Cocaine contaminated water makes salmon swim faster: study

Traces of the illicit drug in their natural environment change how fish move, say scientists.

Juvenile Atlantic salmon swam farther and dispersed more widely as a result of exposure to narcotics, according to the findings.

The study, published in the journal Current Biology, is the first to show the effects of cocaine contamination on fish behavior in the wild rather than in lab conditions.

To understand how pollutants influenced animal movement, an international research team used slow-release chemical implants and acoustic tracking to monitor 105 juvenile Atlantic salmon over eight weeks in Lake Vättern, Sweden.

The fish were assigned to one of three treatment groups: a control group, a group exposed to cocaine, and a group exposed to benzoylecgonine, the primary metabolite of cocaine that is commonly detected in wastewater.

The researchers found fish exposed to benzoylecgonine swam up to 1.9 times farther per week than unexposed fish and dispersed up to 12.3 kilometers (7.6 miles) farther across the lake.

They said the changes became more pronounced over time — indicating that exposure altered how fish used space in a complex natural ecosystem.

Study co-author Dr. Marcus Michelangeli says the findings were important because movement played a central role in how animals interacted with their environment.

He said: “Where fish go determines what they eat, what eats them, and how populations are structured.

“If pollution is changing these patterns, it has the potential to affect ecosystems in ways we are only beginning to understand.”

Cocaine and its metabolites are increasingly detected in rivers and lakes around the world, primarily entering waterways through wastewater systems that were not designed to fully remove the compounds.

While previous research has shown cocaine could affect animal behavior, those studies had been limited to laboratory settings.

The new study provides the first evidence that the effects also occurred in the wild, where animals experienced far more complex environmental conditions.

The researchers also found the cocaine metabolite benzoylecgonine had an even stronger effect on fish movement than cocaine itself.

They say that is "significant" because risk assessments typically focused on the parent compound, even though metabolites were often more common in waterways — suggesting current approaches may overlook important biological effects.

But the team emphasized the findings did not indicate a risk to people consuming fish.

They say exposure levels reflected those already found in polluted waterways, the compounds break down over time, and the fish studied were juveniles well below legal-catch size.

Dr. Michelangeli, of Griffith University’s Australian Rivers Institute, Brisbane, said the study highlighted a broader issue about the types of pollutants entering aquatic ecosystems.

He added: “The idea of cocaine affecting fish might seem surprising, but the reality is that wildlife is already being exposed to a wide range of human-derived drugs every day.

“The unusual part is not the experiment, it’s what’s already happening in our waterways.”

Dr. Michelangeli says future research would aim to determine how widespread the effects were, identify which species were most at risk, and test whether altered movement patterns lead to changes in fish survival and reproduction.