In Sweden, Atlantic salmon are being exposed to cocaine by researchers, leading to surprising behavioral changes. Erin McCallum, an aquatic ecology expert at the Swedish University of Agricultural Sciences, explained the study aimed to assess the impact of cocaine found in waterways worldwide on fish behavior, including in Canada.
Published in Current Biology, the study revealed that juvenile Atlantic salmon in Lake Vättern, Sweden, ingesting cocaine or its metabolite, benzoylecgonine, swam further than sober salmon. McCallum highlighted that increased swimming distances could offer more habitat options and food sources for the fish, but it could also expose them to potential risks like poor habitats and predators.
To conduct the study, researchers used slow-release chemical implants to expose the fish to cocaine or benzoylecgonine and observed their behavior over eight weeks. The results showed that fish exposed to benzoylecgonine swam almost twice as far per week and dispersed over 12 kilometers across the lake. The effects on fish exposed to cocaine were weaker and less consistent.
Mark Servos from the University of Waterloo praised the study as a novel approach outside the lab, emphasizing the importance of understanding the real-world impacts of drugs in fish. McCallum noted that wastewater treatment plants often fail to filter out illicit drugs and their metabolites, leading to these substances entering waterways globally.
A study in 2025 reported the presence of cocaine, tramadol, and codeine in waterways worldwide, while a 2024 study from Brazil found cocaine in wild sharks off the coast of Rio de Janeiro. In Canada, research found opioids and other drugs in small freshwater fish downstream of urban wastewater plants.
Despite the low risk to humans, research suggests potential impacts on fish reproduction due to drug exposure. While technology exists to improve wastewater treatment and remove these contaminants, the high cost presents a challenge for communities. McCallum emphasized the need for further research on drug metabolites and their effects on wildlife to better understand pollution’s environmental impacts.
