The study, appeared in journal Ecological Applications, has implications for brake pads, copper mines, the researchers said.
"A copper-exposed fish is not getting the information it needs to make good decisions," said lead researcher Jenifer McIntyre at the Washington State University' Puyallup Research and Extension Center.
Earlier research showed that copper impacts a salmon's sense of smell. Other research showed that when a salmon's sense of smell is affected, its behavior changes.
McIntyre and her team put the two together, exposing juvenile coho salmon to varying amounts of copper and placing them in tanks with cutthroat trout, a common predator. The results were striking.
Salmon are attuned to smell a substance called Schreckstoff. German for "scary stuff," it is released when a fish is physically damaged, alerting nearby fish to the predator's presence.
In their experiments, conducted in a four-foot-diameter tank, the researchers found that fish that weren't exposed to copper would freeze in the presence of Schreckstoff, making it harder for motion- sensitive predators to detect them. On average, half a minute would go by before they were attacked.
More From This Section
But salmon in water with just five parts of copper per billion failed to detect the Schreckstoff and kept swimming. They were attacked in about five seconds, they said.
"It's very simply and obviously because predators can see them more easily. They're not in lockdown mode,"McIntyre said.
The unwary exposed fish were also more likely to be killed in the attack, being captured 30 per cent of the time on the first strike. Unexposed fish managed to escape the first strike nearly nine times out of ten, most likely because they were already wary and poised to take evasive action.
McIntyre also noticed that the behavior of predators was the same whether or not they had been exposed to copper.
Copper finds its way into streams and marine waters from a variety of sources, including motor vehicle brake linings, pesticides, building materials and protective boat coatings.
Actual amounts will vary from undetectable in rural or forested areas to elevated in urban areas, especially when runoff from a storm washes roads of accumulated brake dust and other contaminants.