Every tire on the road is dosed with a preservative called 6PPD, added to keep the rubber from drying out and cracking as it flexes...Tiny flecks of rubber rub off onto the pavement, and the 6PPD in them meets ozone in the air and changes into a brand new compound, 6PPD quinone. That transformation product, not the raw tire, is what proves deadly to a coho...It takes only a fraction of a part per billion of the quinone to be fatal to a coho, a concentration a single storm easily clears.
On Miller Creek, monitoring across three straight spring storms found about four in five of the young coho gone within a day each time.