Fish kill raises stink

Hundreds of dead fish have surfaced at the Clarrie Hall Dam near Uki, raising concerns about the quality of Tweed Shire's water supply.

More than 500 dead fish have risen to the surface of the Clarrie Hall Dam near Uki. Picture: Jasper Gudgeon

HUNDREDS of dead fish have surfaced at the Clarrie Hall Dam near Uki, raising concerns about the quality of Tweed Shire's water supply.

Uki local Jasper Gudgeon was on a bushwalk with a friend on Thursday afternoon when they noticed the smell of dead fish and rotten eggs coming from the dam.

"We noticed the stench and looked up and there were hundreds of fish all dead there," he said.

"We were both pretty shocked as my friend has lived near there his whole life and has never come across anything like this before.

"It wasn't just half a dozen, they were everywhere all around us.

"It was pretty concerning."

Mr Gudgeon posted his find on social media, prompting concerns about the quality of the drinking water coming from the dam.

"Isn't that what we drink?" one user asked.

"Can't be good having rotting carcasses in the drinking water," another wrote. "Never seen this in 18 years of being on the dam," said another.

But the Tweed Shire Council has played down concerns, dismissing claims the dead fish could impact water quality.

Tweed Shire Council water manager Michael Wraight said the fish kill was a "natural process that happens annually around the beginning of winter".

"The Clarrie Hall Dam experienced a seasonal overturn in the last two weeks," he said.

"The overturn results in deeper de-oxygenated waters being upwelled toward the water surface.

"It is likely that these low dissolved-oxygen levels has resulted in the death of the fish identified."

He said staff had analysed the dam over the past two weeks and found "very low oxygen levels in all levels of the dam".

Mr Wraight said council staff inspected the dam on Friday morning where "about 500 Australian bass were found dead".

He said said the fish would be removed and disposed of appropriately and a more detailed inspection of the dam would be completed throughout the day.

At least 300,000 Australian bass fingerlings have been released in the dam since 1991, with 20,000 counted in 2009.

Water expert Scott Bendell, who has worked on a number of dams in the region, said the stench of rotten eggs at the dam meant the fish kill was likely caused by rotting vegetation.

He said acid sulphate would have rotted away vegetation at the bottom of the dam and then produced methane gas, which would then rise to the surface.

"The fish in that vicinity, their tails get eaten off by the acid sulphate, their gills get eaten away and they suffocate because there's no oxygen in the water," he said.

But Mr Bendell said acid sulphate was in all waterways and there was "nothing to worry about".

"It happens in pretty much all waterways that I know of and it's mostly dams where this occurs," he said.