The Mystery of the Dead Coho Salmon

For decades, Coho salmon were turning up dead in urban streams the Pacific Northwest. The salmon would stop swimming straight, and then die before they had a chance to spawn. Researchers worried that unless they figured out the cause, the species would eventually go extinct. Enter a formidable crew of biologists, modelers, community scientists, environmental chemists. After eventually ruling out the obvious suspects — things like temperature, oxygen levels and known toxins — researchers eventually zeroed in on a prime suspect: chemicals in tires. But the question remained: Which one? If you liked this episode, check out our other episodes on satellites monitoring emissions https://www.npr.org/2023/06/14/1182170193/this-satellite-could-help-clean-up-the-air and how air pollution could create superbugs https://www.npr.org/2023/09/06/1198013055/what-to-know-about-the-link-between-air-pollution-and-superbugs. Want to hear more environmental stories or science mysteries? Tell us by emailing [email protected] mailto:[email protected]! Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ atplus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

• 0:00Speaker 00

  This message comes from Mint Mobile. Ditch overpriced wireless and get premium wireless for less. Go to mintmobile.com/switch to see which plan works for you.

• 0:12Speaker 01

  You're listening to Short Wave from NPR.

• 0:17Speaker 02

  Hey, Short Wavers. Camila Domonoski here in the host chair with a mystery, a decades-long puzzle centering on a string of inexplicable deaths. The victims were koho salmon, showing up dead in urban streams and Puget Sound around Seattle in very suspicious circumstances.

• 0:39Speaker 03

  They show weird symptoms, like they are drunk. So they cannot swim in a straight line. They're like swimming in circles. And usually when they show that symptom, they will be dead in like a few hours.

• 0:51Speaker 02

  That's Jen Yutien, whom you might call one of the lead detectives on this case. He was a postdoc research scientist at the Center for Urban Waters at the University of Washington Tacoma when he was called in to the case. These fish spawn in freshwater, strike off for the open seas, trek back to the place of their birth, but then they die before they can spawn. And this keeps happening for years.

• 1:17Speaker 03

  If this trend continues, they might be like extinct in a few decades. So that's the problem we want to solve.

• 1:25Speaker 02

  And "we" was a lot of people: biologists, modelers, community scientists, environmental chemists. Some researchers rule out all the obvious culprits: temperature, oxygen, known toxins. It's none of those. They need a lead.

• 1:41Speaker 03

  So I think the first big step is our collaborators, NOAA and Washington State University.

• 1:47Speaker 02

  They looked through the data about where fish were dying and started running models.

• 1:52Speaker 03

  They are getting this important clue that the mortality risk of Coho salmon is related to traffic.

• 2:01Speaker 02

  Traffic? It's something to do with the roads, the urban runoff. So after years of work, the list of suspects is narrowing. But what part of the runoff? Enter another team of scientists from the University of Washington, including Zhenyu. They're testing chemicals in runoff.

• 2:20Speaker 03

  For example, things from the concrete, things from the antifreeze, like tire particle leachate, and compare the chemistry of those things versus the water that kills Coho salmon. Based on that comparison, we see that, OK, tire particle leachate is very similar to that.

• 2:39Speaker 02

  Smoking gun. It's coming from tire particles. But tires have tons of chemicals in them, and they need to find which one is the culprit. So today on the show, how a team of researchers finally cracked the case of the Koho salmon and how their discovery is having ripple effects. I'm Camila Domonoski, and you're listening to Short Wave, the science podcast from NPR.

• 3:16Speaker 04

  Stay tuned for the following announcements and messages from our sponsors. This message comes from Total Wine & More. Cool fall nights can make you want to entertain friends outside by a fire. Stop by Total Wine & More and pick up a cranberry-flavored or a crisp apple wine, even a pumpkin spice cream that's perfect in coffee, always at the lowest prices. Find your favorite fall flavors at the lowest prices at Total Wine & More. Spirits are not sold in Virginia and North Carolina. Drink responsibly. B21.

