Salmon given antianxiety drugs take more risks, study finds

An adult Atlantic salmon in Sweden. Photo / Jörgen Wiklund

Young salmon in central Sweden must endure a gauntlet to migrate from their freshwater spawning grounds to the Baltic Sea. Their 28-km route along the Dal River is rife with danger, from northern pike and seabirds to hydropower dams.

Four out of five fish don’t make it.

Some Atlantic salmon recently got an unexpected edge, after scientists dosed them with antianxiety drugs meant for humans, not fish. The drug, clobazam, made the fish more independent and faster, research published last week found - but scientists said the seemingly positive effects could belie hidden or long-term consequences for the fish. And the mere existence of the study speaks to a growing problem: We’re turning our rivers, lakes and oceans into soups of pharmaceutical pollution.

“Any sort of deviation from natural behaviours is likely to carry potential consequences,” such as making salmon easier targets for predators, said Jack Brand, researcher at Swedish University of Agricultural Sciences and lead author of the paper published on Thursday in Science.

Nearly 1000 pharmaceuticals have been detected in waterways around the world, and scientists say they threaten animals and the people who rely on them. Brand and his colleagues focused on one behaviour of a single species - migration in Atlantic salmon - but he said he can easily imagine exponential consequences when expanding the focus to other drugs, species and waterways across the world.“Pharmaceutical pollution is this sort of invisible agent of global change,” Brand said.

Some drugs enter the water after being flushed down the toilet. Other biologically active substances are released into rivers in the pharmaceutical manufacturing process. In river sites across more than 100 countries, over 40% of them contained potentially harmful levels of drugs, according to a 2022 study. The most frequently detected compounds were the anticonvulsant carbamazepine, the diabetes drug metformin and caffeine.

Among the animals affected by drugs in the water are critically endangered eels in Europe, which at one point were exposed to potentially harmful levels of MDMA after public urination during a music festival in England. In India, a drug given to cattle led to massive population declines in vultures that fed on carcasses. Even low levels of fluoxetine, the antidepressant in Prozac, changed the mating behaviour of fathead minnows found throughout the United States.

Researchers have spent the past 10 to 15 years showing trace amounts of psychoactive pharmaceuticals affect the behaviour of animals, including fish, Brand said. But they mostly did so in a laboratory setting, raising the question: Would the same thing happen in the wild?

To help answer that question, Brand and his colleagues divided a group of 279 young salmon into four cohorts and inserted implants that slowly released the antianxiety medication clobazam, the opioid painkiller tramadol, a combination of the two or a control with no drugs. The researchers also implanted acoustic transmitters designed to emit a sound unique to each fish and installed underwater microphones along the migration route to detect those sounds. Finally, the team tracked the fish as they swam a roughly 28km stretch of the Dal River, which first flows into a large reservoir, becomes rapids and then courses through two hydropower dams before discharging into the Baltic Sea.

They conducted their experiment during annual salmon runs in 2020 and 2021 to lessen the impact of natural variation, according to the article.

Their findings surprised Brand. As he expected, the fish exposed to clobazam showed riskier behaviour - separating from other salmon and swimming downriver with abandon. “They were just sort of beelining it through the dams and making their way down the river instead of sitting around the dam and waiting for other shoal mates.”

Yet that bolder behaviour didn’t reduce the number of fish that reached the sea. The clobazam-drugged salmon swam through one of the two hydropower dams 2½ to three times faster than the others, the scientists found, noting that prior research has shown dams increase the danger that young salmon get eaten, exhaust themselves, mistime their entry to the sea or get killed by the dam itself.

Brand warned people about concluding that the drugs helped the fish, at least on a large scale and in the long term. Researchers didn’t track the fish once they made it to the sea, so they don’t know what happened to them after that. The speedy drug-affected loners could have swum right into the path of predators.

Rachel Ann Hauser-Davis, a biologist with the Brazil-based scientific research institution Oswaldo Cruz Foundation, who has examined the effects of carbamazepine and the antianxiety drug clonazepam on zebra fish, said Brand’s study and her own research indicate pharmaceutical pollution is altering ecosystems, even at low levels.

The results of both of their studies reiterate what scientists have been repeating for decades: Pharmaceutical drugs are getting through wastewater treatment facilities and new technology is required to remove them, Hauser-Davis wrote on Wednesday in an email. She called for raising public awareness about improper drug disposal, restraining the use of prescription drugs, and increasing the use of “green pharmacy” practices in which medication is redesigned to more easily and quickly biodegrade.

“We should definitely change both our individual behaviour and systemic practices,” she said.

Brand said finding out what happens to the salmon once they reach the sea is one of his research goals. But first, he’s studying the effects of illegal drugs such as cocaine and amphetamines on Atlantic salmon. He said he plans to release his findings later this year - and keep sounding an alarm he feels hasn’t been heeded.

“Pharmaceutical pollution, and then even chemical pollution more generally,” he said, “is often really underappreciated by the public as a potential global threat.”