A ground-breaking new study published in Science reveals that pharmaceutical pollution, at levels commonly found in the environment, can significantly alter the migratory behaviour of Atlantic salmon (Salmo salar), with potential consequences for population viability and ecological dynamics. The research, led by Dr Jack A. Brand of the Swedish University of Agricultural Sciences (SLU), alongside an international team of collaborators, provides some of the clearest evidence to date that psychoactive pharmaceutical contaminants—specifically the anxiolytic drug clobazam—can affect the behaviour of wild fish populations in complex and unexpected ways.
Using an integrative, multi-year design that combined laboratory assays with large-scale field experiments involving 730 salmon smolts in Sweden’s River Dal, the researchers demonstrated that clobazam accumulates in fish brains and alters their river-to-sea migration. Notably, exposed fish passed through two hydropower dams more rapidly and in greater numbers than control fish. While this might suggest improved migration efficiency, the study found no evidence that survival through the dams was enhanced. Instead, the behavioural changes likely stem from altered shoaling dynamics and heightened risk-taking behaviour induced by the drug.
In predator-exposure trials, clobazam-exposed salmon formed less cohesive shoals—a deviation from typical anti-predator responses where group cohesion increases under threat. This decreased social cohesion may have allowed fish to navigate the dams more individually and quickly. However, this solitary, risk-prone strategy could increase vulnerability to predation in the wild, raising concerns about long-term survival. Previous research has shown that migrating in groups is a key anti-predator strategy in smolts, and solitary fish may face a higher mortality rate when exposed to real-world ecological threats.
These findings underscore the profound and often overlooked effects of human pharmaceutical waste on aquatic ecosystems. The study also adds critical new insight to the One Health agenda, which recognises the interconnectedness of human, environmental, and animal health. For the One Health Breakthrough Partnership (OHBP), this research reinforces the need for integrated approaches to pharmaceutical stewardship—from sustainable medicine use to wastewater innovation—across disciplines and sectors.
Pharmaceutical pollutants have typically flown under the radar in environmental regulation, but this research clearly demonstrates their ability to alter ecologically and evolutionarily significant behaviours. If unmanaged, these impacts could cascade across food webs, migration systems, and fisheries, highlighting an urgent need for enhanced monitoring and policy reform.
The study has already reached beyond academic circles, recently receiving attention on The Late Show with Stephen Colbert. You can catch the segment starting here.