A decades old mystery as to what is killing coho salmon returning to streams in the cities of the Pacific Northwest appears to have been solved, and motor vehicles, or more specifically their tires, have been fingered as the guilty party.

Scientists in Seattle began quantifying major salmon kills in that city more that a decade ago. Subsequent investigations led to a peer-reviewed study published in 2015 in Applied Ecology linking the deaths to runoff from roads and parking lots.

That study noted that reports of major kills of fish or other aquatic life in urban streams were rare, but “a notable exception is the recurring die‐off of adult coho salmon that return from the ocean to spawn each year in large metropolitan areas of northern California, western Oregon and Washington in the USA, and southern British Columbia in Canada.”

Coho migrations into spawning streams are usually triggered by heavy fall rains. The study noted that when the rains started in urban areas, the salmon showed up on schedule and almost immediately began dying.

“Typically within days of arriving at stream reaches suitable for spawning, affected fish become stricken with symptoms that progress from a loss of orientation (surface swimming) to a loss of equilibrium and death on a time‐scale of a few hours,” reported a team of researchers led by Nathanial Scholz of the Northwest Fisheries Science Center. “Year‐to‐year mortality rates within and across urban watersheds are typically high (approximately 50 to 90 percent), as measured by the proportion of unspawned females for an entire annual run.”

The scientists were able to tie the deaths to runoff from urban streets and parking lots, but given the runoff wasn’t killing other fish or stream life, the question remained as to what exactly the killer element murdering coho.

Killer chemistry

The latest study – published Thursday in the Journal Science – fingers what scientists termed “a globally ubiquitous tire rubber antioxidant”, N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine).

Commonly called 6PPD, the chemical is added to rubber in tires to make them last longer. That’s good for motorists but, as it turns out, bad for coho salmon, though it took years for scientists to figure this out.

They had to test a couple of thousand chemicals before they found the one deadly to coho. Why only these salmon remains unclear. It could be the chemical is also toxic to other fish or even other animals and has gone unnoticed as a dangerous pollutant for decades, the researchers noted, or it could be other factors.

The search for the deadly compound was made complicated because it isn’t exactly 6PPD itself doing the killing, but 6PPD-quinone, a compound created when 6PPD reacts with the ozone common in urban environments.

Ozone (03) is a naturally occurring chemical in the earth’s atmosphere, but it builds up in cities when the nitric oxide (2NO) emitted from internal combustion engines is exposed to sunlight. 2NO is an unstable molecule easily split apart by the sun, freeing the oxygen atom that bonds with the natural oxygen in the air (O2) to form O3 – ozone.

“Since 2016, all key measures for particulate matter 2.5 (PM2.5) and ozone pollution have been on the rise (in Seattle). Prior to the 2015 to 2017 monitoring period, Seattle had met federal attainment levels for the allowable number of ‘unhealthy’ PM2.5 and ozone days,” according to the website of the interest group IQAir. “Driving Seattle’s declining air quality is a combination of population growth, new industry and construction, the increased frequency and severity of wildfires, and EPA rollbacks on a variety of emission sources.”

The particulate matter of concern to coho is the tiny bits of rubber constantly wearing off tires and the even tinier bits of 6PPD in that rubber. The reaction of the latter with ozone is what has proven deadly for the fish.

Zhenyu Tian, a research scientist at the Center for Urban Waters at the University fo Washington Tacoma, told Science Daily that unraveling this tangle of chemical interactions was not easy.

A fish mystery

“There were periods last year when we thought we might not be able to get this identified,” he said. “We knew that the chemical that we thought was toxic had 18 carbons, 22 hydrogens, two nitrogens and two oxygens. And we kept trying to figure out what it was.

“Then one day in December, it was just like bing! in my mind. The killer chemical might not be a chemical directly added to the tire, but something related.”

Thus began a search for something similar to the killer in question and that, in turn, led to 6PPD which was found to break down when released into the environment and there reform into multiple new compounds including 6PPD-quinone.

The researchers later identified 6PPD-quinone in creeks in San Fransisco and Los Angeles, as well as Seattle, and expect the deadly chemical is found in greater or lesser quantities in creeks near busy roads all along the Pacific coast.

