By SUZANNE DOWNING
April 23, 2026 – Researchers in Sweden and England report in the journal Current Biology that juvenile Atlantic salmon exposed to trace amounts of cocaine-related pollutants in the wild became significantly more active, swimming farther and dispersing more widely than normal.
The international research team, including scientists from Griffith University, the Swedish University of Agricultural Sciences, and the Zoological Society of London, conducted what is believed to be the first real-world field experiment examining how illicit drug contaminants affect fish behavior in nature.
The study focused on juvenile Atlantic salmon raised in hatcheries and released into Sweden’s Lake Vättern. A total of 105 fish were equipped with tracking transmitters and tiny implanted devices that released either cocaine, its primary metabolite benzoylecgonine, or no substance at all for comparison.
Over an eight-week period, the differences were evident.
Fish exposed to benzoylecgonine, the compound commonly found in wastewater after cocaine use, traveled up to 1.9 times farther per week than unexposed fish, covering an average of nearly 14 additional kilometers. They also dispersed significantly farther from their release point, with some traveling more than 12 kilometers beyond where control fish tended to remain.
Researchers observed that exposed fish maintained higher levels of activity throughout the study, while unaffected fish gradually settled into more typical, stable movement patterns.
The behavioral shift, described by researchers as increased “wanderlust,” could have fisheries consequences. Movement patterns play a critical role in how fish find food, avoid predators, select habitat, and return to spawning streams. Alterations to those behaviors could ripple through entire ecosystems.
Cocaine and its metabolites enter rivers and lakes primarily through human wastewater, as drug residues pass through the body and are not fully removed by treatment plants. Scientists say these substances have been detected in waterways around the world, often at low but biologically active concentrations.
While previous laboratory studies have shown drug-related effects on smaller aquatic species, this research stands out for demonstrating those impacts in a natural environment, where variables such as predators, currents, and food availability come into play.
The study did not measure long-term population impacts, focusing instead on short-term behavioral changes during the salmon’s smolt stage, a developmental period of transition toward ocean migration.
Still, researchers say the findings underscore a growing environmental concern: that trace levels of other pharmaceuticals and illicit drugs may be reshaping wildlife behavior in ways that are difficult to detect but potentially far-reaching.
Scientists note that cocaine can affect dopamine systems in the brain, which are linked to movement and reward behavior, though exactly how these mechanisms play out in wild fish would be speculative at this point.
The broader implication is that human activity is introducing biologically active substances into ecosystems in ways that are only beginning to be understood.
