Tweed Shire Council says regular testing has ruled out pesticides, sewerage overflows and algal blooms as causes for a fish kill at the downstream end of Vintage Lakes on April 1.
Council said the tests confirmed low dissolved oxygen was the cause of hundreds of dead bull mullet, which was restricted to the Port Drive and Traminer Court end of the lakes.
Roads and stormwater manager Danny Rose said the lake, which is a stormwater system was subject to regular run-off from the surrounding area, which made managing the water quality and noxious weeds difficult.
‘The vegetation that can be seen on the surface of the lake is predominantly native Red Azolla, a water fern, which is not targeted for weed removal and on its own is not responsible for the fish kill,’ Mr Rose said.
‘The reasons for deoxygenation in this part of the lake could include extended warm weather increasing water temperatures in the lake, a lack of significant rain over summer to flush the drainage system and the decomposition of vegetation in the lake following significant growth since weed harvesting in late November.
‘It is rare for these factors to coincide and a fish kill has not happened here since 2005,’ he said.
‘The best result would be a large rainfall in the catchment to flush the lake.’
All remaining dead fish were removed and taken to landfill on Wednesday and Thursday this week, and Council will continue to monitor the situation.