It is unclear whether the chemical has been found in water in Alaska streams. Top officials of the Alaska Department of Fish and Game could not be reached for comment on Friday.

But there have over the years been no reports of major salmon dieoffs in streams in Anchorage, the state’s largest and traffic-busiest city. Campbell Creek in the heart of the city is a major coho stream, but most of the runoff that enters it is in the lower reaches.

The creeks heads above the city in Chugach State Park and flows in the creek are generally already significant when they reach the urban area. And Anchorage has never had the ozone levels of Seattle.

“Ozone is more readily formed on warm, sunny days when the air is stagnant,” according to the Environmental Protection Agency. “Conversely, ozone production is more limited when it is cloudy, cool, rainy, or windy.”

Alaska is warming, but Anchorage isn’t exactly famous for its warm, sunny days. The peak of the coho return to Campbell Creek usually comes in mid-August. The average high temperature for mid-August in the city is 65 degrees.

Coho return to Seattle streams about a month later than in Anchorage, but the average high in the Pacific Northwest city at that time remain in the low 70s and temperatures of 80 are not uncommon. 

The temperature there hit 93 on Sept. 11, 2013. Early studies of why coho were dying actually looked at elevated stream temperatures as a cause and then dismissed that idea.

“A forensic investigation spanning nearly a decade ruled out several…potential causes, including conventional water quality parameters (e.g. dissolved oxygen, temperature), habitat availability, poor spawner condition and disease,” the 2015 study that identified runoff as the problem said. “Moreover, an initial geospatial land cover analysis found a significant positive association between the severity of the coho die‐off phenomenon and the extent of impervious surface within a watershed.”

Correlation is not causation, but in this case, further testing showed that to be a case. The big problem then came in tracking down the substance in the runoff that was killing the fish.

The cure

The solution to the problem was offered in the 2015 study.

Reserchers experimented with various water control options and “found that inexpensive filtration of urban runoff through simple columns of sand and soil, a process called bioretention, can completely prevent the toxic effects on fish,” the U.S. Fish and Wildlife Service reported at the time.

“The filtration columns, which are similar to rain gardens gaining ground in the Northwest, are an example of emerging green stormwater infrastructure that should be integrated into future development and redevelopment in order to reverse the trend and help coho recover,” wrote USFWS toxicologist Jay Davis.

There is a difference, however, between that which can be done and that which will get done. The researchers involved with the lastest researchers have dismissed filtration as impractical, citing the extent of pavement with runoff in cities like Seattle.

They’ve suggested “salmon safe” tires might be the way to go.

“Tires need these preservative chemicals to make them last,” Edward Kolodziej, an associate professor i the UW Tacoma Division of Sciences & Mathematics told Science Daily. “It’s just a question of which chemicals are a good fit for that and then carefully evaluating their safety for humans, aquatic organisms, etc. We’re not sure what alternative chemical we would recommend, but we do know that chemists are really smart and have many tools in their toolboxes to figure out a safer chemical alternative.”

Whether that happens only time will tell. Market realities are sure to enter the picture. The salmon kills have so far been limited to the Pacific Northwest, although there are now millions of the cohoes in the Great Lakes where they were first introduced in the 1960s.

Tires companies will likely prove reluctant to change tread compounds unless an economical alternative to 6PPD is found. The compound has long been in use to prevent tires from decaying with age.

According to Eastman, one if its manufacturers, 6PPD:

• “Provides powerful antiozonant and antioxidant properties with excellent high temperature, fatigue and flex resistance to rubber compounds.
• “It gives efficient stabilization for a wide range of solution and emulsion polymers.
• “(It) is a more active antioxidant than quinoline or diphenylamine based antioxidants.
• “(And it) gives better long term fatigue resistance and ozone protection than IPPD. Due to its specific molecular structure and higher rubber solubility, it is less affected by environmental variables, such as heat or leaching, leading to greater durability.”

It is indeed ubiquiotis, too. Along with being added to tire compounds, it is used in belts, hoses, cables, automotive mounts, bushings and other rubber products subject to decay from high temperatures or sunlight.