Drug pollution alters salmon migration

The study, led by the Swedish University of Agricultural Sciences, revealed that commonly detected environmental levels of clobazam — a medication often prescribed for sleep disorders — increased the river-to-sea migration success of juvenile salmon in the wild.

The researchers also discovered that clobazam shortened the time it took for juvenile salmon to navigate through two hydropower dams along their migration route — obstacles that typically hinder successful migration.

Dr Marcus Michelangeli from Griffith University’s Australian Rivers Institute, who was a key contributor to the study published in Science, emphasised the increasing threat of pharmaceutical pollution to wildlife and ecosystems worldwide.

“Pharmaceutical pollutants are an emerging global issue, with over 9srcsrc different substances having now been detected in waterways around the world,” Dr Michelangeli said.

“Of particular concern are psychoactive substances like antidepressants and pain medications, which can significantly interfere with wildlife brain function and behaviour.

Dr Michelangeli noted that the study’s real-world focus sets it apart from previous research.

“Most previous studies examining the effects of pharmaceutical pollutants on wildlife have been conducted under controlled laboratory conditions, which don’t fully capture the complexities of natural environments,” he said.

“This study is unique because it investigates the effects of these contaminants on wildlife directly in the field, allowing us to better understand how exposure impacts wildlife behaviour and migration in a natural context.

“While the increased migration success in salmon exposed to clobazam might seem like a beneficial effect, it is important to realise that any change to the natural behaviour and ecology of a species is expected to have broader negative consequences both for that species and the surrounding wildlife community.”

The research team employed innovative slow-release pharmaceutical implants and animal-tracking transmitters to monitor how exposure to clobazam and the opioid painkiller tramadol — another common pharmaceutical pollutant — affected the behaviour and migration of juvenile Atlantic salmon (Salmo salar) in Sweden’s River Dal as they migrated to the Baltic Sea.

A follow-up laboratory experiment also found that clobazam altered shoaling behaviour, indicating that the observed migration changes in the wild may result from drug-induced shifts in social dynamics and risk-taking behaviour.

Dr Michelangeli explained that predicting the full extent of these impacts remains challenging

“When you consider realistic exposure scenarios where entire ecosystems are exposed — encompassing multiple species and a diversity of contaminants — the potential consequences become even more complex,” he said.

While the recent decline of Atlantic salmon is primarily attributed to overfishing, habitat loss, and fragmentation — leading to their endangered status — the study highlights how pharmaceutical pollution could also influence key life-history events in migratory fish.

Dr Michelangeli pointed out that many pharmaceuticals persist in the environment due to poor biodegradability and insufficient wastewater treatment. However, there is hope.

“Advanced wastewater treatment methods are becoming more effective at reducing pharmaceutical contamination, and there is promising potential in green chemistry approaches,” he said.

“By designing drugs that break down more rapidly or become less harmful after use, we can significantly mitigate the environmental impact of pharmaceutical pollution in the future.”

The study ‘Pharmaceutical pollution influences river-to-sea migration in Atlantic salmon (Salmo salar)’ has been published in Science.