Copper in Waterways Makes Salmon More Vulnerable to Predators

By Chelsea Whyte on July 10, 2012 5:31 PM EDT

coho salmon
Trace amounts of copper in water can 'blind' Coho salmon to nearby predators. (Photo: Creative Commons: Soggydan)

Even the smallest amounts of copper in natural water sources - usually from vehicle brake linings, pesticides, or mining operations - can easily confuse salmon, making them more vulnerable to predators.

"A copper-exposed fish is not getting the information it needs to make good decisions," said Jennifer McIntyre, a postdoctoral research associate in WSU's Puyallup Research and Extension Center whose research is published in the journal Ecological Applications.

She found that the metal affects the fish's sense of smell, stopping them from detecting a compound that usually alerts them to be still in the presence of danger. She exposed young coho salmon to varying amounts of copper and placed them in tanks with cutthroat trout, an aptly named predator of salmon.

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Under normal circumstances, salmon react to a substance called Schreckstoff, German for 'scary stuff', which is released when a fish is physically damaged, creating a smelly alarm system for nearby fish to watch out for predators.

In her experiments, fish that weren't exposed to copper would freeze when they caught a whiff of Schreckstoff, making it harder for predators to sense their presence and giving them an average of 30 seconds before the cutthroat trout detected them.

But the fish exposed to small amounts of copper - just 5 parts per billion - couldn't catch wind of the Schreckstoff, and kept swimming. They were attacked in about five seconds.

"It's very simply and obviously because predators can see them more easily," said McIntyre, according to UPI. "They're not in lockdown mode."

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, reports Zee News. 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. 

With testimony from McIntyre's NOAA colleagues and others, the Washington State legislature in 2010 started phasing out copper brake pads and linings over the next 15 to 20 years, reports Phys.org. According to the state Department of Ecology, brake pads are the source of up to half the copper in the state's urban waterways.

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