Fish in lakes tainted with metals are losing their sense of smell, stoking concern among experts that the problem could devastate populations.
Fish use their sense of smell to find mates and food, and to avoid getting eaten. It helps them navigate their often murky world, and it is necessary for their growth and survival. But when metals contact fish nostrils, the neurons shut down to protect the brain.
Metals already have been linked to impaired reproduction and growth in fish but now they are proving to be “covert toxics,” said Keith Tierney, a University of Alberta assistant professor who did not participate in the new study. “If you can’t smell food, or avoid predators, you’re more likely to die – simple as that.”
Greg Pyle, a professor at the University of Lethbridge in Alberta, said he suspects that impaired sense of smell “has meaningful and profound effects” on many fish species. It may be jeopardizing entire populations of fish, including some endangered species.
“We’ve tested everything from leeches to water fleas to several species of fish,” Pyle said. “Every species and every metal we’ve observed has had effects at low, environmentally relevant concentrations.”
Most contaminated lakes have a metallic mix, making it hard to tease out which pollutants are to blame.
In the latest study, Pyle and his team of researchers took yellow perch that lived in Ontario lakes contaminated with mercury, nickel, copper, iron and manganese, and put them in a cleaner lake. Within 24 hours of basking in the clean water, the fish regained their sense of smell.
This shows “fish from metal contaminated lakes have the ability to recover once the lake recovers,” the authors wrote in the paper published in last month’s Ecotoxicology and Environmental Safety journal.
The researchers used wild fish from two lakes with metal contamination (Ramsey and Hannah lakes) and from a cleaner one (Geneva Lake). Ramsey and Hannah, located in Sudbury, Ontario, are polluted from more than a century of mining, particularly with nickel. Hannah Lake is one of the worst-polluted lakes in the area, while Ramsey is similar to other North American lakes near industrial areas. Geneva Lake is far enough northwest to escape most contaminants.
Just as the clean lake revived the sense of smell for the Ramsey and Hannah fish, Geneva Lake’s perch had decreased smell after just 24 hours of hanging out in the dirtier lakes. Their response times to substances that smelled like their food dropped 75 to 59 percent.
Some metals attack specific neurons in the nostrils that respond to certain smells, Pyle said. Nickel targets the neurons that help fish smell food, while copper – at low concentrations – targets the neurons that help fish avoid predators. At higher concentrations, copper impairs their smell for everything.
“Copper is a poster child for water pollution,” said Nathaniel Scholz, an ecotoxicology program manager at the National Oceanic and Atmospheric Administration's Northwest Fisheries Science Center. “Copper is intensively used as a pesticide, fungicide…It’s found in cars, in boat paint, so boatyards are often contaminated. And it’s often found in industrial discharge and near legacy mining operations. It’s a rare pollutant that’s both agricultural and urban.”