Plastic in the depths: Scientists spent decades on the trail of a mystery toxin killing salmon en masse. A recent breakthrough revealed the culprit

For decades, coho salmon returning from the Pacific Ocean to the creeks and streams of Puget Sound in Washington state to spawn were dying in large numbers. No one knew why. Scientists working to solve the mystery of the mass deaths noticed they occurred after heavy rains.

Toxicologists suspected pesticides, as the main creek they studied ran through a golf course. But no evidence of pesticides was found. They ruled out disease, lack of oxygen and chemicals such as metals and hydrocarbons.

The first real breakthrough happened when they tested actual runoff collected from a nearby road and exposed test salmon to it. The fish died within hours.

Jennifer McIntyre.

“The harder step was delving into what could be in that stormwater,” said Jenifer McIntyre, an assistant professor of aquatic toxicology at Washington State University, who has spent 15 years searching for what was killing the coho, an important species in the Pacific north-west.

It was when they tested car tyre particles – a poorly understood yet ubiquitous pollutant – that they knew they were on the right track. Using a parmesan grater atop a drill, they carefully shaved tiny fragments of tyre and soaked them in water.

“When we tested the tyres it killed all the fish,” said McIntyre. From there, they were able to identify the culprit: a toxic chemical known as 6PPD-quinone, the product of the preservative 6PPD, which is added to tyres to stop them breaking down. The pioneering study, published in 2020, has been heralded as critical to our understanding of what some describe as a “stealth pollutant”.

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The Guardian