Thousands of carp have died in the Menindee Main Weir in far west New South Wales, triggering a mass fish kill that ecologists worry could affect native species as water quality declines.
The NSW Department of Primary Industries (DPI) told Guardian Australia it had received reports of a fish death event in the Lower Darling-Baaka in Menindee.
“It is estimated that thousands of fish, predominantly carp and bony herring, have been affected, with a small number of Murray cod and yabbies also impacted,” a department spokesperson said.
Unlike the 2019 Menindee fish kills, in which hundreds of thousands of native fish were found dead along a stretch of the Lower Darling River in Menindee, the fish are dying not because of a lack of water but because of the sheer volume of carp due to a population boom following the floods.
The DPI said it believed the fish deaths were likely related to low dissolved oxygen levels in the water.
“This appears to be driven by increased biomass in the system as flood waters recede, with high levels of nutrients draining back into the river channel in recent weeks,” the spokesperson said.
“The current hot weather in the region is also exacerbating the risk of hypoxia, as warmer water holds less oxygen than cold water, and fish have higher oxygen needs at warmer temperatures.”
WaterNSW said it had increased monitoring at Menindee lakes in recent months to track water in the wake of the floods.
“Recent results indicate that while dissolved oxygen levels have fallen at key monitoring points within the lakes, the area of most concern at this stage is Lake Wetherell in the vicinity of the main weir,” it said.
The fish kill follows reports from Menindee residents who say they have been concerned for weeks about the “enormous amounts of carp” in the river system.
Menindee photographer Geoff Looney said he saw hundreds of dead fish at the weir, mainly carp and bony herring, with “thousands of carp” trying to get in.
“I do a lot of fishing and I’d never seen so many carp in the river,” he said.
Prof Fran Sheldon, a river ecologist who was on the independent panel on the 2019 Menindee fish kills, said the carp were struggling to breathe because they didn’t cope as well as other fish when water quality declined.
“The carp are really visual because they’re coming up and gasping for air,” she said.
“Whereas the native fish probably do that a bit less, so you don’t see them as much. They’re not probably as obvious to the people there but there would be a lot of native fish out there as well.”
Based on the ecology of the Menindee lakes, Sheldon said, there would likely be a lot of bony bream affected by the low oxygen levels, but “they won’t be at the surface like you see with the carp”.
“Now this will be like a domino effect because the more carp die, the worse the water quality will get and then more die. You’ll start to get deoxygenation because the carp start to rot and when they start to rot that sucks the oxygen out of the water.”
Sheldon said it was not unusual for fish populations to boom during floods, followed by “mass mortality of fish” as flood waters receded, but that she was concerned about the large carp population.
“It just highlights the massive numbers of carp in the Murray Darling Basin, which is an issue,” she said. “These fish really do dominate the biomass now of the system, and they never used to and that in itself is a bit of a tragedy.
“For the system, it’s a good thing that they all die out … If these fish all ended up back in the river, it’d be a complete disaster.”
The DPI said it would continue to work with Water NSW and would undertake sampling between Weir 32 and Menindee Main Weir of the fish population once flooding receded.
