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Hatched in quiet streams and rivers, salmon undertake a perilous journey to reach the open ocean where they become mature adults. Over millions of years, generations of young salmon have migrated great distances, in some cases traveling hundreds of miles from freshwater systems to the sea. However, modern salmon face a hurdle that was unknown to their ancient ancestors: pharmaceutical pollution that changes their migration behavior.
Recently, researchers discovered that when a drug called clobazam accumulates in salmon’s brains, migrating fish reach the ocean sooner and navigate dam obstacles faster. On the surface, this change might seem helpful to salmon. However, any deviation from normal animal behavior through human activity — particularly when psychoactive substances are involved — is a red flag, and the full extent of how drug pollution may alter salmon health, behavior and reproduction is still unknown, scientists reported April 10 in the journal Science.
Clobazam, which is commonly found in wastewater, belongs to a group of medicines called benzodiazepines, which depress the central nervous system. The drug is used to prevent epileptic seizures, for short-term treatment of anxiety and to treat anxiety-related sleep disorders. But because neural wiring in fish resembles that of mammals, fish are highly susceptible to the effects of drugs that tweak human neurochemistry, said Dr. Christopher C. Caudill, a professor in the department of fish and wildlife sciences at the University of Idaho.
“Humans share a large amount of biological architecture with fishes — our physiology and anatomy are remarkably similar. Thus, it is intuitive that psychoactive drugs alter the behavior of both fishes and humans,” Caudill, who was not involved in the research, told CNN in an email.
Prior research showed that benzodiazepines could alter behavior in Atlantic salmon (Salmo salar), but they did so under conditions unlike those experienced by wild salmon, said study coauthor Dr. Marcus Michelangeli, a lecturer in the School of Environment and Science at Griffith University in Queensland, Australia.
“Those studies were largely conducted in laboratory settings, only tracked movement over short distances — less than 100 metres (328 feet) — or used drug concentrations much higher than what salmon would typically encounter in the wild,” Michelangeli said via email.
“Our study took a different approach. We followed the entire river-to-sea migration of juvenile salmon in a natural river system, using drug concentrations that match what fish are actually exposed to in the environment.”
The field investigation’s findings highlight the growing risks pharmaceutical pollutants pose to wildlife populations across the globe, according to Michelangeli.
Just keep swimming
For the new study, the scientists performed trials with more than 700 young salmon, or “smolts,” in the laboratory and in the field. The research team used sound-transmitting tags to remotely track hundreds of smolts in 2020 and 2021 as the fish navigated the Dal River in central Sweden.
Migrating smolts swim downriver into a reservoir, hurtle over rapids and crest two dams before finally reaching the Baltic Sea. The journey takes 10 to 13 days.
Two major classes of pharmaceuticals — benzodiazepines and opioids — “are commonly detected in rivers and streams worldwide, including in Sweden, where our study was conducted,” Michelangeli said.
Time-release implants in the smolts dispensed two drugs from these classes: clobazam and tramadol. Fish received clobazam, or tramadol, or both. A control group of smolts received implants with no drugs in them at all.
“These two drugs are known to interact chemically when taken together in humans, and they often co-occur in the environment,” Michelangeli said. “This made them a good test case to explore how pharmaceutical mixtures might affect animal behaviour.”
Along with the field trials, the scientists ran a laboratory-based study on 256 smolts to confirm that the implants worked as intended and that the drugs were lingering in the fishes’ bodily tissues and brains.
When the researchers tracked the migrating salmon with transmitters, they found that more clobazam-exposed salmon reached the Baltic than any of the other fish. Compared with the control group, more than twice as many salmon with clobazam implants made it to the sea.
Lab experiments showed that clobazam affected shoaling behavior, in which smolts stick close together to evade predators. Under the influence of clobazam, fish swam farther apart even when a predator was near, “suggesting that the drug may reduce natural fear responses,” Michelangeli said.
Less fear, more risk
Fish with clobazam implants were also faster at getting past two hydropower dams along their migration route — about two to eight times faster than fish in the other groups. These dams are notorious death zones, where churning turbines can swiftly reduce smolts to salmon tartare.
By diminishing fear in smolts, clobazam might briefly benefit the fish by boosting their migration success. But the drug could also increase their vulnerability to ocean predators, decreasing their chances of surviving long enough to return home to spawn, Caudill said.
“The transition from freshwater to saltwater is one of the most dangerous times in the life of a salmon because they experience many new predators in the ocean,” he said. Drug-exposed and risk-taking salmon may be more likely to reach the Baltic, but less likely to ever leave it alive.
Caudill’s research investigates how environmental change affects fish ecology and evolution. In future work, he said, “I do plan to consider the potential for behavioral effects from pharmaceutical pollution.”
Further study will clarify how behavioral changes from drug pollution affect long-term survival, reproduction and how populations change over time — in salmon and in other wildlife that are vulnerable to pharmaceutical contaminants.
“While more drug-exposed salmon may reach the sea, it doesn’t mean they’re healthy or that the population benefits in the long term,” Michelangeli said.
“The bottom line is we need to be cautious with this interpretation. Changing behaviour with pharmaceuticals — even unintentionally — could reshape whole populations in ways we don’t yet understand.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.
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