Salmon Are Getting Dosed With Cocaine Via Waterways: It’s Changing How They Move

Flushing drugs down the drain might seem like the end of the story, but for Atlantic salmon in Sweden, it’s just the beginning. Researchers tracking more than 100 juvenile salmon in one of Sweden’s largest lakes found that fish exposed to a common cocaine byproduct swam up to roughly 1.9 times farther per week than unexposed fish. They also drifted up to about 12.3 kilometers (roughly 7.6 miles) farther from where they started. Cocaine-related pollution, flushed into waterways through human wastewater, appears to be driving the difference.

Published in the journal Current Biology, this is the first study to track how cocaine pollution affects fish movement outside a laboratory. Every prior experiment on this topic took place in tanks and controlled settings that can’t replicate the unpredictable conditions of a real lake, with its currents, predators, temperature swings, and competition for food. If these findings hold true more broadly, altered movement patterns could disrupt where fish feed, how they encounter predators, and how populations spread across a region, adding yet another potential stressor for a species already losing ground to pollution, climate change, and habitat destruction.

Tracking Drugged Salmon Across a Real Lake

A team led by researchers at the Swedish University of Agricultural Sciences chose Lake Vättern in Sweden, a natural lake spanning more than 1,900 square kilometers, as their testing ground. They divided 105 two-year-old hatchery-raised Atlantic salmon into three equal groups of 35 fish.

One group received a slow-release implant containing cocaine. A second group received an implant containing the drug’s primary breakdown product, benzoylecgonine, a substance the human body produces after cocaine use and one of the most commonly detected cocaine-related compounds in waterways worldwide. The third group received a blank implant with no drug, serving as the control. Rather than mimicking waterborne chemical exposure directly, the implants were designed to simulate environmentally realistic dosing levels, the kind fish might experience near wastewater discharge points in high-use areas.

All 105 salmon were then released into Lake Vättern and tracked for eight weeks via a network of underwater receivers. A parallel laboratory experiment with 63 additional fish confirmed the drugs were reaching the brain at trace concentrations broadly consistent with what fish might absorb in waterways where treated wastewater regularly flows in, though the researchers cautioned that real-world comparisons across species remain limited. Control fish showed no detectable levels of either substance.

What the Cocaine-Exposed Salmon Actually Did

During the first two weeks after release, all three groups behaved similarly, swimming actively and covering large distances. That’s a typical pattern for hatchery-raised fish adjusting to a new environment. As weeks passed, the groups split apart.

Control fish steadily became less active and stayed in the southern portion of the lake, closer to the release site. Fish exposed to the cocaine byproduct kept moving. By the final two weeks, byproduct-exposed fish were swimming an estimated 13.7 kilometers farther per week than control fish, nearly double the distance. They also turned up on tracking receivers in the central and northern portions of the lake, far from where they started. Cocaine-treated fish showed a similar but less dramatic trend, swimming roughly 5.3 kilometers more per week than controls during the final two weeks, though that estimate carried significant uncertainty.

Why the Breakdown Product Mattered More Than Cocaine Itself

One of the study’s most notable results was that the byproduct drove bigger behavioral changes than cocaine itself. Risk assessments for pollutants typically focus on the original drug rather than what it becomes after passing through the human body and into waterways, yet benzoylecgonine is routinely found at higher concentrations than cocaine in rivers and lakes around the world. This gap could have consequences for how environmental contaminants are assessed across species.

Earlier lab research in other species has shown that this byproduct can trigger stronger biological effects than cocaine, including greater cellular stress and disruption of energy-related processes. Whether the same mechanisms operate in Atlantic salmon is still an open question.

A Growing Problem With No Easy Fix

Cocaine is among the most commonly detected illegal substances in waterways globally. Standard wastewater treatment plants were never designed to filter out these compounds, meaning they flow into rivers, lakes, and coastal waters largely unchecked. Previous research had already found cocaine and its byproducts in wild sharks off Brazil, in small crustaceans in British rivers, and in the brains of European eels in laboratory tanks. None of that work showed the pollutants changing how fish actually move in a real ecosystem.

Atlantic salmon populations have fallen sharply in recent years, driven by chemical pollution, habitat degradation, and shifting environmental conditions. For a species already fighting for survival, an invisible chemical nudge from human drug use washing through sewage systems may be one more obstacle it simply doesn’t need. As the researchers concluded, cocaine pollution “may be added to the growing list of stressors affecting fish in the wild, with yet unknown consequences for long-term population persistence.”

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Disclaimer: This article is based on a peer-reviewed study. The findings reflect the results of a single field experiment conducted on hatchery-raised fish and should not be taken as definitive conclusions about all salmon populations or all cocaine-polluted waterways. The research does not constitute medical, environmental, or policy advice.

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