Fallout from Australia’s Huge Wildfires Is Choking Rivers
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As gruesome as the scene had been on land that January day, Pearce’s main concern then, and in the ensuing weeks, has been for a fish, the Macquarie perch. This species, which grows to about 16 inches in length and can live for 25 years, was once abundant and commercially fished throughout the Murray River basin. Today it’s endangered and only found in a handful of sites across southeast Australia, including in this 3.7-mile section of the Mannus Creek. Even before the bush fires, Pearce had been thinking about moving perch out of the creek and into captivity because of declining water quality caused by drought; the fires made that urgent. He managed to net nine perch in the days following the fires—at least some were still alive, he was glad to find—and arranged to return with some colleagues to net more.
Then, on January 19, thunderstorms dumped large amounts of rain in the Mannus Creek catchment area. It was a relief for many, putting out fires and watering parched cattle paddocks, but the downpours triggered disaster in the creek. When Pearce arrived in Bogandyera Nature Reserve with his fish-netting teams around midday on January 20, they watched in “complete and utter despair” as huge volumes of ashy sediment, like dark, runny mud, came down the stream. “My initial feeling was that everything was going to die,” said Pearce, “I just didn’t know how anything was going to survive in that thick soup.”
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Luiz Silva and Katie Doyle, freshwater fish scientists at Charles Sturt University, have now documented 14 sites where ash-related fish kills occurred across southeastern Australia in recent months.
“We have never seen that extent of fire-related fish kills anywhere in the world,” said Silva, adding that he and Doyle documented fish mortality along more than 40 miles of the upper Murray River, even in its large main channel, where pollution would normally be diluted.
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At Mannus Creek, one of the primary impacts of the large pulse of ash was a rapid decline in dissolved oxygen levels in the water. With an electronic probe, Pearce documented oxygen levels dropping from an already-low 7.5 milligrams per liter to 0.06 milligrams per liter on January 20—virtually zero. “Everything was trying to get out of the creek,” he recalled. “Freshwater shrimp, yabbies [Australian freshwater crayfish], mayfly larvae, freshwater snails—they were all crawling out.”
Pearce even saw dragonfly larvae emerge from the muck and try to molt into their flying, adult forms. They were too immature, and died while he watched. “I found dead tadpoles and Gambusia [small, non-native mosquitofish], which was surprising, because nothing kills Gambusia normally,” he said. “I even saw platypus swimming about at the surface, all stressed, in the middle of the day. You never see them in the middle of the day.”
Elsewhere in New South Wales, Silva and Doyle found large, dead fish that appeared to have asphyxiated, their gills blackened and clogged with ash. Some had red eyes and appeared to have been “cooked” in fire-heated water. Pulses of ash can cause rapid changes in pH, for example making usually acidic rivers extremely alkaline within a short time, with lethal impacts on river life, said Doyle. Other experts are estimating that hundreds of thousands of fish, at a minimum, have died in southeastern Australia during this fire season.
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When I visited Mannus Creek this week with Pearce, the creek’s water was beginning to clear up, though it was still very dirty. Although he has seen a few Macquarie perch in recent days, the deep pools that provide cool water for them to find refuge during hot spells were filled with ash and sediment, as were the gravel beds they need to lay their eggs. No breeding habitat for the species was visible. Pearce said that over time, successive floods would normally flush out the sediment. But with the fire burning so hot in the creek catchment, there was little vegetation regrowth to hold the topsoil. The next rains might dump more sediment in the stream rather than cleaning it.
As for the plans to breed threatened fish in captivity, this is easier said than done. Pearce told me that female Macquarie perch are difficult to bring into breeding condition in captive facilities. Lintermans pointed out that the stocky galaxias’ breeding biology is virtually unknown. Fisheries managers will have to try to breed it through trial and error, and they might fail.
https://www.wired.com/story/fallout-from-australias-huge-wildfires-is-choking-rivers/
