For 20 years, the mystery behind what was killing coho salmon in Seattle’s urban creeks seemed unsolvable. Every time it rained, fish would begin swimming in circles, floating belly up just a few hours later. Scientists looked into heavy metals, various chemical contaminants, water temperature and oxygen levels. But none of these things were the problem.

So when toxicologists at Washington State University published research in the journal Science late last year unmasking the culprit, their findings made headlines. It was a chemical no one had heard of before: 6PPD-quinone.

6PPD-quinone is a toxic by-product of N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD), an antioxidant added to car tires to prolong their life. As 6PPD leaches out, it is oxidized into 6PPD-quinone.

When environmental chemist Cassandra Johannessen read the Washington study, she thought of the samples she’d analyzed from Toronto’s Don River and Highland Creek as part of her master’s degree at Trent University. She had been looking for a tire additive called hexamethoxymethyl‑melamine (HMMM) – which is a potential fish toxin and which she did find.

“Then we thought, we already have these water samples. Let’s take a look again and see if we can find 6PPD-quinone,” says Johannessen.

That second look turned up the chemical in both rivers. In fact, in the Don, it was at more than twice the concentrations that killed the Seattle salmon and levels remained elevated for up to 14 hours after rainstorms. Johannessen and her team published their findings in a pair of studies over the summer and fall.

She was working from samples collected by the province during and after rainstorms in 2019 and 2020 and did not look for dead fish in the river at the time of the storms. Still, Johannessen says 6PPD-quinone is likely affecting the Don River and Highland Creek salmon, along with other aquatic wildlife in the area.

While coho and other Pacific salmon are not native to the rivers, they were introduced in the Great Lakes region starting early in the last century for recreational fishing. They were meant to replace the Atlantic salmon which became locally extinct in 1898. (In 2006, Atlantic salmon were reintroduced.)

Shortly after Johannessen published her research, University of Saskatchewan toxicologist Markus Brinkmann showed 6PPD-quinone in samples taken from snow facilities, snowmelt puddles, and about a dozen outfall sites along the South Saskatchewan River.

Brinkmann’s team was surprised to find 6PPD-quinone at levels comparable to what Johannessen had found in Toronto.

“Saskatoon is significantly smaller and has less traffic,” he says. “But maybe it’s because Saskatoon has long dry periods punctuated by big storms – there is time for the compound to accumulate between storms.”

Brinkmann and his team ran toxicology tests on algae and invertebrates and found 6PPD-quinone was not very toxic. They now plan to turn their attention to fish.

Both Brinkmann and Johannessen point out that because playgrounds and some sporting equipment is made from crumbled recycled tires, traffic may not be the only source of 6PPD-quinone.

Governments on both sides of the border are starting to investigate the chemical – earlier this year, the National Contaminants Advisory Group of Fisheries and Oceans Canada announced funding for researchers to study the impact of 6PPD-quinone and other tire by-products on Canadian salmonid populations. DFO wants to know exactly how toxic these chemicals are to other aquatic species.

“The major take away is that this is a new area of research. We have much to learn and we are still scratching the surface,” says Johannessen. “But international studies are beginning to show tire-derived chemicals are ubiquitous in urban water systems.”