The study, conducted by researchers at Sveriges lantbruksuniversitet (SLU), found that fish exposed to benzoylecgonine swam up to 1.9 times farther per week than unexposed fish. They also dispersed up to 12.3 kilometers farther across the lake, and these changes became increasingly pronounced over time, according to the study published in Current Biology. Salmon that ingested cocaine residues became significantly more active and daring, according to the study from SLU.
The cocaine metabolite benzoylecgonine had a greater impact on fish movement than cocaine itself, the study found. Fish exposed to cocaine showed a similar pattern but the effect was weaker and less consistent. Cocaine is broken down mainly into benzoylecgonine after consumption, according to research from multiple sources.
Cocaine and its metabolites are increasingly detected in rivers and lakes globally, entering via sewage systems, multiple studies have shown. Cocaine is one of the most detected illicit drugs in aquatic environments worldwide, according to research. Average surface water concentrations are about 105 ng/L for cocaine and 257 ng/L for benzoylecgonine, with maximums in the thousands, according to a global analysis. Wastewater treatment plants are not designed to fully remove cocaine and its metabolites, according to research.
Evidence from other species underscores the widespread nature of contamination. Cocaine has been detected in the muscles and liver of wild sharks off the coast of Rio de Janeiro in a 2024 study from Brazil. Human drugs are making their way into rivers, lakes, and oceans worldwide, according to research from two sources. These compounds target brain systems shared across many animals, so even small amounts can affect wildlife, according to research.
The long-term ecological consequences of these behavioral changes for salmon populations remain unknown. The concentrations of cocaine and benzoylecgonine used in the study and how they compare to real-world environmental levels have not been disclosed. It is also unclear whether there are observed effects on other aquatic species beyond salmon, what specific mechanisms cause benzoylecgonine to have a stronger effect than cocaine, and what measures could be taken to reduce pharmaceutical pollution in waterways.