Cocaine-Fueled Wild Salmon Swam Twice as Far as Sober Ones

Cocaine pollution can affect the behavior of fish—altering, for example, the way Atlantic salmon move through their environment, prompting them to swim farther and disperse over a wider 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 the wild, 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 primarily through wastewater treatment systems. Although previous research has shown that cocaine pollution can affect animal behavior, this evidence was 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 juvenile 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 that is 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 analyses, the team found that, compared with the control group, fish exposed to benzoylecgonine swam up to 1.9 times farther, dispersing at the end of the experiment about 20 miles from the release point.

"The location of the fish determines what they eat, what eats them, and how populations are structured," said coauthor Marcus Michelangeli. “If pollution is altering these patterns, it has the potential to affect ecosystems in ways we are only now beginning to understand.”

In addition to showing how cocaine pollution has changed the way salmon use space in a natural ecosystem, the new study found that the most pronounced effect was observed not so much in the group exposed to cocaine itself, but in that exposed to its metabolite. This result has implications for monitoring, since 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 might have effects on fish might seem surprising, but the reality is that wildlife is already exposed to a wide range of human-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 alterations in behavior translate into changes in survival and reproduction.