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Byron Shire
November 29, 2022

Fish kills in Tweed Estuary following rain

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Council has been removing fish affected by the fish kill from the river. Photo supplied.

A fish kill appears to have been triggered in the Tweed River Estuary around Murwillumbah following the weekend’s rain.

Inspections by Council officers Tuesday morning revealed dead fish on the water’s surface between Condong and the Commercial Road boat ramp in Murwillumbah. It has been estimated that between 2-3000 dead fish have died including bream, whiting and gar.

‘Preliminary water quality testing by Council has shown that dissolved oxygen levels and pH is low in the river,’ Tweed Shire Council’s waterways program leader Tom Alletson said.

We believe that runoff from floodplain areas affected by acid sulfate soil is the most likely cause of the fish kill. 

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, he said.

‘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.

‘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 the DPI (Department of Primary Industries) page on acid sulfate soils. 

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  1. I run in the cane fields and live on the river. All the cane drains were low or completely dry just prior to the rain. They were being dug out whilst dry- therefore exposing the soil to the air. Is there not some way that the drains between the cane that lead into the river cannot be closed off until there is sufficient water in the cane fields system so that the leaching does not occur in the river during rain periods? It looks like the river between Qarry Rd and Condong Bridge has been chlorinated. It is a weird milky green colour. I thought council had implemented a policy not to dig out the drains any more?

  2. It is more likely a Neonicotinoid Insecticide run off from the cane fields, though I doubt that the water will be tested for that. It will come out in some years that the same Insecticide is killing our local lorikeets as well as the world’s bees. Most Australian Beekeepers are aware of this.

  3. I would agree that acid sulfate coming from the flood plains is mostly from cane farms and cane drains and these need to be managed. Add to that the toxic pesticides. However in the upper fresh water catchment a vast amount of floating weeds probably azola and salvinia has been growing on the mostly nitrogen rich pollution particularly cow and pig manure. Large volumes of these floating weeds would have been flushed into the estuary along with unknown volumes of fresh water algae. All of this vegetable matter breaking down in the salt water estuary is known to cause reduced oxygen levels in the water causing fish kills. Some people are working hard to control riverbank weeds fencing off cattle from the banks and planting riparian trees. We have suggested the reduction of large populations of cattle and pigs near the weir pool and above the dam. The water weeds do a good job in using up the pollution but need to be scooped out of the water before they wash into the estuary.


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