This is scientific American 62nd Science. I'm Julia Rosen. Got a minute?

Each fall, thousands of coho salmon flock to northwest rivers to spawn, but many never get the chance, especially near big cities like

Seattle.

And in some of these urban areas, up to 90% of the females were dying before they spawned,

which is not a good thing for a population.

Julian Spromberg, a toxicologist affiliated with the Northwest Fisheries Science Center.

Researchers suspected these deaths were partly a matter of bad timing.

The fish often reached streams during the first showers of the

rainy season which flush chemicals from roads and parking lots into the water.

Now Spromberg and her colleagues have produced the first direct evidence that this runoff kills

coho salmon. Their study is in the Journal of Applied Ecology. The researchers found that fish

exposed to stormwater from Seattle area highways quickly grew sick and died.

Surprisingly though, the salmon didn't seem to mind taking a dip in a cocktail of common road pollutants,

including hydrocarbons and metals.

That detail suggests the killer ingredient in runoff

may be a different kind of chemical or a lethal combination of several compounds.

There's a whole lot of stuff in here that we haven't been able to measure or don't have the

capabilities of measuring at this point.

However, Spromberg says there's a way to help the fish even before scientists hunt down the culprit.

Her team also found that filtering runoff through just a few feet of soil made stormwater safe for salmon.

Cities can implement this simple form of clean water technology by building more

systems including roadside rain gardens to collect runoff from paved areas and

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