Fish kill in Tweed River near Murwillumbah

January 23, 2020

THE weekend’s rain seems to have triggered a fish kill in the Tweed River around Murwillumbah.

Inspections by Tweed Shire Council officers on January 21, 2020 revealed dead fish on the water’s surface between Condong and the Commercial Rd boat ramp in Murwillumbah. Council’s Waterways Program leader Tom Alletson said the numbers of dead fish were estimated to be 200 to 300, with the affected species including bream, whiting and garfish.

“Preliminary water quality testing by Council has shown that dissolved oxygen levels and pH is low in the river,” Mr Alletson said. “We believe that runoff from floodplain areas affected by acid sulfate soil is the most likely cause of the fish kill in the Tweed River.

“Acid sulfate soil is a naturally occurring type of soil found extensively in low-lying areas of the Tweed River floodplain, and across vast areas of the east coast of Australia. When exposed to oxygen, the mineral pyrite, which is found in these soils, reacts to form sulphuric acid.

“After periods of prolonged drought when the ground water table is low and acid sulfate soils become exposed to the air, more acid is formed. When a large downpour flushes this acid into the river, it is common for fish kills to occur.”

Council will continue monitoring to try to isolate the floodplain drains where poor-quality water is entering the estuary. A crew has been removing dead fish since Wednesday afternoon.

“It is possible that the quality of water flowing off the flood plain could get worse in coming days,” Mr Alletson said. “How this may affect fish and other aquatic life cannot be predicted at this time, however, it is likely that there will be some dead fish within the river for days to come.”

For more information about acid sulfate soils and the effects on aquatic life, visit dpi.nsw.gov.au/fishing/habitat/threats/acid-sulfate-soils