Hundreds of thousands of fish have died after recent flooding in northern New South Wales caused “severe deoxygenation” of rivers, with researchers alarmed at discovering carcasses of species that traditionally tolerate lower oxygen levels.
Scientists are still researching the full of extent of the destruction to marine life along the Richmond river, where multiple fish kill events occurred following flooding in late February and early March. The flooding led to a total lack of oxygen in a 60km stretch of the waterway, between Coraki and Ballina on the northern NSW coast.
While data on the effects of the latest flood events at the end of March is not yet clear, Southern Cross University researchers who had been undertaking long-term monitoring of the Richmond river in a project with the NSW Department of Planning and Environment believe the final fish death toll will be in the hundreds of thousands.
Fish kills from deoxygenation occur when floodwaters overtop the riverbanks and spill out onto lower lying parts of the floodplain, covering grass and vegetation, which depletes all oxygen in the river as it decays and rots in the overlying water.
Prof Damien Maher, who is leading the university’s research project, which includes installing logger machines to measure oxygen levels, said no fish could survive in the 60km stretch of the Richmond river without oxygen.
He said researchers had already discovered the remains of species normally tolerant of low oxygen levels, such as carp. Most of the dead fish washed out to sea.
“We also saw a bunch of dead eels, and they normally survive these events,” Maher said.
Cassie Price, from the non-profit group OzFish, which gathers data on waterways to support recreational fishers, was also surprised when she heard reports of mulloway fish among those killed.
“The unusual thing about this fish kill is that some of the fish are really big species that can go offshore normally,” she said. “Some of these fish have been up to two metres long.
“We suspect they became trapped between bodies of water, when they’ve come in to feed off smaller fish, and they’ve not been able to get back out after waters receded.
“The oxygen was dead zero, there were no pockets of good water for them to go.”
Prof Scott Johnston, a landscape hydrogeochemist from Southern Cross University, said deoxygenation was not uncommon in lower parts of the Richmond river after large floods, and that the fish kills were just the “tip of the ecological iceberg”.
He said human modification of the lower floodplain had unintentionally made fish kill events more frequent and severe, and warned of a “second wave” of impact as floodplains began to drain.
“Large areas of the floodplain have acid sulphate soils in them at fairly shallow depth. So once the flood recedes and the surface water drains away, it starts pulling the shallow ground water from the wetlands into the river, which has high levels of sulphuric acid and metals,” Johnston said.
Metal contaminants could lead to secondary fish kills, he said.
Maher said it would take months for fish to begin repopulating and “years and years until the rivers resemble anything like how it was prior to the floods”.
Conversely, Maher said there could yet be a positive impact on the ecosystem as the severity of upheaval could lead to a “reset”.
“Over time what you have is an accumulation of sediments through erosion on the riverbed. Back in pre-European times, the whole catchment was forested, so that erosion was quite minimal,” he said. “With everything washed into the ocean, we’ve essentially kind of gone back to that state, in that we don’t have a whole heap of that fine silt sediment in the system now.”
That could lead to river beds being rockier and having clearer water once they had naturally restored, Maher said.
“If there are ways to stop the excess sediment in the upper catchment that’s been moved around through these landslides and prevent that from getting back into the river, we may actually be looking at a river system that’s partly reverted back towards its natural state.”