In Sweden, Atlantic salmon are being exposed to cocaine by researchers, resulting in the fish exhibiting unusual behavior. Erin McCallum, an aquatic ecology associate professor at the Swedish University of Agricultural Sciences, explained that while it may seem unconventional to study the effects of cocaine on fish, the presence of cocaine and other substances in water bodies globally prompted the research team to investigate its impact on fish populations.
A recent study published in Current Biology revealed that juvenile Atlantic salmon exposed to cocaine or its metabolite, benzoylecgonine, in Sweden’s Lake Vättern showed increased swimming activity compared to sober fish. McCallum emphasized that enhanced swimming abilities could provide fish with more foraging opportunities, but it also exposes them to potential risks such as encountering unfamiliar habitats or predators.
To conduct the study, researchers utilized slow-release chemical implants to administer cocaine or benzoylecgonine to the salmon and monitored their movements over an eight-week period. The results showed that fish exposed to benzoylecgonine swam nearly twice the distance per week and dispersed over 12 kilometers farther across the lake. Fish exposed to cocaine exhibited similar but weaker effects.
Mark Servos, a biology professor at the University of Waterloo, commended the research for its innovative approach outside the laboratory setting. He emphasized the importance of understanding the subtle yet significant impacts of drugs in wild fish populations, as these substances can enter waterways through wastewater systems.
McCallum highlighted that residual drugs, including cocaine, enter water bodies through human waste and wastewater treatment plant inefficiencies. She noted that pharmaceuticals and their metabolites, such as caffeine, are commonly found in surface waters globally. Notably, a study from 2025 reported the detection of drugs like cocaine, tramadol, and codeine in waterways worldwide.
Additionally, a 2024 study in Brazil discovered cocaine residues in wild sharks off the coast of Rio de Janeiro. In Canada, researchers found opioids and other drugs, including fentanyl and methadone, in small freshwater fish collected downstream of urban wastewater treatment plants.
While the risk to humans is minimal, concerns remain about the potential reproductive impacts on fish due to drug exposure. Servos emphasized the need for further research to assess the long-term effects on fish populations and the environment. Although technology exists to improve wastewater treatment to remove drug residues, the cost implications must be weighed against the benefits.
McCallum stressed the importance of investigating not only the presence of drugs in water bodies but also the metabolites they produce, as these compounds had the most significant effects on the studied salmon. She emphasized the need for increased research on the impacts of pollution on wildlife to better understand and mitigate potential ecological consequences.