Normally, fish confronted with oxygen-poor water would swim elsewhere. As the floodwaters receded, these bacteria and microorganisms stripped the much-reduced river of oxygen, causing the already-large numbers of fish to suffocate. For the Darling-Baaka, that came in the form of mass floods earlier this year that led to a boom in fish populations and washed soil and decaying plant matter into the river, causing a boom in bacteria and microorganisms. That kind of over-extraction, says Grafton, creates “unhealthy” rivers that are vulnerable to extreme weather. According to a recent study by Grafton, the amount of water in the Darling-Baaka has declined rapidly, with most of the reduction caused by excessive water extraction by upstream farms that draw on the river’s water for irrigation. Often, the reason rivers lose oxygen is human interference in the surrounding ecosystems. As they lay dead and dying in the water, invasive carp feasted on their bodies. In the Darling-Baaka, most of the fish that died were bony herring, a native species. A third of freshwater fish species are threatened with extinction, according to a 2021 report by sixteen global environmental organizations. Consequently, fish die-offs are contributing to a growing biodiversity crisis. “You get the native fish dying off sooner,” says Grafton, and more resilient invasive species take their place. Suffocation often doesn’t affect all fish equally. “Ultimately, the fish death was because there wasn’t enough oxygen in the water,” says Quentin Grafton, the director of Australian National University’s Centre for Water Economics, Environment and Policy. In both Texas and the Darling-Baaka, as with most other die-offs, the fish died in a mass suffocation. In many places, the millions of carcasses were so densely packed that observers could hardly see the water, which had turned murky green from the swiftly-rotting flesh. In March, the Darling-Baaka River in Australia flashed silver with fish bodies. Texas is not the only place to see mass fish deaths recently.
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