Carcasses of around 100 Murray cod found in one of NSW's biggest dams

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The carcasses of up to 100 Murray cod are lining the banks or floating on the surface of Blowering Dam on the Southern Slopes of New South Wales.

Key points:

  • Up to 100 Murray cod, believed to be male, have died at Blowering Dam in southern NSW
  • Fisheries officers are conducting post-mortem exams and water-quality tests to determine the cause
  • One theory is a recent cold snap caused deoxygenated water to rollover the fish as they guarded eggs

Fisheries NSW officers are investigating the cause, but local anglers are asking if it is similar to the “Armageddon” of fish kills seen on the Lower Darling River last summer.

“Some local fishos were up there a week or so ago and noticed a few during a night session, so they posted a video on YouTube which I spotted,” Riverina angler Rhys Creed said.

“The boys said they could smell them before they could see them.”

Mr Creed said he spoke to the Department of Primary Industries about what he saw in the video but then wanted to see for himself.

“The DPI said the kills were in two specific bays on the western side of the dam, so I was thinking it was just angler mishandling, but when I went up and did a bit of investigating, I found fish throughout the dam on both sides,” he said.

“They reckon there was up to 50 or so fish, but after I did a scout around in areas they didn’t look, I believe it could be closer to 100, which isn’t good at all.

“These are big fish that take a long time to grow to that size — they’re trophy fish that people travel a long way for.”

Cause of deaths unknown

Mr Creed said the fish he saw measured between 80 and 110 centimetres in length, which he believed indicated they were male.

“Female Murray cod are usually between 1.2 and 1.3 metres but nothing that big is dead.”

A DPI spokesman said Fisheries NSW officers had collected samples from the dead fish, which also included golden perch and carp, and taken water-quality readings.

“The cause of the fish deaths is unknown at this stage,” the spokesman said in a statement.

“Localised fish kills in impoundments after rainfall events are not uncommon.

“DPI Fisheries does not expect further mortalities in Blowering Dam, however fish communities right across NSW are under stress due to ongoing drought conditions.

“Drought, high temperatures and drying watercourses are expected to place fish populations under severe pressure this summer.”

Possible blackwater event

But this has nothing to do with drought or rainfall, according to Mr Creed.

“I’m no scientist, and I know we’re still waiting for DPI to come back with the actual results, but from my point of view, the cause is what’s called rollover,” he said.

“Rollover usually happens coming into winter, and it’s where the top water cools and swaps with the bottom water, which hasn’t got any oxygen, which means deoxygenated water sits in the shallows.

“Usually fish will move deeper, but we’ve just had breeding season for Murray cod and it’s the males that protect the eggs, which are laid in the shallows.

“So instead of moving deeper, these males have stayed on their nests, like good, protective parents, but have died as a result.”

The reason it happened now rather than at the start of winter, Mr Creed believes, is because of the region’s recent erratic weather.

“Three weeks ago, we had a week where temperatures were between 35 and 40 degrees, and then we had a massive cold snap,” he said.

“It went from 40 degrees to 17, with overnight temperatures of nearly zero at Blowering.

“And that’s what’s done it.”

Although devastated, Mr Creed said he was relieved it was only a small-scale event that wouldn’t stop anglers from doing what they love.

“I don’t think it’ll slow up the population, because there’s a lot of big fish in there — thousands, even tens of thousands.

“They’re elusive, though, so there could be more than we believe or there could be less, but I still think it’ll be a top fishing spot.”