obstructions to fish migration, absence of fish screens on pumps, and contamination from cold water.”