The life of a juvenile salmon would be enough to have anyone reaching for an anti-anxiety pill.
The tiny fish run a deadly gantlet as they swim from freshwater to the ocean, dodging ravenous predators and whirring hydropower turbines while pushing through sluggish reservoirs backed up behind dams. On one Swedish river, approximately one out of every 10 fish complete the journey.
Just as some people use medication to tamp down panic attacks, new research suggests the drugs can also alter salmon behavior in ways that increase their ability to navigate these perils.
The discovery about the anti-anxiety medication clobazam, which appeared last week in the journal Science, is some of the most compelling real-world evidence that pharmaceutical pollution can alter ecosystems in potentially significant ways. And this is just one of hundreds of different drugs making their way from sewers to the open water.
“Our findings raise important questions about how pharmaceutical pollution alters migration behavior and survival in the wild,” said Jack Brand, a researcher at the Swedish University of Agricultural Sciences.
For years, scientists have been documenting changes in animal behavior in response to drug pollution. Many of these pharmaceuticals aren’t removed by typical wastewater treatment, and flow into waterways after being excreted in people’s urine or feces. But much of the research on their effects on animals has been done in labs, where conditions might not replicate the real world.
Brand and colleagues tackled that problem by taking 279 lab-reared juvenile Atlantic salmon, implanting them with tiny radio trackers, and then putting slow-release drug capsules inside some of them. Drugged fish got either clobazam, the opioid pain killer tramadol, or a cocktail of both drugs, all at levels found in fish in other research.
Then they put the little fish into the River Dal, in central Sweden, and used underwater radio receivers to monitor their movements down a 28-kilometer stretch of river to the Baltic Sea.
The fish dosed with just the anti-anxiety drug showed distinctly different patterns. They sped past one dam between 2.5 and 3 times faster than the other fish and passed another dam as much as 8 times faster. The number of clobazam-fueled fish that made it all the way to the sea was more than double their sober counterparts, the scientists reported.
The different outcomes might be related to how the anti-anxiety medication alters fish behavior in the face of dangers. Salmon smolts confronted with a threat such as a predatory pike will group together, seeking safety in numbers. In lab tests, this same group of scientists found that young salmon confronted with a pike didn’t clump together as much if they had been dosed with anti-anxiety medication. That risk-taking behavior might have contributed to the speedier and more successful journey down the River Dal.
But that doesn’t mean salmon should start popping pills before embarking on their journey.
“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,” cautioned Marcus Michelangeli, a scientist at Griffith University in Australia who worked on the study.
The potential downsides of such drugs were underscored by a new survey in the International Journal of Environmental Research and Public Health. Scientists in Switzerland found that nine of 35 of the most common prescription drugs sold in that country have turned up in Swiss rivers at levels exceeding environmental quality thresholds. The most common categories were painkillers such as diclofenac and antibiotics such as ciprofloxacin.
The researchers behind that study suggested the results could be used to help guide physicians toward prescribing environmentally safer drugs if they offered the same benefits to patients. Such “eco-responsible medicine” can benefit patients “directly, by avoiding over-medication, but also indirectly, by promoting a healthier environment, which is essential for well-being,” said Nicolas Senn, a scientist at University of Lausanne.
Who knows. Maybe that could reduce the amount of clobazam people are taking and flushing into rivers.
Brand, et. al. “Pharmaceutical pollution influences river-to-sea migration in Atlantic salmon (Salmo salar).” Science. April 10, 2025.
Charmillot, et. al. “Developing an Ecotoxicological Classification for Frequently Used Drugs in Primary Care.” International Journal of Environmental Research and Public Health. Feb. 16, 2025.
Image: ©Anthropocene Magazine
