How scientists approached the (salmon) mass murderer

The salmon was dying and no one knew why.

About 20 years ago, ambitious restoration projects brought smelted salmon back to the city streams of Seattle. But after it fell, the fish showed strange behavior: they were lined up on one side, rolled over, swimming in a circle. They die within hours – before spawning, taking with them the next generation. In some streams, up to 90 percent of smelted salmon is lost.

“The collision with these sick fish was quite astonishing,” said Jenifer McIntyre, now a toxicologist and professor at Washington State University, who is a member of a team that finally solved the mystery of dying salmon years ago around Puget Sound. “We argued intensely in those early years, what could be the reason for that?”

The team’s results were published in the journal Science on Thursday.

The investigation began with a forensic investigation. Was there a metal or some other chemical in the water? Nothing was found. Do you have a problem with the temperature? No. Maybe a lack of oxygen? The salmon seemed to be suffocating, but they had plenty of air. There was no evidence of disease or pesticide exposure. But the lack of a connection to the rain and all the other explanations made Dr. McIntyre and his team focus on the flow of roads.

In collaboration with a local fish hatchery run by the Suquamish tribe, it was decided to test the theory and expose the fish to chemicals made from chemicals known to be on the road, such as heavy metals and hydrocarbons from engine oil. But salmon had no effect, even at surprisingly high concentrations.

Scientists have decided to try the real things again, the actual course. Luckily for them, the water channel descending from the elevated road was emptied into the parking lot of the Northwest Fisheries Science Center, where some members of the team worked. On a rainy day in 2012, stainless steel tanks were filled with a clear dark liquid from the spout. This time, the salmon showed strange symptoms and died instantly.

– What’s in that mix? Dr. McIntyre recalled the thinking. “It’s just the water that’s on the way, that’s what we’re going through in our rain boots.” It must be something people don’t measure regularly, he thought.

Enter Edward P. Kolodziej, an environmental engineer and chemist at the University of Washington. His laboratory used a machine called a high-resolution mass spectrometer to compare the chemical composition of the highway runoff with water collected from two urban streams where salmon are dying. The samples split the chemicals associated with the tire particles. So the team did a test by soaking the shredded tire in water. The salmon died.

They got closer to the answer, but the tire water still contained more than 2,000 chemicals. To unravel the mystery, they had to identify the specific culprit. Dr. Kolodziej and other researchers diligently narrowed the space, separated the tire solution into various chemical combinations, and then tested them on fish. They came up with 200 chemicals on their list with a Venn-diagram type approach. Whenever a substance known in the literature to be toxic to fish was identified, individual chemicals were purchased and tested.

“We almost made a bet in the lab to see if the chemical we thought he was doing with it would kill the fish,” Dr. Kolodziej said. – And never. Not flame retardants. Not plasticizers. Not a lot of others you’ve never heard of.

“We’re stuck,” said Zhenyu Tian, ​​a researcher who does a lot of analysis.

“Depressed,” Dr. Kolodziej said.

Then a Ph.D. student, Haoqi Nina Zhao, proposed a new method for isolating chemicals that led to a primary suspect. But they couldn’t try it because they didn’t know what it was.

“It’s almost like a fingerprint,” Dr. Tian said. “But you don’t really know who it is because your fingerprint in your database doesn’t exist.”

Dr. Tian “aha!” one morning the moment came. To the conjecture that the mysterious chemical was converted from a substance originally added to a tire, he sought a compound with the same carbon and nitrogen molecules, ignoring oxygen and hydrogen, as the latter is more likely to change when a chemical is converted. The Environment Agency found a match in a report on tire rubber: an antioxidant called 6PPD.

The researchers ordered the smallest possible quantity, about a pound of purple pellets. When the material was oxidized, the resulting chemical looked the same as one that worked so hard to insulate it from the tire water. It was time to test this version, 6PPD-quinone on salmon.

“I find the destruction of the fish incredibly sad,” Dr. Kolodziej said. – You’re just watching these fish fight. And yet you are happy to understand why.

The killer was the 6PPD quinone of the tires at the outlet of the roadway.

“The analysis they did is truly amazing,” said Nancy Denslow, a professor and director of aquatic toxicology at the University of Florida who was not involved in the study. He also praised the large number of authors. “It’s amazing to see large groups come together to solve problems,” he said. “Group science is fantastic.”

Their answer raises so many questions that Dr. McIntyre, a toxicologist who watched disoriented salmon in streams 15 years ago, now has more work to do.

He has upcoming research on how road runoff will affect some other fish species (not nearly as dramatically, but there are still consequences). The team is talking to the tire industry and hopes manufacturers will be willing to look for a substitute preservative. Scientists are concerned about the wider health effects of chemicals in tires, including humans, especially because tires are often recycled to produce artificial turf for sports fields. “It seems to me that these finer particles can be inhaled,” Dr. McIntyre said. – Now this washout is on the lung tissue.

While chemicals have always surrounded us (plants themselves are chemical factories), in the last hundred years they have been made synthetically by humans. “We are synthesizing them a little faster than we are able to hold,” Dr. Kolodziej said.

“I think the vast majority of these are okay, but bad acting chemicals are swimming out there,” he said. – And a long, slow and difficult process to identify them.