Skip to content

What is 6PPD and why is it killing our coho?

Researchers are raising alarm after dozens of coho salmon killed by chemicals

The coho couldn’t have seen it coming.

As West Vancouver Secondary students and volunteers from West Vancouver Streamkeeper Society surveyed the banks of Brothers Creek below Inglewood Avenue in late October, they found more than 40 carcasses of otherwise healthy looking coho salmon – far too many for so early in the spawning season.

It was a tragic turn after what had started to be very promising results in their annual attempt to keep tabs on the local salmon population.

John Barker, past president and volunteer with the stewardship group, walked the creek banks examining the dead coho. Three of the females he inspected still had full egg sacks, indicating they hadn’t had a chance to spawn.

“So, this is just devastating,” he said. “In 20 years with the Streamkeepers, I’ve never seen an event like this.”

What killed the fish?

Barker said the society’s contacts within Fisheries and Oceans Canada told them it was likely a case of urban runoff mortality syndrome, and the culprit was most likely 6PPD-quinone.

The compound derives from an additive sprayed onto vehicle tires to extend their life and prevent them from cracking. But as the substance sluffs off and accumulates on the roadway, it is exposed to ground level ozone and starts to break down into much more toxic components – ones that aren’t captured by our current regulatory processes. When it rains, the 6PPD-quinone is washed into the stormwater system and, eventually, fish-bearing creeks.

Researchers in Washington State first identified the link between urban runoff and coho lethality in the 1990s, and since then, the body of evidence has grown significantly.

6PPD-quinone is considered the second-most toxic chemical to aquatic species ever identified. As little as one microgram (which is one millionth of a gram) of 6PPD per liter of water is enough to kill coho. A single tire, ubiquitous as they are, has enough to provide one billion lethal doses.

Soon after they are exposed, the coho show signs of being disoriented, losing their balance and swimming in circles.

“And there’s no recovery from it,” Barker said. “It looks like it’s a fairly sudden death.”

Curiously though, other species of salmon are less susceptible. Pink and chum salmon don’t appear to be affected at all, at least not in the short term.

Exactly how the chemical kills the coho and why it isn’t as dangerous for other species is one of the many things DFO scientists are studying at the Pacific Science Enterprise Centre lab on Marine Drive in West Vancouver.

They’re also trying to learn how much of it is showing up in the creeks and when.

Tanya Brown and a team of researchers and volunteers have been collecting water samples from about 70 creeks around coastal B.C. anytime there is a forecast for five or more millimetres of rain following a 48-hour dry period. When the stormwater begins to flow, they collect another sample to compare, and then one after the rain subsides to provide further data. The samples are then frozen and taken to a lab in Sidney for analysis.

The samples collected during the October rains coming down when the fish were killed in Brothers Creek won’t be analyzed for another few weeks, but Brown said they already know very well the risks that 6PPD poses to coho and that it does spike in local streams when it rains.

“What our data is revealing is that, on average, during a rain event around sites around Metro Vancouver, we’re exceeding expected lethal concentrations for coho salmon,” she said. “We’re seeing up to a 90-fold increase in 6PPD concentrations.”

The Canadian government is funding the research as part of a larger project looking into environmental factors that impact the endangered Southern Resident killer whales. That includes the salmonids they prey on to survive.

But why now? Bad timing. Worse climate.

6PPD isn’t a new chemical on tires and its risks to coho have been documented since the 1990s. What has changed though is our climate.

It used to be that late-summer or early-fall rains would flush the chemicals out of the creek system before the adult coho returned. Now, with drought conditions lasting right into October, the toxic 6PPD-quinon and the coho are arriving in the creeks at the same time, Barker said.

“Everything is dry as can be and then we get one of these massive storms that comes in and just overloads us with water,” Barker said. “The loss is tragic and frightening to think of the future.”

Barker, Brown and others concerned about fish are turning their minds to how similar tragedies might be prevented in the future.

Brown said her hope is that her research can help identify “hotspots” where significant amounts of 6PPD are entering fish-bearing creeks, which would allow us to engineer proper mitigation measures.

A UBC study from 2023 found rain gardens or other biofiltration systems that include rocks, soil and plants can successfully collect and filter out about 95 per cent of the 6PPD found in stormwater, along with countless other nasty contaminants that turn up in runoff.

“There are 30,000, high-volume chemicals on Canada’s marketplace, but this is one flagship chemical among many other road runoff-associated chemicals,” Brown said.

While it still isn’t conclusively known whether the 6PPD in Brothers Creek is coming from the Highway 1 or other local streets, it isn’t too soon to start working on those types of plans now, Brown said.

“I don’t think we need to wait for the research here to know what the mechanism of action is or to understand it. We know that this chemical is causing lethality to coho salmon at the concentrations that we’re detecting in our salmon-bearing streams,” she said. “Municipalities are reaching out and they’re concerned and they want to help and take action towards mitigating the effects of this and other chemicals.”

Barker said he will most certainly be raising the possibility with District of West Vancouver council.

“It sounds like a fairly hefty undertaking but at the expense of fish, this is well worth exploring,” he said.

At the source

Capturing and attenuating the toxic chemicals in stormwater may only be part of the strategy though. There is a growing movement afoot to target urban runoff mortality syndrome before the rubber hits the road.

In August, the Yurok, Port Gamble S’Klallam, and the Puyallup Tribes in Washington State petitioned the U.S. Environmental Protection Agency to “establish regulations prohibiting the manufacturing, processing, use, and distribution” of the chemical under the Toxic Substances Control Act “in order to eliminate the unreasonable risk” it presents to the environment. The petition had the support of the Washington, Oregon, Vermont, Rhode Island, and Connecticut state governments.

On Nov. 2, the EPA wrote back to the tribes, stating that “while the agency cannot commit to a specific rulemaking timeframe or outcome,” it would begin a process to better regulate 6PPD.

Although the risks of 6PPD are known very well to fisheries scientists and ecologists, it’s not a topic of household discussion. The sooner that changes, the more likely it is we’ll see more action in Canada, Brown said.

“The more that we know about it, I think that there’s going to be more pressure on our regulators as well as on the tire companies to do their part to help mitigate the risks,” Brown said.

While it’s too late for the dozens of fish killed by the runoff in October, Barker said there is some good news. The students and volunteers were back out on the creek on Nov. 5 and found more fish arriving in greater numbers and preparing to spawn, apparently unaffected by the late-October plume of 6PPD. It means Brothers Creek still has the potential to be home for a thriving population of coho and other species, if only we can keep our chemicals out of their gills.

“This thing has done its damage and been washed away. It’s gone and it’s over. Anything arriving after that pulse is going to survive,” Barker said. “We’ve got coho coming through in good numbers and pink and chum salmon in fairly good numbers.”

[email protected]