Traces of cocaine detected in rivers and lakes appear to accumulate in the brains of salmon, affecting their behaviour and raising concerns about the impact on fish populations, according to a new scientific study.
Young Atlantic salmon that were experimentally exposed to the substance and its main breakdown product showed increased mobility, covering greater distances and spreading across wider areas of a lake. The finding suggests that such pollutants can influence where fish move, their feeding habits, and their vulnerability to predators.
Although it is not entirely clear how pollutants from wastewater treatment plants affect aquatic ecosystems, scientists warn that fish may burn more energy or face greater exposure to danger. The need for more food could push them toward greater exposure to predators.
“To a large extent, we don’t know the consequences, but I expect there will be trade-offs,” says Dr. Jack Brand from the Swedish University of Agricultural Sciences. “They might end up in worse condition or have to compensate by seeking out much more food, which means they’ll spend more time exposed.”
A Growing Environmental Risk
Scientists have previously warned that pharmaceutical pollution represents a significant and growing threat to biodiversity. They have also called on pharmaceutical companies to develop greener drugs that break down more easily in the environment.
Concerns are heightened by earlier reports of trout “addicted” to methamphetamine and perch that lost their fear of predators due to antidepressants. In 2019, tests on freshwater shrimp in Suffolk revealed traces of dozens of drugs, including cocaine, methamphetamine, antidepressants and antipsychotics, though no clear conclusions were drawn about their effects.
The Lake Vättern Study
To investigate the impact of cocaine pollution, Brand and his research team implanted Atlantic salmon with slow-release devices delivering realistic levels of the substance or its metabolite, benzoylecgonine. A third group received implants without any drug and served as the control group. All fish were fitted with acoustic transmitters.
The salmon were released into the southwestern corner of Lake Vättern — Sweden’s second largest lake, also home to large pike. Researchers tracked their movements for two months using sensors placed around the lake.
Over time, all the fish became less active; however, those exposed to cocaine or its metabolite remained more active toward the end of the study. In the final two weeks, cocaine-exposed salmon swam 5 kilometres more than the control group, while those exposed to the metabolite covered nearly 14 kilometres extra.
Remarkably, the metabolite had the greater effect — fish exposed to it moved 12 kilometres further north than the others. “It was really the metabolite, which we know occurs at higher concentrations in nature, that had the much more profound effect on fish behaviour and movement,” Brand noted.
He added that if risk assessments do not include compounds such as metabolites and derivatives, a significant portion of the environmental risk to animals may be going undetected.
The Need for Better Wastewater Management
Professor Leon Barron, head of the emerging chemical contaminants group at Imperial College London, stressed that it is crucial to determine whether similar effects are seen in fish exposed to pollutants naturally in the wild. These effects, he noted, must also be compared with those of other common chemicals detected in aquatic organisms.
“Better management of wastewater, and specifically reducing raw sewage discharges, could help reduce risks to wildlife and their ecosystems,” he said.
Although existing wastewater treatment effectively removes many illegal drugs such as cocaine and benzoylecgonine, one of the main sources of pollution remains untreated sewage — which can enter waterways through sewer overflows during storms or from incorrectly connected household pipes.
