Scientists gave cocaine to salmon and you will absolutely believe what happened next

Cocaine contamination can influence the behavior of fish-change, for example, the Atlantic Ocean salmon move through their environment, prompting them to swim further and spread over a larger area.

So finds a recent study by a research team coordinated by Griffith University, the Swedish University of Agricultural Sciences, the Zoological Society of London, and the Max Planck Institute of Animal Behavior and published in the journal Current Biology. The findings provide the first evidence that the effects of cocaine contamination on fish behavior occur not only under laboratory conditions, but also in nature, where animals are exposed to much more complex environmental conditions.

Cocaine and its metabolites have been detected with increasing frequency in rivers and lakes around the world, entering waterways mainly through wastewater treatment systems. Although previous research has shown that cocaine exposure can affect animal behavior, this evidence has been limited to laboratory conditions. A 2024 study by the Oswaldo Cruz Institute in Brazil showed that even sharks are exposed to cocaine, but little is known about its effects on animals in the wild.

To understand more about it, the authors of the new study surgically implanted small devices that slowly released chemicals into 105 young Atlantic salmon in Lake Vättern in Sweden. They were then divided into three groups: a control group, which was not exposed to substances; a group exposed to cocaine; and a group exposed to benzoylecgonine, the main metabolite of cocaine commonly detected in wastewater. The researchers also attached small tags to the fish so they could monitor their movements over a two-month period. From subsequent analysis, the team found that fish exposed to benzoylecgonine swam up to 1.9 times farther compared to the control group and dispersed about 20 miles from the release point at the end of the experiment.

“The location of the fish determines what they eat, what they eat, and how populations are structured,” said co-author Marcus Michelangeli. “If pollution changes these patterns, it has the potential to affect ecosystems in ways we are only beginning to understand.”

In addition to showing how cocaine pollution changed the way salmon use space in a natural ecosystem, the new study found that the most pronounced effect was not so much observed in the group exposed to cocaine itself, but in those exposed to its metabolite. This result has implications for monitoring, as the metabolites are often more common in waterways and current risk assessments generally focus on the main compound, potentially neglecting important biological effects.

“The idea that cocaine can have effects on fish may seem surprising, but the reality is that wildlife is already exposed to a wide variety of man-made drugs on a daily basis,” said Michelangeli. The researchers’ next step will be to be able to determine how widespread these effects are, identify which species are most at risk, and test whether changes in behavior translate into changes in survival and reproduction.

This story originally appeared on WIRED Italy and was translated from Italian.