Researchers tracked 105 juvenile Atlantic salmon released into Sweden’s Lake Vättern after fitting them with slow‑release implants that emitted cocaine, benzoylecgonine, or nothing. Fish exposed to benzoylecgonine swam up to 1.9 times farther per week (nearly 14 km) and dispersed about 12 km farther from the release site (~60% increase) than controls. The metabolite had stronger effects than cocaine itself, raising concerns because metabolites are often more prevalent in waterways. Experts note implants aren’t a perfect mimic of environmental exposure, but the findings suggest drug pollution could alter wildlife behaviour and ecosystem dynamics.
Cocaine Pollution Alters Salmon Behavior — Metabolite Makes Juveniles Swim Farther, Study Finds
Illustration of a fly fisherman battling a leaping Atlantic Salmon (Salmo salar) on a pristine river (by W Blackwood Law, Canadian), 1950.(GraphicaArtis/Getty Images)
Exposure to cocaine-related compounds in waterways appears to change how far juvenile Atlantic salmon travel, a new field study shows. Researchers released tagged fish into Sweden’s Lake Vättern after fitting them with slow‑release implants that delivered either cocaine, benzoylecgonine (the main cocaine metabolite), or nothing. The results, published in Current Biology, point to unexpected ecological consequences from widely detected drug residues in aquatic environments.
Study Design and Main Results
The team, led by aquatic ecologist Jack Brand at the Swedish University of Agricultural Sciences, tracked 105 captive-reared juvenile Atlantic salmon (Salmo salar). The fish were split into three equal groups of 35: one implanted with devices releasing cocaine, one releasing benzoylecgonine, and a control group with inert implants. After release, acoustic tags recorded their movements around the lake.
Salmon exposed to benzoylecgonine swam substantially farther than controls—on average up to 1.9 times farther per week (nearly 14 km). They also dispersed more widely from the release site, reaching as much as ~12 km farther than nonexposed fish, an increase of about 60 percent. Benzoylecgonine produced stronger movement effects than the parent drug, cocaine.
“Many aquatic organisms in human-impacted environments are living in a dilute cocktail of biologically active chemicals,” said Jack Brand. “We’re only really scratching the surface.”
Why It Matters
There are several potential ecological consequences. Fish that swim farther may expend energy that would otherwise fuel growth and foraging, risk entering unsuitable habitats, or encounter different predators and prey. Because benzoylecgonine is often found at higher concentrations than cocaine in wastewater-affected waters, its outsized behavioral effects raise questions about how environmental risk assessments are conducted.
Experts not involved with the work noted the study’s advance over lab-only experiments but cautioned that slow‑release implants do not perfectly mimic real-world exposure via contaminated water. Still, similar behavioral and physiological impacts have been reported in European eels, crayfish and other aquatic animals, and recent surveys have detected cocaine residues in wild fish, including sharks.
Public Health Note
Brand and colleagues emphasize that contaminant levels measured in these fish are far below thresholds of concern for human consumers. Typical culinary uses—whether a roasted fillet or lox on a bagel—remain safe.
Bottom line: Drug metabolites from human wastewater can alter animal behavior in the wild, and benzoylecgonine in particular may change how juvenile salmon move and disperse—effects that could ripple through freshwater ecosystems.
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