• 3:48Speaker 05

  Hey, it's Ray from Car Talk. Are you tired of all the depth and thoughtful care that goes into NPR shows? Want some good old-fashioned goofing around and stumbling to figure out what's going on? Well, I've been taking occasional car questions again. You can hear them by signing up for NPR Plus, along with lots of other bonus content. Just go to plus.npr.org.

• 4:11Speaker 06

  Throughline from NPR, Finding Catharsis Amid the Trauma of World War II.

• 4:17Speaker 07

  I crept into an unbelievable world where everything was natural and benign and possible.

• 4:23Speaker 06

  Finnish artist Tove Janssen and her children's classic, The Moomins. Listen to NPR's Throughline in the NPR app or wherever you get your podcasts.

• 4:34Speaker 02

  So knowing that tires have tons of chemicals in them that would need to be narrowed down, Zheng Yu and his team step in and start testing batches.

• 4:42Speaker 03

  You can think this as cutting a loaf of bread, right? Like you cut them into thin slices. You have something abnormal in the bread, but you don't know where it is. So by cutting into slices, you can test each slice individually so that you can narrow down the scope from like hundreds to tens to one. That's the logic.

• 5:01Speaker 02

  And one by one, they narrowed down the options until they found the deadly chemical. But that chemical, whatever was killing the salmon, it wasn't a tire additive. It didn't match anything put into tires. They still had no idea what it was. So Zheng Yu and his team were stuck. They'd zeroed in on this chemical killing the salmon. They knew it came from the tires somehow, but it just didn't match any tire additive. Zheng Yu says, yeah, that's the scientific process for you.

• 5:37Speaker 03

  You are trying different possibilities, trying different assumptions, and the assumption for additive just didn't work.

• 5:45Speaker 02

  OK, new assumption. If this wasn't a chemical added to tires, maybe it's a chemical that's produced when a tire additive reacts with something. Chemists call that a transformation product. But how do you find that? They were kind of stuck for months until Zheng Yu had an idea. Now, at this point, it was 2020. The pandemic had started.

• 6:11Speaker 03

  We are not allowed to work a long time in the lab. So I was like running a little bit more and taking a shower. And that's kind of like where, yeah, got that. Had the idea in the shower? Yeah, I probably should start that morning run again, but I don't have time for now.

• 6:27Speaker 02

  Zhenyu's boss, if you're listening, give this dude some more time to take runs. But anyway, this was his eureka moment during his post-run shower, the location of so many great ideas. He realized how to find the additive.

• 6:43Speaker 03

  Typically for transformation, you will have oxygen and hydrogen changed. But the carbon structure will be most likely stay and also the nitrogen may be not changed.

• 6:55Speaker 02

  They knew their mysterious murderous chemical, the product after a transformation, was made of very specific amounts of carbon, nitrogen, hydrogen, and oxygen. So he searches for a tire additive with the same carbon and nitrogen makeup as their murderous chemical.

• 7:16Speaker 02

  And they found it, a tire additive called 6PPD that helps tires last longer because it's an antioxidant. It reacts with ozone to make 6PPD quino. That has more oxygen bonds and it kills the Coho. This breakthrough got a lot of attention because, well, everyone knew, yeah, probably not great that all these tire particles are full of all these chemicals.

• 7:42Speaker 03

  Those things can be bad, but they are generally in a, you know, the mode of action is like chronic, right? Like a long-term kind of attack. But this thing is like killing a big fish in hours. So that is why it's kind of like getting more attention and like changing the way people think.

• 8:01Speaker 02

  Changing the way people like Nick Malden think. He's with a company called Emissions Analytics in the UK. They used to study tailpipe emissions. These days, they do a lot of work on tire emissions. All the particles that go into the air and waterways as your tires wear away.

• 8:19Speaker 06

  People have only really started looking at this since 2020. And they did that for a specific reason because of a link to the death of the Coho salmon. That was what made people start looking. Now that people have started looking. We're realising problem all along has been much bigger than expected.

