A major clean-up effort is under way in Australia after millions of fish died in a river in western New South Wales (NSW). Fish will be cleared from “high density areas”, but it will not be possible to remove all the carcasses, police say. Police reassured local residents that the water supply remained “high quality”. The deaths are thought to have been caused by low oxygen levels in the river after a recent heatwave. An emergency hub has been set up in the town of Menindee in western NSW to coordinate the response and monitor water quality. Describing the operation as “very challenging and significant”, NSW Police Commander Brett Greentree said the event was “unprecedented in terms of the millions of fish which have died.” “Our purpose is making sure Menindee has clean water supply... I’m comfortable we’re in a good spot regards to water quality at the moment,” he told reporters. Commander Greentree said contractors with specialized skills would use “a netting procedure” to remove the fish. “But I need to be very upfront with the community and say ‘will every fish be removed?’ I don’t think so, from the information I’ve had,” he added. Posting a Facebook video showing rotting fish lining the riverbank, local resident Graeme McCrabb wrote “the worst is still coming”. Temperatures in the area reached 40C (104F) at the weekend. Meanwhile, volunteers from conservation charity OzFish have begun a search and rescue operation to retrieve as many surviving native fish from the river as possible. Describing the smell as “putrid”, Braeden Lampard told the Australian Broadcasting Corporation his team were transferring rescued fish to a holding tank. He estimated 85% of the dead fish were native species such as Bony Bream and Golden Perch, with the rest consisting of non-native types such as carp. Cassie Price, OzFish’s director of programs, said most of the floating remains would sink to the river bed within 48 hours. “It would be pretty unlikely to get most of [fish] biomass out of the river. It will sink down, which will cause a bit of a nutrient spike, which is not good for the water quality either,” she told the BBC. “It’s likely to cause algal blooms, which will cause more issues for a while,” she added. The deaths were caused by hypoxic blackwater, a naturally occurring phenomenon that causes extremely low oxygen levels, police said. State government agencies said they were releasing higher quality water to boost dissolved oxygen levels, and would work with federal agencies to find the underlying cause. The latest event follows another mass fish die-off in the same area that occurred in similar conditions in 2018. The Darling-Baaka River forms part of the Murray-Darling Basin, which is Australia’s largest river system. Its ecosystem has faced pressure from drought and increased human use in recent years, while last year brought severe flooding after intense rainfall and storms. Authorities said the latest die-off had been exacerbated by “significantly increased numbers of fish in the system” as floodwaters receded. — BBC