Cocaine Pollution Gives Atlantic Salmon Wanderlust: Drug Byproducts Turn Juvenile Fish into Long-Distance Swimmers in Real Lakes

A groundbreaking field study has revealed that trace levels of cocaine and its primary metabolite—routinely flushed into rivers and lakes worldwide—can dramatically alter the behavior of young Atlantic salmon in their natural environment. For the first time outside a laboratory, researchers showed that exposure to environmentally realistic concentrations of these compounds makes juvenile salmon swim farther, disperse more widely, and remain more active over time. The effects were strongest from benzoylecgonine, the stable breakdown product of cocaine that often persists at higher levels in wastewater than the drug itself.

The Growing Problem of Pharmaceutical and Illicit Drug Pollution

Standard wastewater treatment plants are not designed to filter out cocaine, its metabolites, or many other human pharmaceuticals. As a result, these compounds enter aquatic ecosystems globally through sewage outflows. Benzoylecgonine, in particular, is one of the most commonly detected cocaine-related pollutants in surface waters. While laboratory studies had previously hinted at behavioral changes in fish exposed to such drugs, their relevance to wild populations remained unclear—until now. Atlantic salmon (Salmo salar) populations are already in steep decline due to habitat loss, climate change, overfishing, and other stressors. Any additional disruption to their movement, foraging, or survival could have serious ecological and economic consequences.

Study Design: Bringing the Lab into the Lake

Led by Jack A. Brand and Michael G. Bertram of the Swedish University of Agricultural Sciences (SLU), an international team conducted the experiment in Lake Vättern, Sweden’s second-largest lake (surface area ~1,912 km²). In April 2022, they released 105 hatchery-reared two-year-old Atlantic salmon smolts (average mass ~110 g) fitted with two devices:

• Slow-release chemical implants delivering either cocaine, benzoylecgonine, or a control (coconut oil only) at doses mimicking real-world pollution levels (~50 μg g⁻¹ body weight).
• Acoustic telemetry tags to track their positions via an array of underwater receivers.

The fish were divided into three equal groups (35 each) and monitored continuously for eight weeks in the wild. Brain tissue analysis later confirmed that both compounds accumulated at detectable levels: cocaine-treated fish averaged ~42.85 ng g⁻¹, while benzoylecgonine-treated fish averaged ~33.74 ng g⁻¹. No drugs were detected in controls. This implant-and-telemetry approach was a major innovation, allowing controlled exposure while capturing natural behaviors in a large, complex ecosystem—far beyond the artificial confines of lab tanks.

Key Findings: Hyperactive and Far-Ranging

All fish gradually became less active as the study progressed, consistent with natural settlement patterns. However, clear differences emerged, especially in the later weeks:

• Movement: Benzoylecgonine-exposed salmon swam up to ~1.9 times farther per week than unexposed controls—equating to an extra ~13.7 km per week in weeks 7–8. Cocaine-exposed fish also moved more, but the effect was weaker and less consistent.
• Dispersal: Benzoylecgonine-treated fish spread up to ~12.3 km farther from the release site than controls by the study’s end. Their maximum dispersal distance increased notably, while controls stayed relatively close to the starting point.
• Space use: Exposed fish (particularly those on the metabolite) explored more unique areas of the lake and maintained higher activity levels toward the end of the tracking period.

These changes suggest cocaine-derived pollutants can rewire fish spatial ecology, potentially altering where they feed, hide from predators, or interact with other species.

Why Benzoylecgonine Had the Stronger Effect

Surprisingly, the metabolite drove more pronounced behavioral shifts than the parent drug. Benzoylecgonine is more persistent in the environment and may interact differently with fish physiology (e.g., via vasoconstriction or oxidative stress pathways observed in earlier lab work). The researchers note this finding challenges assumptions in environmental risk assessments, which often focus more on the original compound.

Broader Implications for Wildlife and Conservation

“Where fish go determines what they eat, what eats them, and how populations are structured,” said co-author Dr. Marcus Michelangeli. “If pollution is changing these patterns, it has the potential to affect ecosystems in ways we are only beginning to understand.” Altered movement could increase energy expenditure, predation risk, or disrupt migration timing—adding yet another pressure on already vulnerable salmon stocks. The study’s authors emphasize that cocaine pollution joins a growing list of “psychoactive pollutants” (including pharmaceuticals like antidepressants) that may subtly reshape aquatic life.They call for better wastewater treatment technologies, updated regulatory frameworks that account for drug metabolites, and further field research on long-term survival, reproduction, and population-level impacts.

Limitations and Next Steps

The experiment used hatchery-reared fish, which may behave slightly differently from wild counterparts. Tracking was limited to the first eight weeks, and the receiver array covered only part of the massive lake. Future studies plan higher-resolution tracking, tests on wild fish, and assessments of fitness consequences (e.g., growth or predation). Ambient lake levels of cocaine and benzoylecgonine were trace and unlikely to have influenced results.

Conclusion

This research marks a pivotal advance in understanding how human drug use reverberates through ecosystems. Cocaine pollution isn’t just a curiosity—it’s a real, measurable driver of behavioral change in one of the planet’s most iconic fish species. As lead author Jack Brand noted, the unusual part isn’t the experiment; it’s what’s already happening in our waterways every day.

Citations

1. Brand, J.A., Palm, D., Cerveny, D., Michelangeli, M., Bose, A.P.H., McCallum, E.S., Hellström, G., Fick, J., Brooks, B.W., Brodin, T., & Bertram, M.G. (2026). Cocaine pollution alters the movement and space use of Atlantic salmon (Salmo salar) in a large natural lake. Current Biology, 36(8), 2018–2027.e4. https://doi.org/10.1016/j.cub.2026.03.026 (Open access).
2. Griffith University News. (2026, April 21). Cocaine pollution alters salmon behaviour in the wild. https://news.griffith.edu.au/2026/04/21/cocaine-pollution-alters-salmon-behaviour-in-the-wild/
3. The Guardian. (2026, April 20). Cocaine pollution in rivers and lakes may disrupt behaviour of salmon, study suggests. https://www.theguardian.com/science/2026/apr/20/cocaine-pollution-may-disrupt-behaviour-of-salmon-study-suggests
4. The New York Times. (2026, April 20). These Salmon Got High on Cocaine. That Wasn’t the Craziest Part. https://www.nytimes.com/2026/04/20/science/cocaine-salmon-sweden.html
5. Science. (2026, April 20). Cocaine pollution gives salmon wanderlust. https://www.science.org/content/article/cocaine-pollution-gives-salmon-wanderlust
6. Phys.org. (2026, April 20). Cocaine pollution alters salmon behavior in the wild, study shows. https://phys.org/news/2026-04-cocaine-pollution-salmon-behavior-wild.html

All data and conclusions are drawn directly from the peer-reviewed paper and associated press materials published April 20, 2026.