Cocaine pollution from humans is altering the behavior of juvenile Atlantic salmon in the wild, according to a study published in Current Biology. Fish exposed to the cocaine metabolite benzoylecgonine swam up to 1.9 times farther per week and dispersed up to 12.3 kilometers wider across a lake than unexposed fish.
The study, conducted by researchers at the Swedish University of Agricultural Sciences (SLU), is described as the first to demonstrate effects of cocaine contamination on fish behavior in the wild, the researchers said. Over eight weeks, the team used 105 juvenile Atlantic salmon in Lake Vättern, Sweden, to examine how exposure to cocaine and its primary metabolite, benzoylecgonine, affected their movement and boldness. The findings, published in Current Biology, showed that fish exposed to cocaine were bolder and swam almost twice the distance compared to unexposed fish, while those exposed to benzoylecgonine showed even more pronounced changes, swimming up to 1.9 times farther per week and spreading over wider areas.
Interestingly, the cocaine metabolite benzoylecgonine had a greater impact on fish movement than cocaine itself, the study found. This suggests that current risk assessments may be missing crucial biological effects by focusing on the parent compound rather than its metabolites, the researchers said. Benzoylecgonine is more stable and persists longer in the environment, potentially leading to prolonged exposure for aquatic organisms.
Cocaine and its metabolites enter waterways primarily through sewage systems, as human consumption leads to excretion of the drug and its breakdown products. Cocaine is one of the most detected illicit drugs in aquatic environments worldwide, according to research from multiple sources. A global analysis found average surface water concentrations of about 105 ng/L for cocaine and 257 ng/L for benzoylecgonine, with maximum concentrations reaching thousands of ng/L. Human drugs are making their way into rivers, lakes and oceans around the world, raising concerns about their ecological impact.
The findings add to a growing body of evidence that pharmaceutical and illicit drug contamination affects wildlife. A 2024 study from Brazil found cocaine in the muscles and liver of wild sharks off the coast of Rio de Janeiro. These compounds target brain systems shared across many animals, so even small amounts can affect wildlife, according to research. Changes in behavior are often one of the earliest and most sensitive indicators of environmental effects on wildlife, researchers have noted.
The altered behavior carries ecological risks. Professor Tomas Brodin of SLU said that increased movement makes fish more vulnerable to predators. Dispersing over larger areas may also affect feeding, reproduction, and exposure to other stressors. However, eating the fish is not dangerous for humans because concentrations are low and drugs disappear from the fish as they grow, the researchers said.
To address the issue, researchers are calling for improved water treatment and new regulations to remove drug residues from waterways. Current wastewater treatment plants are not designed to eliminate such contaminants, and upgrading infrastructure could reduce the ecological impact. The study highlights the need for broader monitoring of pharmaceutical and illicit drug pollution in aquatic environments.
Several unknowns remain. The long-term ecological consequences of altered fish behavior due to cocaine pollution are not yet understood. It is also unclear how widespread cocaine contamination is in aquatic environments globally, as monitoring is limited. The specific concentrations of cocaine and benzoylecgonine used in the study were not disclosed, and the mechanisms by which benzoylecgonine affects fish behavior require further investigation. Additionally, whether other drugs or contaminants have similar behavioral effects on wildlife remains an open question.