Science The mass die-offs, which can occur naturally, are becoming more common around the world—and scientists say humans are to blame. Here’s why, and how we can help prevent them.
Dead fish float among reeds along the shore of Kleiner Jasmunder Bodden on the island of Rügen, Germany in September 2022. Fish die-offs like these are often caused by low oxygen levels.
PHOTOGRAPH BY DANIEL MÜLLER, AGENTUR FOCUS/REDUX
In June, the beaches of southeast Texas were carpeted in dead fish. As the decomposing bodies lay along the shore, waves of predators picked through them. Soon, according to the Texas Parks and Wildlife Department’s Kills and Spills Team, all that was left were “shredded skeletons”.
The devastation was one of several recent mass deaths to cause concern among scientists and environmentalists about the health of fish in rivers and oceans around the world.
Fish die-offs can happen naturally, following extreme weather like droughts or natural blooms of algae. But experts say that by disrupting the ecosystems that typically keep such die-offs in check, humans are making the die-offs worse, disproportionately affecting native fish, destroying habitats, and poisoning water. Here’s what you need to know.
What are fish die-offs?
Texas is not the only place to see mass fish deaths recently. In March, the Darling-Baaka River in Australia flashed silver with fish bodies. 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 both Texas and the Darling-Baaka, as with most other die-offs, the fish died in a mass suffocation. “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.
Suffocation often doesn’t affect all fish equally. “You get the native fish dying off sooner,” says Grafton, and more resilient invasive species take their place. Consequently, fish die-offs are contributing to a growing biodiversity crisis. A third of freshwater fish species are threatened with extinction, according to a 2021 report by sixteen global environmental organizations.
In the Darling-Baaka, most of the fish that died were bony herring, a native species. As they lay dead and dying in the water, invasive carp feasted on their bodies.
Disrupting the flow of water
Often, the reason rivers lose oxygen is human interference in the surrounding ecosystems. 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…
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