I've seen studies where they tracked wild Norwegian salmon, and found that 90% died before spawning. As far as I understood it, this was interpreted as "natural death", but it's the same figure as the one in the article. It seems like a very plausible explanation that those deaths also largely occurred due to the same poisoning from tires.
I'd theorize it has, but nobody has been able to coordinate such a focused investigation.
It's surprising they did not check toxicity on other salmon species. Either it's specific to the coho salmon, which is maybe unlikely, or other salmon species are not stocked in streams near cities?
Fish in general have been undergoing a mass extinction event in recent decades. We normally blame it on overfishing, but effects like this could have been right under nose the entire time and simply missed because nobody was looking for it.
They have to start somewhere. I'm sure we'll see more papers in the future.
The salmon swim up Cedar River in Renton, WA which a heavily used road follows and crosses the river few times. That gives plenty of opportunity for runoff to get into the river.
BTW - the Cedar River also flows past Renton Airport which serves small aircraft which use aviation gasoline. That fuel is still allowed to contain tetra-ethyl lead. This is another environmental issue that should be addressed.
It rains a lot there, not in other places.
The paper also has data for Los Angeles, with concentrations begin similar.