Exposure to low concentrations make salmon susceptible to predation
By Summit Voice
SUMMIT COUNTY — Researchers have long known that dissolved metals can be toxic to fish at relatively low concentrations. Some trout species, for example, can be poisoned by very low levels of zinc.
Now, some new research shows that tiny amounts of copper — from brake linings or mining operations, for example — affect salmons’ sense of smell so much that they can’t detect a compound that normally alerts them to danger.
“A copper-exposed fish is not getting the information it needs to make good decisions,” said Jenifer McIntyre, a postdoctoral research associate in Washington State University’s Puyallup Research and Extension Center.
Just trace amounts of copper can affect salmon to where they are easily eaten by predators, McIntyre said. She based her study in part on previous research showing that copper impacts a salmon’s sense of smell, which affects its behavior.
McIntyre exposed 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 her experiments, conducted in a four-foot-diameter tank, 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.
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.
“It’s very simply and obviously because predators can see them more easily,” McIntyre said. “They’re not in lockdown mode.”
The unwary exposed fish were also more likely to be killed in the attack. The salmon exposed to copper were captured 30 percent of the time on the first strike.
The salmon not exposed to copper 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.
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. According to the state Department of Ecology, brake pads are the source of up to half the copper in the state’s urban waterways.
McIntyre used concentrations of between 5 and 20 parts per billion but has sampled highway runoff with 60 times as much copper. Copper’s effect is mediated by organic matter, which can make the metal unavailable to living things.
“My scenarios are potentially more like a hard-rock copper mining situation than storm water runoff, which typically carries dissolved organic matter along with the copper and other contaminants,” McIntyre says.
Alaska’s proposed Pebble Mine, for example, would produce tens of billions of pounds of copper near Bristol Bay, the largest sockeye salmon fishery in the world.
McIntyre’s research was conducted for a University of Washington doctorate with colleagues at UW and the National Oceanic and Atmospheric Administration and appears in the latest issue of the journal Ecological Applications.