Clobazam Alters Salmon Migration in Sweden’s River Dal

Every spring, juvenile Atlantic salmon leave their freshwater homes and begin a perilous journey to the sea – dodging predators, navigating dams and relying on finely tuned instincts to survive. But new research, published in Science, suggests that even trace amounts of human medication in rivers could be quietly rewriting the rules of this ancient migration.

A team of international researchers led by the Swedish University of Agricultural Sciences tracked salmon in Sweden's River Dal and discovered that exposure to the anti-anxiety drug clobazam boosted their migration success. While the drug helped fish navigate dams more quickly, it also altered their social behavior in potentially risky ways.

Pharmaceutical pollution in rivers

Across the world, rivers, lakes and streams are quietly accumulating a cocktail of pharmaceuticals – the chemical residue of modern medicine. More than 900 different active pharmaceutical ingredients have now been detected in freshwater ecosystems globally, and the concentrations are only expected to rise.

These drugs enter the environment through various pathways. Some are excreted from the body and pass through wastewater treatment plants, which often lack the capacity to fully remove them. Others seep into waterways via agricultural runoff, improper disposal or manufacturing waste. Unlike many natural pollutants, pharmaceuticals are built to persist – designed to remain stable, active at low doses and target specific biological pathways. In the environment, this becomes a problem.

“Pharmaceutical pollutants are an emerging global issue,” said co-author Dr. Marcus Michelangeli, a lecturer in ecology at Griffith University. “Of particular concern are psychoactive substances like antidepressants and pain medications, which can significantly interfere with wildlife brain function and behaviour.”

Benzodiazepines and opioids – medications commonly used to treat anxiety, pain and sleep disorders – don’t just linger in the water; they accumulate in the tissues of fish and other aquatic organisms, including their brains. Even at extremely low concentrations, they can disrupt key neurological processes, altering how animals move, socialize and respond to threats.

Lab-based studies have shown that exposure to these drugs can change everything from a fish’s shoaling tendencies to its ability to avoid predators. Prior work also shows benzodiazepines like clobazam dampen stress responses in fish.

“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,” said Michelangeli.

What happens when fish encounter these drugs in the wild, amidst shifting water temperatures, predators, human infrastructure and interactions with other species? And how might such exposures influence key life events – like reproduction or migration?

For migratory species such as Atlantic salmon, the stakes are especially high. Once abundant across Europe and North America, many salmon populations have experienced dramatic declines in recent decades. Overfishing, habitat fragmentation and climate change have all played a role, but chemical pollution is increasingly recognized as an additional threat. Understanding how pharmaceutical contaminants influence migration behavior and survival in the wild is not only vital for conserving these species but also for protecting the ecosystems they help sustain.

Tracking salmon and sleep drugs in the wild

In what is now the largest field-based study of its kind, a team tracked 730 juvenile salmon, or smolts, as they made their journey from freshwater to the sea. The study, conducted over two spring migration seasons in Sweden’s River Dal, focused on two drugs that regularly turn up in aquatic ecosystems: clobazam, a benzodiazepine prescribed for anxiety and sleep disorders, and tramadol, an opioid painkiller. Both are designed to act on the brain and nervous system – and both are found in rivers worldwide at trace but biologically active levels.

To mimic real-world exposure, researchers used slow-release pharmaceutical implants, surgically inserted into the fish to deliver environmentally relevant doses of the drugs. Fish were randomly assigned to one of four groups: control (no drug), clobazam, tramadol or a clobazam-tramadol mixture. All were fitted with acoustic telemetry tags to monitor their movements over a 28 km stretch of the river, including 2 hydropower dams.

“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 behavior and migration in a natural context,” said Michelangeli.

Smolts exposed to clobazam were more likely to reach the Baltic Sea than those in other groups.

On average, these fish also passed through the dams two to three times faster than other groups at both dams, suggesting that the drug may have altered their behavior in ways that helped them overcome physical barriers more quickly. At Älvkarleby, clobazam fish passed in about 8 hours vs up to 64 hours in the mixture group.

However, among the fish that completed the journey, there was no major difference in overall migration speed – meaning clobazam did not necessarily make fish swim faster, just more effectively through key chokepoints.

In contrast, tramadol alone had little measurable impact on migration success or dam passage speed, performing similarly to controls.

The clobazam-tramadol mixture also did not perform as well as clobazam alone, but fish in this group still had higher success than controls.

To probe the behavioral changes behind these patterns, the team conducted a follow-up laboratory experiment. They tested whether clobazam influenced the way juvenile salmon formed shoals.

When threatened by a common predator (the northern pike), clobazam-exposed fish displayed weaker shoaling behavior than control fish. The drug appeared to increase risk-taking, potentially encouraging fish to stray from the safety of the group.

Ecological implications and looking ahead

At first glance, the findings of the salmon study might seem like good news: more fish making it to sea sounds like a win. But dig deeper, and a more complex and concerning picture emerges. The increased migration success of clobazam-exposed salmon was driven by changes in their behavior, likely due to reduced shoal cohesion and increased risk-taking. In the wild, such changes don’t come without trade-offs.

Riskier behavior may help fish overcome barriers, but it could also make them more vulnerable to predators, disrupt the synchronization of group migration or alter the timing of key life-history events such as sea entry or spawning.

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

Clobazam didn’t just change fish behavior; it changed their migratory outcomes, potentially reshaping an important phase in the salmon life cycle. The interactions between different drugs, species and stressors could have ripple effects that science is only beginning to untangle.

“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,” said Michelangeli.

Addressing the issue will require a multi-pronged approach. One route is upgrading wastewater treatment infrastructure to better filter out pharmaceutical residues before they reach rivers and lakes. Another lies in the design of the drugs themselves.

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

Looking ahead, scientists say more field-based research is urgently needed to understand the behavioral effects of pharmaceutical contaminants and trace their long-term consequences for population survival, reproductive success and ecosystem stability.

Reference: Brand JA, Michelangeli M, Shry SJ, et al. Pharmaceutical pollution influences river-to-sea migration in Atlantic salmon (Salmo salar). Science. 2025. doi: 10.1126/science.adp7174

This article is a rework of a press release issued by Griffith University. Material has been edited for length and content.