Residents from the New South Wales central west town of Lake Cargelligo first started to notice fish dying in their local lake two weeks ago.
Since then, officials have estimated thousands of bony herring, a native Australian fish, have perished. They suspect a drop in water temperature at the lake has triggered what is known as “winter die off”.
Dr Adam Kerezsy, an aquatic ecologist who lives in the area, agreed that bony herring can often die in colder weather – however the scale of this event was noticeable.
“We [fish biologists] are starting to think it may be a combination of the cold weather and a reasonably fast draw-down of the level of Lake Cargelligo to 50% of capacity,” Kerezsy said.
He described the bony herring as “important for fresh water ecosystems because they provide a food source for large fish such as yellowbelly and cod and fish-eating birds such as pelicans, cormorants, terns and gulls”.
Kerezsy said the lake had been “pretty much full” until April, and then the water levels dropped quickly.
He said it had been the earliest onset of winter he has experienced in Lake Cargelligo since 2018, with cold weather coming on fast in the last week of May.
An investigation by the NSW Department of Primary Industries and Regional Development (DPIRD) may help aquatic ecologists such as Kerezsy determine if other factors also influence fish kills.
A spokesperson said officials visited the lake on 8 July and “estimated that thousands of bony herring have been affected”.
“Bony herring can become stressed or die when water temperatures drop. This is known as ‘winter die off’ and DPIRD fisheries staff have confirmed that this is the suspected cause of this fish kill event,” the spokesperson said.
A spokesperson for the Lachlan Shire council said drinking water to the town would not be affected as the town was currently accessing its water supply from a bore – “and not the Lachlan River, which is the lake”.
The town does take water from the bore and the lake which allows for flexibility, especially during a drought, they said. “However at the moment the bore is being used.”
The NSW Department of Climate Change, Energy, the Environment and Water said they had not received any request for assistance from the council.
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“Water from Lake Cargelligo is fed to a water treatment plant where it undergoes extensive processes to ensure it is safe to drink,” a department spokesperson said.
Local Richard Famorcan, 45, said that he and a group of friends visited the lake on Sunday to spot birds and saw thousands of dead fish.
Famorcan said in his two years living in the area he had never seen a mass fish kill event.
He posted a video on Facebook showing scores of dead fish lining the shore.
“Me and some of my friends decided to go the lake to just watch the group of pelicans and other birds,” said Famorcan, who sometimes fishes at the lake.
Kerezsy said the die-off is proving very popular with local birds, with thousands capitalising on the dead fish.
But there could be longer term consequences of the fish kill, he warned.
“It will be interesting to see if this large die off results in a depleted food resource later on, for example in spring, and whether it contributes to poor water quality.”