• 8:38Speaker 02

  Now, to be clear, some researchers into air and water quality have always been worried about tyre emissions. But the general public, regulators, the auto industry were much more focused on tailpipes. And regulations to cut tailpipe emissions have, in fact, been super effective. They've come down a lot. So much that Nick's company, as well as peer-reviewed research, have found that tyre emissions are now, on some metrics, a bigger problem than tailpipe emissions.

• 9:08Speaker 06

  To put it in context, the typical emissions from a vehicle, round numbers are about 100 milligrams of rubber material per kilometre driven, roughly speaking. And the maximum limit you're allowed on particles out of an exhaust pipe, tailpipe, is five milligrams.

• 9:31Speaker 02

  Five versus 100. And to add a wrinkle, this realisation is coming right as electric vehicles are becoming more popular. And they're heavier. Heavier vehicles mean more tyre emissions as you drive. One researcher told me, just picture using a rubber eraser. The harder you press, the more you wear it down.

• 9:52Speaker 06

  So you've got two things going on simultaneously. We're realising how big the problem was originally, and the evolution of technology is making that existing problem worse.

• 10:02Speaker 02

  Now, he is not saying that EVs are worse than gas cars overall. Tailpipes produce different kinds of pollution. We're just talking about one kind here, particulates. And the biggest benefit of EVs is that they reduce carbon emissions to reduce climate change. That benefit is clear, even if you factor in making batteries and charging vehicles. And when it comes to tyre emissions specifically, it's complicated. EVs have pros and cons. While EVs are heavier, which makes these emissions worse, they also have regenerative braking.

• 10:34Speaker 06

  So that's the way you use the motor as a dynamo to slow the vehicle down instead of the friction brakes. That has the potential, if you use that a lot. If you drive really conservatively and use that regenerative braking, you can actually get tire emissions lower than an old internal combustion engine.

• 10:53Speaker 02

  So like I said, complicated. But suffice to say, tire wear pollution is a problem for all road vehicles, including EVs. So what's the solution? There are lots of ways to help, like start with the tires. We can change the chemicals in them to be less toxic. We can get fewer particles in the first place, tweak the way roads are designed, or encourage people to drive less aggressively. That makes a huge difference, actually. Or, of course, drive less. Or after we've made particles, street sweeping and filtering runoff can help animals like the salmon. And then there are ideas, like the one they're working on at the Korea Institute of Machinery and Materials.

• 11:37Speaker 02

  I reached out to ask about some testing that they've done. And Seokwan Lee told me they're also working on collection systems to suck up and store road dust and tire particles a car would otherwise send into the air. He said it's a two-part system.

• 11:51Speaker 06

  The first part is ingesting system. So the second part is the collector.

• 11:59Speaker 02

  Is it like taping a vacuum cleaner on the back?

• 12:02Speaker 06

  Yeah, it's similar. The big principle is same for the vacuum cleaner. Yeah, that's right.

• 12:07Speaker 02

  We'll see where that goes. In the meantime, the world makes billions of tires every year. Producing them, using them, and disposing of them are all sources of pollution. And as tailpipes get cleaner and electric vehicles get more popular, this is kind of the new frontier for reducing emissions on roads, cutting down on pollution that doesn't come from tailpipes.

• 12:37Speaker 02

  Before we head out, if you liked this episode, make sure to follow us if you haven't already. That way, you never miss a new episode. And if you liked this episode, check out our episodes on satellites monitoring emissions and how air pollution could create superbugs. We'll link to them in our show notes. This episode was produced by Jessica Young and edited by our showrunner Rebecca Ramirez. It was fact-checked by Tyler Jones, and Kwasi Lee was the audio engineer. Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Camila Domonoski. Thanks for listening to Short Wave from NPR.

• 13:26Speaker 00

  Stay tuned for the following announcements and messages from our sponsors.