What is a fish kill, and why are they happening more often?

ByPete McKenzie

Published July 12, 2023

• 7 min read

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. 

That kind of over-extraction, says Grafton, creates “unhealthy” rivers that are vulnerable to extreme weather. 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. As the floodwaters receded, these bacteria and microorganisms stripped the much-reduced river of oxygen, causing the already-large numbers of fish to suffocate.  

Normally, fish confronted with oxygen-poor water would swim elsewhere. But much of the Darling-Baaka has been dammed with enormous weirs, says Fran Sheldon, a hydrologist at Griffith University, meaning many of the fish were trapped. 

Richard Kingsford, a river ecologist at the University of New South Wales, says rivers under similar pressure from human interference elsewhere in the world could also see large die-offs. As an example of an ecosystem at risk, he points to the Colorado River, which for the last 23 years has been hit by severe drought caused by climate change and the over-extraction of water for farming and drinking. 

“It’s across the world,” Kingsford says. “These rivers are drying from the bottom up.” 

Nutrient overload

It matters what humans put into the water, not just what they take out. Farms and industrial sites, for example, can often leach fertilizer or waste into nearby rivers or lakes, leading to massive algae blooms that coat the water. As the algae die, they feed bacteria, which then consume oxygen that fish would otherwise use to breathe and cause them to suffocate. 

In the middle of last year, for example, canals along the Oder River in Poland turned toxic after local farms and sewage systems released waste laden with nutrients into the water. Officials believe the waste fuelled a bloom of golden algae that led to the death of thousands of fish, forcing both Poland and Germany to haul hundreds of tonnes of rotting flesh from the water. 

Higher temperatures

Another cause of fish die-offs is rising temperatures, which are often due to climate change. Warmer water holds less oxygen, explains James Renwick, a climate scientist at Victoria University of Wellington. 

Rising temperatures can kill fish in both rivers and oceans, but according to Renwick, they have a particularly bad impact on marine fish. “Fisheries are incredibly sensitive to temperature, because temperatures don’t vary much,” he says. The average global sea surface temperature has increased by roughly 1.8 degrees Fahrenheit over the last four decades. “A one degree change in surface temperature is a big deal in most parts of the global ocean,” says Renwick.

Oxygen loss due to rising temperatures has caused mass die-offs around the world. Last year, in New Zealand, marine salmon farms were struck by a rapid surge in water temperatures, forcing farm operators to dump more than a thousand tonnes of suffocated fish in nearby landfills. Similarly, in June, thousands of fish carpeted the Gulf Coast of Texas, killed by sudden ocean warming that left them without air to breathe. 

Poisoned water

The most direct cause of fish die-offs, however, is water that has been poisoned with chemicals or toxic substances. 

This is most common in countries with poor environmental safeguards. In March last year, for example, the Madagascan government granted the mining company Rio Tinto permission to release hundreds of millions of gallons of wastewater contaminated with acidic effluent and aluminum into the Mandromondromotra River. Days later, thousands of dead fish washed up on the shores of connected lakes, devastating local communities that rely on fishing to survive. 

But laws don’t always prevent disaster. In August 2019, a steel mill in northwest Indiana broke environmental laws by releasing cyanide and ammonia into the Little Calumet River, killing thousands of fish in the process. 

How can we prevent future fish die-offs?

Limiting fish die-offs will require significant changes to how humans interact with rivers and oceans, experts say. The first priority, according to Grafton and Kingsford, is to build a more sustainable relationship between human communities and the ecosystems they rely on, by policing the things humans dump into lakes and rivers, and limiting the amount of water humans pull out. 

The second priority, according to Renwick, is to accelerate the fight against climate change. “If we let warming continue, marine heatwaves will become more common and more intense, and there’ll be more of these kinds of die-offs,” he says. “We need to stop burning fossil fuels immediately, or as soon as we can, because these problems are only going to get worse.”

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