A rescue operation is under way to save the freshwater crayfish affected by poor water quality following significant flooding along the Murray River.
Murray crayfish and other aquatic animals are in danger of suffocating as dissolved oxygen levels plummet in the southern Riverina region of New South Wales.
While some rivers along flood-affected communities begin to peak, Murray crayfish have been sighted in their hundreds climbing up trees and escaping the water.
The NSW Department of Primary Industries (DPI) Fisheries launched a rescue and relocation operation to protect the Murray crayfish, which is a nationally listed threatened species, in response to the growing number of reports.
The director of freshwater environment at DPI Fisheries, Cameron Lay, said levels of dissolved oxygen across the Murray-Darling Basin are declining after record amounts of rainfall, particularly in the last month.
“It’s the highest-level recorded rainfall in the basin in contemporary history,” he said.
“In times of significant flooding, we often get a phenomenon called hypoxic blackwater, where you get a rapid breakdown of organic matter that’s been covered with water, and that basically sucks all the oxygen out of the water.”
Lay said one of the first signs of hypoxic blackwater occurring is often the crayfish will “literally walk out of the river” because there’s not enough oxygen in the water for them to breathe.
“Unfortunately, it’s usually the sign of more challenging circumstances to come for the native fish that can’t breathe air and they obviously can’t walk out of the river,” he said.
“We’re quite concerned about conditions over the coming days and weeks because we have seen quite significant fish kills in the past associated with hypoxic blackwater, which is a consequence of flooding.”
On the Murray River, Lay is leading a team of fisheries officers who are rescuing crayfish as they leave the water and transporting them to the Narrandera Fisheries Centre.
He said Murray crayfish is one of the few species they have management options for during these circumstances.
In the 24 hours to Thursday last week, Lay’s team had rescued 158 Murray crayfish and he said they can look after at least 8,000 crayfish at the facility.
“It might not sound like a huge amount of animals but for a threatened species which is confined to relatively small sections of the Murray River, that’s quite an ecologically significant number.”
Lay said the Little Murray River at Swan Hill, which is an anabranch of the Murray River, is discharging very low-oxygen water.
“[It’s] close to zero, in terms of what we will call dissolved oxygen levels, and that’s why it’s causing significant issues for fish.”
A Moulamein grower, Jeremy Morton, was in Swan Hill on Wednesday and tweeted a photo of a crayfish crawling out of the river.
Murray Crayfish everywhere along the bank in Swan Hill pic.twitter.com/K468ZdMeQH
— Jeremy Morton 🌾🐑🚜🐂 (@Jeremy_Morton_) October 25, 2022
Morton, who is also the chairman of the National Irrigators Council, said it often takes a few days for dead fish to start floating to the surface but he’s already seen a couple of photos.
“They ended up with the mouth wide open and their gills quite extended because they’re obviously just trying to breathe, and eventually suffocate. It must be a horrible way to die.
“So we won’t know the impact until well after the event but at some point we were going to see dead floating fish in the rivers.”
According to the Murray–Darling Basin Authority (MDBA), prevention is not possible and mitigation options are very limited, given the scale of natural flooding.
The senior director of Environmental Management at MDBA, Dr Janet Pritchard, said they were worried about warming temperatures as summer approaches.
“All microorganisms go crazy and start using up all the oxygen in the water, and that’s where we start to have problems with crayfish crawling out or fish dying.”
Pritchard said even though there are regulations in place to protect our water security, Australia has “one of the most highly variable climates around the world”.
However, as climate change continues to drive more extreme and extensive natural disasters like floods, she said we need to be better prepared.
“That’s definitely something I don’t think we can manage out of, especially not events this size,” she said.
Pritchard said the Murray crayfish are particularly vulnerable to predation when they leave the water.
Lay said it’s quite distressing having to manage some of the short-term impacts of “stressed and dying fish” but there is hope.
“Floods do bring a wonderful opportunity for recovery of fish species and for river health in general.”
The deputy director general at DPI Fisheries, Sean Sloan, said all NSW laws relating to taking Murray crayfish continue to apply.
“Fishers and community members are encouraged to report any fish deaths or observations through the Fishers Watch phone line on 1800 043 536.”
