Fish in lakes tainted with metals are losing their sense of smell, stoking concern among experts that the problem could devastate populations.
But if the fish can just get into cleaner water – even if they’ve been exposed to pollutants their whole life – they start sniffing things properly again, according to new research out of Canada.
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.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
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.
Similar results have been reported with minnows and perch, with metals apparently reducing their ability to escape predators.
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.”
Copper use has more than doubled in the United States over the past three decades, according to a 2012 report from the Copper Development Association.
Copper and other metal contaminants are a factor in the poor survival of the West Coast’s coho salmon, which are endangered or threatened in most of the region, Scholz said.
Young coho salmon exposed to low levels of copper did not evade predators – cutthroat trout – nearly as well as unexposed salmon, according to a lab study by Scholz and colleagues.
This is concerning, Scholz said, because they are listed as endangered or threatened throughout most of the Northwest United States.
The problem is “likely to be widespread in many freshwater aquatic habitats,” according to a NOAA report. The report said that increases in salmon response time to smells came within 10 minutes of exposure in some cases.
Some pesticides also affect fish smell, including atrazine and chlorpyrifos, according to research by Oregon State University and Canadian scientists, respectively.
Adding to the concern, Tierney found that zebrafish hung out where the herbicides entered their water, instead of avoiding it. The fish seemed to think that there was more food where the chemicals were because of excessive nutrients and bacteria.
Pyle said one way to mitigate the problem is cleaning up contamination near spawning sites, as embryos are sensitive to the metals. Pyle said hatching in clean water, even if the fish ends up in dirty water, bolsters the chance it will maintain its sense of smell.
“You and I can communicate and learn about our environment from seeing and listening,” Pyle said. “But when you’re living in water, you get a lot better info from molecules dissolved in your immediate surroundings. It’s crucial for them.”
This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.