In Sweden, Atlantic salmon are being exposed to cocaine by researchers, leading to unexpected behavior changes. Erin McCallum, an aquatic ecology associate professor at the Swedish University of Agricultural Sciences, explained the rationale behind this unconventional study. With the presence of cocaine and other substances in global waterways, including Canada, researchers aimed to investigate how these substances affect fish in their habitats.
A recent report published in Current Biology revealed that juvenile Atlantic salmon in Sweden’s Lake Vättern, when exposed to cocaine or its metabolite benzoylecgonine, exhibited increased swimming distances compared to sober counterparts. McCallum highlighted that swimming farther provides salmon with more opportunities to explore different habitats and food sources, but also exposes them to potential risks like encountering poor habitats or new predators.
To conduct the study, researchers utilized slow-release chemical implants to administer cocaine or benzoylecgonine to the salmon, tracking their movements over eight weeks. The findings showed that fish exposed to benzoylecgonine swam nearly twice the distance per week and dispersed over 12 kilometers farther across the lake. Although fish exposed to cocaine displayed similar behavior, the effects were slightly weaker and less consistent.
Mark Servos, a biology professor at the University of Waterloo, praised the study as a novel approach outside traditional laboratory settings. He emphasized the importance of understanding the subtle yet significant changes these substances may induce in wild fish populations. McCallum highlighted that residual drugs, including cocaine, enter waterways through human waste, as treatment plants often cannot completely filter out these substances.
Notably, a 2025 study reported the presence of drugs like cocaine, tramadol, and codeine in water bodies worldwide. In Brazil, a 2024 study discovered cocaine in the muscles and liver of wild sharks off the coast of Rio de Janeiro. Similarly, in Canada, research found various substances, including fentanyl and methadone, in small freshwater fish near urban wastewater outflows.
While the risks to humans are minimal, concerns remain about the impact of these drugs on fish reproduction. Servos stressed the need for further research to comprehend the full implications of these environmental contaminants. McCallum underscored the necessity of investigating drug metabolites’ effects on wildlife, as they may have significant yet overlooked consequences on ecosystems.
The study in Sweden advocates for more comprehensive research on drug impacts in water bodies and their metabolites, especially given their potential effects on wildlife. The findings suggest that understanding the effects of pollution on the environment requires a closer examination of metabolites, which are often overlooked in current monitoring practices and research efforts.