The salmon were dying and no one knew why.
About 20 years ago, ambitious restoration projects brought coho salmon back to urban streams in the Seattle area. But after the rain, the fish displayed strange behaviors: sideways, turn around, swim in circles. Within hours, they would die – before spawning, taking the next generation with them. In some streams, up to 90 percent of coho salmon have been lost.
“It was quite astonishing to come across these diseased fish,” said Jenifer McIntyre, now a toxicologist and professor at Washington State University who is part of a team that years later finally solved the mystery. dying salmon around Puget Sound. “During those early years we debated intensely, what could be the cause?”
The team’s results were published Thursday in the journal Science.
The investigation began with a forensic examination. Was it a metal or some other chemical in the water? Nothing they couldn’t find. A problem with the temperature? No. Maybe a lack of oxygen? The salmon looked like it was suffocating, but it had a lot to breathe. There was no evidence of illness or exposure to pesticides. But the connection to the rain and the lack of any other explanation led Dr McIntyre and his team to focus on the runoff from the roads.
Partnering with a local fish hatchery run by the Suquamish tribe, they decided to put the theory to the test, exposing the fish to a mixture they created of chemicals they knew were in the runoff from the roads. , such as heavy metals and hydrocarbons in motor oil. But the salmon was not affected, even at surprisingly high concentrations.
The scientists decided to try again with the real stuff, the real runoff. Luckily for them, a downpipe from an elevated road emptied in the parking lot of the Northwest Fisheries Science Center, where some of the crew worked. One rainy day in 2012, they filled stainless steel containers with a translucent dark liquid coming out of the spout. This time the salmon exhibited the bizarre symptoms and died quickly.
“What’s in this mix?” Dr McIntyre remembered thinking. “It’s just water on the road, that’s what we go through with our rain boots.” This must be something that people don’t measure regularly, she told herself.
Enter Edward P. Kolodziej, environmental engineer and chemist at the University of Washington. His lab used a machine called a high-resolution mass spectrometer to compare the chemical composition of freeway runoff to that of water collected from two urban streams where salmon were dying. The samples shared chemicals related to the tire particles. So the team prepared a test concoction by dipping the shredded tire tread in water. The salmon is dead.
They were getting closer to the answer, but the water in the tires still contained over 2,000 chemicals. To solve the mystery, they had to identify the specific culprit. Dr Kolodziej and other researchers painstakingly reduced the field, separating the tire solution into different chemical combinations, and then testing them on fish. With a Venn diagram-type approach, they narrowed their list down to 200 chemicals. Whenever they identified one that was known in the literature to be toxic to fish, they bought it and tried that individual chemical.
“We would almost take bets in the lab on whether the chemical we thought we were making would kill the fish,” said Dr Kolodziej. “And it never did. Not flame retardants. Not plasticizers. Not a bunch of others that you’ve never heard of.
“We were stuck,” said Zhenyu Tian, a research scientist who has conducted numerous analyzes.
“Depressed,” said Dr. Kolodziej.
Then a doctorate. The student, Haoqi Nina Zhao, suggested a new way to separate the chemicals that led to a prime suspect. But they couldn’t test it because they didn’t know what it was.
“It’s almost like you have a fingerprint,” Dr. Tian said. “But you really don’t know who it is, because in your database that fingerprint doesn’t exist.”
The “aha!” From Dr. Tian the time came one morning. Guessing that the mysterious chemical had transformed from a substance initially added to the tire, he looked for a compound whose carbon and nitrogen molecules matched, ignoring oxygen and hydrogen, as the latter are more likely to change when a chemical changes. In an Environmental Protection Agency report on tire rubber, he found a match: an antioxidant called 6PPD.
The researchers ordered the smallest amount possible, about a pound of purple lozenges. When they oxidized the substance, the resulting chemical looked like the one they had worked so hard to isolate water from the tires. It was time to test this version, 6PPD-quinone, on salmon.
“I find it incredibly sad to see fish dying,” said Dr Kolodziej. “You just watch these fish struggle. And yet, you are happy to understand why.
The killer was the 6PPD-quinone from the tires in the pavement runoff.
“The analysis they did is really amazing,” said Nancy Denslow, professor and director of aquatic toxicology at the University of Florida who was not affiliated with the study. She also praised the large number of authors. “It’s wonderful to see large groups of people coming together to solve problems,” she said. “Group science is fantastic.”
Their answer, however, raises so many questions that Dr. McIntyre, the toxicologist who observed disoriented salmon in streams 15 years ago, now has even more work to do.
She has research to come into how road runoff affects certain other fish species (not as dramatically, but there are still consequences). The team is in conversation with the tire industry and hopes manufacturers will be ready to look for a replacement preservative. Scientists are concerned about the wider effects of chemicals in tires on health, including on humans, especially because tires are often recycled to make artificial turf for athletic fields. “It seems to me that there could be an inhalation of these finer particles,” Dr. McIntyre said. “Now you’ve got this leaching out of lung tissue.”
While chemicals have always surrounded us (plants themselves are chemical factories), over the past hundred years humans have made them synthetically. “We synthesized them a little faster than we can keep up with,” said Dr Kolodziej.
“I think the vast majority of these are good, but there are bad chemicals from actors floating around there,” he said. “And it’s a long, slow and difficult process to identify them.”