For the first time, scientists have shown that cocaine pollution doesn’t just alter fish behavior in a lab — it does so in the wild, with consequences that could ripple through entire ecosystems.

An international team led by Griffith University and the Swedish University of Agricultural Sciences tracked 105 juvenile Atlantic salmon over eight weeks in Lake Vättern, Sweden’s second-largest lake. The fish were divided into three groups: one received slow-release cocaine implants, another received benzoylecgonine (cocaine’s primary metabolite), and a third served as a control. All were fitted with acoustic transmitters and monitored via sensors around the lake.

The results, published in Current Biology, were striking. In each of the study’s final two weeks, cocaine-exposed salmon swam roughly 5km further than controls. But the real surprise was the metabolite group: those fish swam nearly 14km more per week — about twice as far — and dispersed up to 12.3km farther across the lake.

“It was really the metabolite, which we know occurs at higher concentrations in the wild, that had the much more profound effect on fishes’ behaviour and movement,” said Dr Jack Brand of the Swedish University of Agricultural Sciences. That matters because environmental risk assessments typically focus on the parent compound, potentially missing a large chunk of the actual threat.

Cocaine and its metabolites enter waterways primarily through wastewater systems not designed to fully remove them. A major source is raw sewage from storm overflows and household plumbing misconnections.

More swimming means more energy burned and more time exposed to predators. “Where fish go determines what they eat, what eats them, and how populations are structured,” said co-author Dr Marcus Michelangeli of Griffith University’s Australian Rivers Institute. “If pollution is changing these patterns, it has the potential to affect ecosystems in ways we are only beginning to understand.”

The exposure levels mirrored those already found in polluted waterways worldwide. As Michelangeli put it: “The unusual part is not the experiment, it’s what’s already happening in our waterways.”

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