Drug pollution in water is making salmon take more risks – new research

“Out of sight, out of mind” is how we often treat what is flushed down our toilets. But the drugs we take, from anxiety medications to antibiotics, don’t simply vanish after leaving our bodies. Many are not fully removed by wastewater treatment systems and end up in rivers, lakes and streams, where they can linger and affect wildlife in unexpected ways.

In our new study, we investigated how a sedative called clobazam, commonly prescribed for sleep and anxiety disorders, influences the migration of juvenile Atlantic salmon (Salmo salar) from the River Dal in central Sweden to the Baltic Sea.

Our findings suggest that even tiny traces of drugs in the environment can alter animal behaviour in ways that may shape their survival and success in the wild.

A recent global survey of the world’s rivers found drugs were contaminating waterways on every continent – even Antarctica. These substances enter aquatic ecosystems not only through our everyday use, as active compounds pass through our bodies and into sewage systems, but also due to improper disposal and industrial effluents.

To date, almost 1,000 different active pharmaceutical substances have been detected in environments worldwide.

Particularly worrying is the fact that the biological targets of many of these drugs, such as receptors in the human brain, are also present in a wide variety of other species. That means animals in the wild can also be affected.

In fact, research over the last several decades has demonstrated that pharmaceutical pollutants can disrupt a wide range of traits in animals, including their physiology, development and reproduction.

Pharmaceutical pollution in the wild

The behavioural effects of pharmaceutical pollutants have received relatively less attention, but laboratory studies show that a variety of these contaminants can change brain function and behaviour in fish and other animals. This is a major cause for concern, given that actions critical to survival, including avoiding predators, foraging for food and social interaction, can all be disrupted.

Lab-based research has provided useful insights, but experimental conditions rarely reflect the complexity of nature. Environments are dynamic and difficult to predict, and animals often behave differently than they do in controlled settings. That’s why we set out to test the effects of pharmaceutical exposure in the wild.

As part of a large field study in central Sweden, we attached implants that slowly released clobazam (a common pharmaceutical pollutant) and also miniature tracking transmitters to juvenile Atlantic salmon on their seaward migration through the Dal.

The Dal is a large river in central Sweden that flows into the Baltic Sea. Michael Bertram

We found that clobazam increased the success of this river-to-sea migration, as more clobazam-treated salmon reached the Baltic Sea compared with untreated fish. These clobazam-exposed salmon also took less time to pass through two major hydropower dams that often delay or block salmon migration.

To better understand these changes, we followed up with a laboratory experiment which revealed that clobazam also altered how fish group and move together – what scientists call shoaling behaviour – when faced with a predator.

This suggests that the migration changes observed in the wild may stem from drug-induced shifts in social dynamics and risk-taking behaviour.

What does this mean for wildlife?

Our study is among the first to show that pharmaceutical pollution can affect not just behaviour in the lab, but outcomes for animals in their natural environment.

While an increase in migration success might initially sound like a positive effect, any disruption to natural behaviour can have ripple effects across ecosystems.

Even seemingly beneficial changes to animal behaviour, like faster passage through barriers, can come at a cost. Changes to the timing of migrations, for instance, might lead fish to arrive at the sea when conditions are not ideal, or expose them to new predators and risks. Over time, these subtle shifts could influence the dynamics of entire populations and threaten the balance of ecosystems.

Pharmaceuticals are vital for keeping people and animals healthy. But the accumulation of these drugs in rivers and lakes demands smarter approaches to keeping waterways clean.

One part of the solution is upgrading wastewater treatment plants. Some advanced methods such as ozonation, which involves bubbling ozone gas through wastewater to break down pollutants, can be effective at removing pharmaceuticals. But such advanced treatment systems are often prohibitively expensive to install and out of reach for many regions.

Another promising avenue is green chemistry: designing drugs that break down more easily in the environment or become less toxic after use. Our team has recently highlighted this as a key step toward reducing pharmaceutical pollution in the environment.

Stronger regulations and better drug disposal practices can also help to prevent medications from ending up in waterways in the first place.

There’s no single fix, but by advancing and integrating science, technology and policy, we can help to protect wildlife from the unintended effects of pharmaceutical pollution.