Copper leaching into a salmon's habitat makes the fish more vulnerable to predators, according to new research from Washington State University.

WSU scientist Jenifer McIntyre found that the metal, which can find its way from mines, pesticides, building materials and brake linings into the salmon's home via storm water runoff, messes with the salmon's sense of smell, preventing it from sniffing a warning chemical released by other fish.

A copper-exposed fish is not getting the information it needs to make good decisions, McIntyre said in a statement on Tuesday. Her research was just published in the latest issue of the journal Ecological Applications.

An injured salmon releases a chemical in the water called schreckstoff -- German for scary stuff -- that alerts other fish that a predator is close by. (Other animals, including minnows and aphids, emit alarm chemicals too. Even some plants will secrete an alarm pheromone to warn other plants if they're damaged; this will trigger the production of defensive chemicals that will make the plants less tasty or even poisonous to discourage whatever's looking to munch on them.)

McIntyre experimented by exposing young coho salmon to different amounts of copper and then putting them in tanks with one of the salmon's predators, the cutthroat trout. The fish that weren't dosed with copper and could smell the shreckstoff would go motionless when they smelled the alarm chemical, and evaded attack by the trout for an average of half a minute.

But a miniscule amount of copper - just five parts per billion - was enough to mask the scheckstoff for other fish, which kept swimming and would get attacked by the trout within an average of five seconds.

It's very simply and obviously because predators can see them more easily, says McIntyre. They're not in lockdown mode.

The finding raises concerns about how copper mining projects like the proposed Pebble Mine in southwest Alaska, near some of the largest runs of salmon in the world and the world's largest sockeye salmon fishery.

McIntyre's experiments used copper concentrations somewhere between 5 and 20 parts per billion, but she says she's sampled runoff water with 60 times more copper in it than the amount she uses in her experiments.

Anywhere that we have human development, developments with lots of impervious surfaces, lots of pavement and asphalt, and roofs that water can't seep through, then we're talking about the possibility of toxic storm water runoff, McIntyre said.

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