2 Chinook Salmon

What’s Killing Killer Whales, a review of stranded orcas found that half their deaths were due to malnutrition. Studies show this is the No. 1 cause in decline of the Southern Resident killer whale population, reliant on an ever-decreasing supply of Pacific salmon.  Analyses of their poop show that Chinook is indeed “King” as it’s found in 80% of all samples. However, Coho—at certain times of year—is also an essential species, at up to 15% of their diet.

So it’s worrying that each fall, many of the Coho salmon returning to Puget Sound’s urban streams, such as those in Seattle, are dying before they spawn. These deaths are not new, or even news. What’s been causing this die-off is a decades-old mystery. With agricultural pesticides, household chemicals and pharmaceuticals ruled out as sources, stormwater run-off was long alleged, but a sample contains a dazzling array of over 2,000 toxicants, and the usual suspects did not line up.

"Most people think that we know what chemicals are toxic, and all we have to do is control the amount of those chemicals to make sure water quality is fine," said co-senior author Edward Kolodziej of University of Washington. "But, in fact, animals are exposed to this giant chemical soup and we don't know what many of the chemicals in it even are."

A group of coho salmon smolts.
Photo courtesy of user "Cacophony". Creative Commons.

A recently published paper finally puts a finger on the perpetrator. After two years of painstaking lab work with state-of-the-art technology to eliminate thousands of options, it was narrowed down to a chemical compound that could not be found on any manufacturers’ list. So next they honed in on unknown, but possible chemical by-products of known chemicals of the “ingredients” of the likely origin.

So what’s the source? Vehicle tires. In fact, a chemical used to prevent tires from cracking and degrading—ironically added as a safety measure for us—and it’s highly toxic and often fatal to Coho salmon.

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Small microscopic crumbs of tire are left on our roadways, and chemicals leaching from them find their way into stormwater drains after heavy rains, arriving unfiltered in our urban streams. Not just in downtown Seattle; the team confirmed these chemicals in other urban neighbourhoods from Washington to California.

Ultimately, this finding could result in a modification by tire manufacturers. Though the scientists don’t want an immediate change to the law to make this happen, as any substitute needs to have non-lethal impacts on our wildlife, and so would need government, scientists and industry to work together towards a safe solution.

On an individual level—in addition to driving less—we can ask our decision makers to support methods that capture and filter stormwater, such as using rain gardens. A previous study published in 2015, showed that filtering stormwater run-off through columns of soil and sand improved the water quality to such an extent that it removed the toxic chemical—now known to be 6PPD-quinone—that kills Coho.