Quirks and Quarks9:21Scientists are exposing fish to cocaine — what happened next won’t surprise you
Atlantic salmon in Sweden are getting high off cocaine. And they are getting the drug from an unlikely source: scientists.
“It does sound a little bit absurd to test how cocaine affects fish behaviour,” said Erin McCallum, a Canadian researcher and associate professor of aquatic ecology at the Swedish University of Agricultural Sciences.
But with cocaine — and other substances — detected in waterways across the globe, including in Canada, McCallum and other researchers at the university wanted to understand its impact on fish in those waterways.
The report, published in Current Biology last month, found that when juvenile Atlantic salmon in Sweden’s Lake Vättern ingested cocaine or its metabolite, benzoylecgonine, they swam farther than their sober counterparts.
“If you’re swimming further in the natural environment as a salmon, you basically have more potential opportunities to use different habitats. You could find different sources of food,” McCallum told Quirks & Quarks host Bob McDonald.
“But this could also expose you to more risk. You could potentially find yourself in an area with poor habitat or potentially expose yourself to new predators.”
Higher and farther
To conduct the study, researchers had to get the juvenile salmon high.
To do this, they used slow-release chemical implants, which exposed the salmon to either cocaine or benzoylecgonine, which is the metabolite that ends up in human waste along with any cocaine the body didn’t use.
They then tracked those salmon over eight weeks. What they found was the fish exposed to benzoylecgonine swam almost twice as far per week and dispersed just over 12 kilometres farther across the lake. The fish exposed to cocaine acted the same, though the effects were weaker and less consistent.
Mark Servos, a professor in the department of biology at the University of Waterloo, says what these researchers have been able to do is novel, and an important step outside of the lab.
“The problem is, we can measure these things in wastewater, we can measure them in surface waters, but we really don’t know what they do in wild fish,” said Servos, who was not involved in the study.
“It gets us closer to the reality that there may be potential for some very subtle, but important changes in the fish.”
Cocaine sharks, too
McCallum says the human body doesn’t actually consume all of a drug, such as cocaine, and what’s left ends up going into the toilet and down the drain. McCallum says many wastewater treatment plants can’t filter out all the illicit drugs, pharmaceuticals and their metabolites, sending them into waterways.
“Many pharmaceuticals have been measured in surface waters near wastewater treatment plants, even caffeine from the coffee that we drink every day,” said McCallum.
And it’s not just Swedish fish that are finding themselves under the influence. According to a 2025 study published in the Science of the Total Environment, these drugs are being detected in waterways worldwide, with cocaine, tramadol, and codeine appearing most frequently.
A 2024 report from Brazil found cocaine in the muscles and liver of wild sharks off the coast of Rio de Janeiro.
And in Canada, a report by Servos and other researchers at the University of Waterloo published in April found substances, including fentanyl, methadone and venlafaxine in small freshwater fish collected from rivers receiving treated urban wastewater outflow.
“There’s very little research that’s been done on these compounds, because it’s only recently that we’ve started to think of them as environmental contaminants,” Servos said.
“If we can understand some of them, we can use them as benchmarks to be able to interpret what the potential is for other types of chemicals to harm the environment.”
More research needed
While there isn’t a major risk to humans, says McCallum, as you’d need to ingest a lot of water near a wastewater treatment plant to experience any sort of effects from these compounds, there are other risks to fish.
One of the chief concerns, Servos says, is the impact these drugs could have on a fish’s ability to reproduce, which has been observed in research settings but not yet tested in nature.
Still, he says it’s not time to take action yet. While technology exists that could improve wastewater treatment filtration to remove some of these drugs, it’s expensive. He says communities must weigh the risk of these drugs, versus the cost of upgrading their facilities.
“We are early on in the research to understand the implications of these compounds.”
McCallum says the research in Sweden shows further examination is needed, not just of the drugs in waterways, but also the metabolites produced by those drugs, as that’s what had the greatest effect on the salmon they studied.
“It’s just not part of active monitoring programs and we don’t do a lot of research to understand what impacts they might have on wild animals,” she said.
“So it really kind of highlighted to me that metabolites are something that could be having an effect that we’re really not looking at right now when we’re trying to understand the impacts of pollution in the environment.”
