Copper — invisibility cloak for salmon eaters

Coho salmon lose their trouble-is-a-brewin’ nose when swimming in copper-polluted waters / Flickr: USFWS Pacific

By John Upton

Salmon face a litany of badass predators during their short but meandering lives. After hatching, baby salmon must dodge hungry fish as they swim downstream and into the open ocean. Once in the ocean, they are preyed upon not only by fish and fishermen, but also by sea lions and other marine mammals. After they’ve grown for two to three years, the fish must make the perilous stream run one more time, this time against the current and often into the mouths of waiting bears, in a desperate bid to reach their spawning grounds.

The salmon can’t fight back or use poisons or barbs to defend themselves. Instead, when a predator is near, they lock down, freezing their movements in hopes that they will become invisible to the marauder.

But what happens when the marauder becomes invisible to the salmon? New research suggests that humanity’s wont to pollute has handed such an invisibility cloak to creatures that feast on salmon.

Salmon rely on their keen sense of smell to detect predators. They don’t so much smell the predators, instead they smell a chemical alarm dubbed Schreckstoff that’s released from the shredded bodies of their attacked and battered brethren. But the sense of smell is compromised when the fish swim in water polluted by copper, a common pollutant that flows into streams from mines, farms, buildings and roads.

Washington State University researcher Jen McIntyre set up an experiment to determine whether copper pollution leaves salmon more vulnerable to predators. Into tanks she deposited juvenile coho salmon and a predator species named cutthroat trout, along with Schreckstoff and small amounts of copper.

Salmon that swam in clean water froze in the tanks and managed to escape initial strikes by the trout nine times out of ten. But salmon that were swimming in copper kept blithely on swimming, and they were captured on the first strike 30 percent of the time, often within five seconds.

“They’re not in lockdown mode,” McIntyre said in a statement that coincided with publication of her results in the journal Ecological Applications. “Predators can see them more easily.”

That’s nifty for hungry fish, but not so promising for the populations of salmon that are clinging to survival in polluted waterways around the world.