Anti-anxiety drugs disrupt salmon migration in the wild, new study finds

Image credit | Jörgen Wiklund

Unlike previous lab-based studies, this large-scale experiment took place in a Swedish river and combined realistic pharmaceutical exposure with state-of-the-art telemetry to track the behaviour of 279 juvenile salmon (smolts) during their seaward migration. The salmon were exposed to either the anti-anxiety drug clobazam (a benzodiazepine), a common painkiller, both, or neither. The drugs were delivered via slow-release implants, at doses mimicking concentrations previously measured in wild fish from polluted rivers.

The researchers found that clobazam-exposed salmon crossed migration barriers two to eight times faster than the other groups. Surprisingly, a higher proportion—more than double—of these fish reached the sea alive. But is that good news?

“At first glance, it may seem like a positive effect,” says Prof. Thoré, who contributed to the data analysis, interpretation, and publication of the study. “But such behavioural changes could carry hidden costs. Moving faster might mean the fish take more risks or use up energy less efficiently—something that could compromise their chances of surviving the return journey to spawn. Not to mention the knock-on effects this may have on other species and the wider ecosystem.”

This is the first time the behavioural effects of psychiatric drugs have been tested at large scale on migrating fish in their natural habitat. Prof. Thoré was involved in the project during his postdoctoral research at the Swedish University of Agricultural Sciences (SLU) and remains actively engaged in the collaboration today.

“This is part of a long-term partnership between UNamur and SLU,” he says. “We’re working together on several projects to better understand how pharmaceutical pollutants affect wild animal behaviour and ecology, and how we can mitigate these effects. It’s a productive collaboration, and I see it evolving into a long-term, structural link between our institutions.”

Pharmaceutical residues such as clobazam are frequently detected in European rivers—including Belgian waterways. A 2022 global survey found that one in four rivers worldwide contains pharmaceutical concentrations considered unsafe for aquatic life. Rivers in Brussels were ranked among the top 20% most contaminated.

“Drugs like clobazam are designed to act on the brain in low doses—and they do the same when fish absorb them,” says Prof. Thoré. “Our findings show that even very low, environmentally relevant concentrations can alter migration and behaviour in a species that’s ecologically, economically, and culturally important, like salmon.”

As Prof. Thoré explained in an interview with De Standaard:

Eli Thoré is an assistant professor and expert in animal behaviour and environmental pollution research at the University of Namur (Belgium), where he leads the Laboratory of Adaptive Biodynamics (LAB) as part of the Research Unit in Environmental and Evolutionary Biology (URBE). He is also a member ot the Institute of Life, Earth and Environment (ILEE). His team takes an integrative approach to understanding how animals respond to environmental changes, particularly those driven by human activity, including pharmaceutical pollution. By focusing on animal behaviour alongside its underlying mechanisms and broader ecological consequences—and by connecting these different scales—his team strives to advance scientific knowledge and contribute to thriving ecosystems that can catalyse sustainable development.

Read the article published in Science: Pharmaceutical pollution influences river-to-sea migration in Atlantic salmon (Salmo salar)