Mass fish kill chokes waterways in Byron region

Thousands of flathead, bream, whiting, and schooling prawns have died in a devastating fish kill in the aftermath of Tropical Cyclone Alfred, choking the waterways of coastal and riverside towns in northern NSW.

Residents spared from the worst effects of Alfred say the catastrophe has now arrived as the fish die gasping in blackwater starved of oxygen. Even hardier creatures such as eels and mud crabs are emerging from the deoxygenated water and dying on the shore.

Thousands of dead fish litter the canals of Ballina due to oxygen-starved blackwater.Credit: Danielle Smith

“We’ve lived here for five years, and during this time, we’ve had a flood, we’ve had a cyclone and a fish kill,” said Lucy van der Hoeven, who lives on a canal in Ballina, a coastal town at the end of the Richmond River.

“The flood was tough, but this would have to be the toughest … to see all that death and sadness. I mean, just to see these beautiful, beautiful fish die – it’s killing us.”

Ballina resident Wendy Sharpe said the scale of the devastation was heading towards the worst fish kill they’d seen. She’s been cleaning up dead fish every day for a week – including about 2000–3000 on Saturday morning.

Wendy Sharpe and Les Clarke said they’d cleaned up thousands of dead fish in one morning.Credit: Danielle Smith

“I think that picking up fish early is making a difference,” she said. “You can’t be inside and have your doors open because the air is so ripe.”

Cyclone Alfred pelted much of the Northern Rivers with relentless rainfall this month. The resulting floodwater from low-lying plains, clogged with rotting plant material, has started draining into the Richmond River and onto Ballina.

When too much plant matter enters waterways, bacteria decomposing the material consume most of the available dissolved oxygen, producing swampy “blackwater” that smells of sewage and suffocates aquatic animals en masse.

Reports of dead fish from the Tweed to Crescent Head began last Monday, said Cassie Price, chief executive of fish habitat charity OzFish.

“Healthy water should have at least five milligrams of oxygen per litre,” Price said. “Fish experience distress when it falls below four milligrams per litre and start to die at two milligrams a litre.”

Readings from last week found levels as low as 0.4 milligrams, and some monitoring had come back with oxygen levels of nil. Price said fish kills usually last three to four days, but this event was unusually drawn out.

“At the start of last week we were talking about tens of thousands of fish. Now we’re talking about hundreds of thousands.”

Price said OzFish is tracking another pulse of deadly blackwater from the Bungawalbin sub-catchment, south-west of Ballina, which is moving towards already badly hit areas. “If there’s anything left to die, it’ll happen over the next week,” she said.

Mass fish kills became common in the region after the conversion of nearby wetland into artificially drained agricultural fields, said Professor Damien Maher, a specialist in hydrobiogeochemistry from Southern Cross University in Lismore. Major fish kills have struck after floods in 2008, 2017 and 2022.

Joshua McPherson from Lismore Council helping clean up the fish kill in Ballina.Credit: Danielle Smith

Part of the problem is a vast drainage network dug across the Tuckean Swamp, which lies 25 kilometres upstream from Ballina.

The 6000-hectare area was once one of Australia’s largest wetlands and known anecdotally as the “Kakadu of the south”, Maher said.

But over the decades the wetlands were drained, and flood-tolerant plants were replaced by crops, sugarcane and pasture grass.

“As soon as those plants are underwater they start dying and breaking down and consuming oxygen,” Maher, who has studied the area’s fish kills for years, said.

The Tuckean Swamp lies 25 kilometres from Ballina and is a source of blackwater after floods.Credit: Richmond River Floodplain Prioritisation Study, UNSW

The drainage system means oxygen-depleted blackwater water rushes quickly into main waterways such as the Richmond River, sparking fish kills. Possible solutions include removing the drains and restoring the wetlands.

“The longer you can hold the water up on the floodplain and allow that to be released to the river slowly over time, that lessens the impact in terms of fish kills,” Maher said.

“But it’s a complicated issue in terms of social, political and economic pressures. We have people earning their living from farming on the floodplain. It’s not as simple as filling in the drains.”

OzFish is researching the best way to manage and restore the wetland.

More fish deaths could come later this week.Credit: Danielle Smith

“Healthy swamp bordering rivers acts as a sieve, or a filter, that reduces the blackwater entering the waterway,” Price said.

Meanwhile, locals continue to help clean up the piles of rotting fish while the stench of blackwater and decay leaches into homes via drainpipes.

Lismore Council worker Joshua McPherson headed to Ballina over the weekend to lend a hand. He said council staff and residents disposed of four tonnes of fish on Saturday morning.

“And that wasn’t all of them. There were plenty on the bank, a lot in the water.”

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