Lake Cargelligo fish kill related to cold temperatures, NSW Fisheries says

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3 days ago

A mass fish kill at an inland lake in western New South Wales is likely a phenomenon called "winter stress syndrome", New South Wales Fisheries says.
The bodies of thousands of the small native species bony bream, or bony herring, washed ashore at Lake Cargelligo in early July, historically when water temperatures were at their lowest.
Residents said what began as a small number of fish escalated to a mass fish kill covering the banks of the lake near the township.
Lachlan Environmental Water Advisory Group acting chair Mal Carnegie said he had not witnessed an event of that scale before.
"We're seeing colder than normal temperatures, certainly than say the last 10 years," Mr Carnegie said.
Staff from the NSW Department of Primary Industries and Regional Development (DPIRD) travelled to the site, about 200 kilometres west of Parkes, on July 8.
The air temperatures in Lake Cargelligo at the beginning of July reached as low as 0.5 degrees Celsius, well below the long-term mean July minimum of 3.6C.
No recording of the water temperature at the time of the mass deaths exists, but Fisheries staff tested the surface water temperature at 10.5C on the day of the site visit.
The fish can typically tolerate water temperatures between 9C and 38C.
DPIRD Fisheries staff said the suspected cause of the fish kill event was climate-related.
There was a significant influx of pelicans and other waterbirds at the time, which quickly consumed the dead animals.
The species is susceptible to what is known as winter die-off, when their immune systems cannot cope below a certain water temperature.
"This is a known phenomenon," director of freshwater fisheries and threatened species Cameron Lay said.
"It's often not a dramatic change in temperature, it can just be a point where the fish's immune system gets to a point where it no longer functions effectively.
"They can very quickly be overrun by parasites or viruses or other diseases around the fish, which normally they're not susceptible to."
The species is a key food source in the Murray-Darling Basin for some larger native species, including the Murray cod and the golden perch.
Mr Lay said it was unlikely the large fish death event would have an impact on the wider system, with survey data suggesting large numbers remained throughout the basin.
"They breed in enormous numbers in good years and they can be incredibly prolific, however the flip side of that is they can die off in relatively large numbers when conditions aren't perfect."
In June, shortly before the mass fish death event, WaterNSW announced it was starting more work to improve flood-damaged and old embankments at Lake Cargelligo.
The water authority dropped the lake's water levels from approximately 57 to 53 per cent capacity, its lowest point since March 2024.
The storage will continue to be lowered to 50 per cent, where it will remain until September while works take place.
Mr Lay said it was "unlikely" that Water NSW's lowering of Lake Cargelligo triggered the fish kill.
DPIRD scientists were also able to rule out common diseases, including red spot disease, which bony bream is known to be susceptible to.
Local fish ecologist Adam Kerezsy said it still did not explain why Lake Cargelligo was so affected, when other basins such as nearby Lake Cowal were not.
"There's still an element of, yeah OK, it got cold, we know they die off in the cold, but why have we had such a severe event?" Dr Kerezsy said.
He hoped DPIRD would use it as an opportunity to gain more knowledge about the species' response to cold temperatures.
But the department said it was not a key priority, and it was focused on studying more threatened or iconic fish that experienced concerning declines over the past century.