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Cocaine in rivers is changing salmon behaviour, new study finds and the results are alarming
Salmon exposed to substances like cocaine travelled 1.9 times farther per week than their unexposed counterparts. Those exposed specifically to the metabolite swam approximately 7.6 miles farther over the study period, a difference researchers described as substantial, given the environmental concentrations involved.
A new scientific study has found that Atlantic salmon exposed to cocaine and its chemical byproduct in their natural habitat swim almost twice as far as unexposed fish, a finding that researchers say raises serious concerns about the growing presence of illicit drugs in the world’s waterways.
The research, conducted jointly by scientists at Australia’s Griffith University and the Swedish University of Agricultural Sciences, tracked the movements of 105 wild Atlantic salmon in Sweden’s Lake Vättern over two months. Fish were exposed to either cocaine or benzoylecgonine, the primary metabolite the drug produces in the liver, while a control group received neither substance.
What the research found
The results were striking. Salmon exposed to substances like cocaine travelled 1.9 times farther per week than their unexposed counterparts. Those exposed specifically to the metabolite swam approximately 7.6 miles farther over the study period, a difference researchers described as substantial, given the environmental concentrations involved.
Perhaps most surprisingly, it was the metabolite rather than the drug itself that produced the most pronounced behavioural change. This is significant because benzoylecgonine occurs at higher concentrations in wild waterways than cocaine does, meaning real-world exposure may be more impactful than laboratory-based risk assessments have previously assumed.
The fish exposed to both substances also dispersed farther north into the lake over time, suggesting the drugs influence not just how much salmon move but where they go, with potential knock-on effects for feeding patterns, energy expenditure, and vulnerability to predators.
Why this matters beyond the lab
Study co-author Marcus Michelangeli from Griffith University’s Australian Rivers Institute stressed that the findings reflect a genuine and growing environmental problem rather than a novel experiment. Cocaine use has been rising globally, with United Nations figures estimating approximately 25 million people used the drug in 2023 alone. As usage increases, so does the volume entering sewage systems and, from there, natural water bodies.
Existing wastewater treatment can remove much of the cocaine that enters the system, but raw sewage discharged through storm overflows and household plumbing misconnections bypasses that process entirely, delivering untreated pharmaceutical and illicit drug content directly into rivers and lakes.
Associate professor Michael Bertram at the Swedish University of Agricultural Sciences described drug contamination as not merely a public health issue but a concrete environmental challenge requiring improved wastewater monitoring and treatment infrastructure.
A wider pattern of concern
The salmon study does not stand alone. Research published last month found that sharks in the Bahamas carry detectable traces of cocaine, caffeine, and prescription painkillers in their bloodstreams. A separate 2024 study identified cocaine and benzoylecgonine in sharks off the coast of Brazil.
Scientists have previously documented freshwater shrimp in UK rivers carrying traces of dozens of pharmaceutical and illicit compounds, while other research has linked antidepressant contamination to altered risk perception in perch.
The cumulative picture that emerges from this body of research is one of ecosystems increasingly saturated with human-derived chemical compounds, with consequences for wildlife behaviour that are only beginning to be understood.
