It has to be stressful: an obstacle course of giant dams, rushing rapids and hungry predators.
That’s what juvenile salmon can face when they migrate out to the salty sea from the freshwater rivers and streams where they hatched. But it turns out that a very specific kind of pollution might be giving some fish an edge, at least on part of the journey.
According to a new study published on Thursday in the journal Science, young salmon exposed to anti-anxiety drugs in the water made it past dams faster. But ecologists are doubtful that it means a survival advantage.
Pharmaceutical pollution is rampant. Nearly 1,000 drugs and their byproducts have been detected in the world’s waterways, including in surprising places like Antarctica. They enter the environment as direct pollution from drug producers, from people flushing unused medications, and from human and animal waste.
Scientists have been studying the effects of these drugs on wildlife for years, but there is still much to learn about how animals respond to the “cocktail of different pharmaceuticals” they’re exposed to, said Michael G. Bertram, a behavioral ecologist at the Swedish University of Agricultural Sciences and a leader of the new study.
There’s a particular interest in psychoactive drugs, like anti-anxiety and antidepressant medications, because they are designed to modify behavior.
Previous studies have found that benzodiazepines, a class of drugs used to treat anxiety and insomnia, can affect migration in Atlantic salmon. They can also decrease stress responses in other fish species.
In short, fish lose their anxiety just as humans do.
But those findings were mostly from lab experiments. The real world is more complicated. “We know a lot from laboratory studies that drugs can change animal behavior, but how that plays out in natural ecosystems is very hard to forecast,” Dr. Bertram said.
Dr. Bertram’s team wanted to test whether lab-based findings held up in the wild. They focused on salmon’s smolt phase, when juvenile fish migrate to the ocean.
They exposed salmon smolts to a common anti-anxiety medication, an opioid painkiller, or a mix of both, in doses similar to what the fish might encounter in the wild. There was also a control group that swam only in uncontaminated water. They then equipped the fish with tracking tags and released them into a river in Sweden. The smolts traveled about 17 miles over one to two weeks, navigating a predator-filled reservoir, rapids and two hydropower dams before reaching the Baltic Sea.
“It’s quite a perilous journey,” Dr. Bertram said.
Smolts that had been exposed to the anti-anxiety drug were more likely to reach the sea than the other groups. Surprisingly, there was no statistical difference in how quickly smolts in any group reached the sea. But there was a difference in how quickly smolts navigated the dams, with fish in the anti-anxiety group moving through hours faster than the control group.
That could be explained by a change in risk-taking behavior, the researchers thought. Lab-based studies had found that anti-anxiety medications increased boldness. The new study ties that to life or death in the wild. “Here, we’re actually showing that there is a direct change in behavior that then alters survival,” Dr. Bertram said.
“It’s interesting to see how one problem impacts how they deal with another problem,” said Olivia Simmons, a salmon ecologist at the Norwegian Institute for Nature Research who was not involved in the study. “These bolder fish could just be going faster because they’re less inhibited.”
The researchers also tested whether boldness changed how the smolts formed schools, a natural defense against predators. They drugged a new batch of fish and filmed their schooling behavior in a tank, with and without a predator. Smolts treated with the anti-anxiety medication formed looser schools, even as the predator, a northern pike, was circling.
That result supported the idea that the anti-anxiety drug made the young salmon more likely to take risks and, possibly, to navigate dams on their own. Dr. Bertram said he wanted to test this idea in the wild with more advanced trackers that would let him see if smolts are traveling in groups or alone, or if a smolt was eaten.
Scientists said they were concerned that drugs in waterways appear to be influencing critical fish behaviors. The ripple effects in ecosystems of this “unnatural selection,” as Dr. Bertram put it, are unknown.
For example, salmon could change their migration timing, arriving when temperatures are wrong or resources are limited. An increase in salmon could put too much pressure on their prey species, throwing an ecosystem out of balance. And if risky behavior continues in the smolts’ new environment, when they are surrounded by predators, the fish could be less likely to survive in the long run and reproduce.
“It’s like playing poker,” said Giovanni Polverino, a behavioral ecologist at the University of Tuscia in Viterbo, Italy, who was not involved in the study published on Thursday but has collaborated with Dr. Bertram on other research.
“The more risks you take, the more chances you have to lose everything,” he said. “In this case, the fish’s life.”
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