Researchers from Sveriges lantbruksuniversitet and Imperial College London conducted the study in Lake Vättern, Sweden, using innovative methods to track fish behavior over eight weeks. They implanted 105 juvenile Atlantic salmon with slow-release chemical implants and acoustic tracking devices to monitor their movements in real-world conditions. The fish were divided into three treatment groups: a control group, a group exposed to cocaine, and a group exposed to benzoylecgonine, which is the primary metabolite of cocaine. This approach allowed scientists to observe how these substances influence salmon in a complex natural ecosystem rather than controlled lab settings.
Key findings from the study show that benzoylecgonine had a profound impact on salmon movement. Fish exposed to this metabolite swam up to 1.9 times farther per week than unexposed fish and dispersed up to 12.3 kilometers (7.6 miles) wider across the lake. According to the study, the cocaine metabolite benzoylecgonine had a greater impact on fish movement than cocaine itself. The changes in fish behavior became increasingly pronounced over time, suggesting exposure altered how fish occupied space within a complex natural ecosystem, as noted by researchers.
The study highlights a global context where cocaine and its metabolites are increasingly detected in rivers and lakes worldwide, entering waterways primarily through sewage systems not designed to fully filter them out. Cocaine is one of the most detected illicit drugs in aquatic environments worldwide, according to research. A global analysis found average surface water concentrations of about 105 nanograms per liter for cocaine and 257 nanograms per liter for benzoylecgonine, with maximum concentrations reaching thousands of nanograms, according to research.
Scientific concern is mounting because even low concentrations of cocaine and benzoylecgonine are a concern because they target brain systems shared across many animals, potentially affecting wildlife, researchers say. According to research, changes in behavior are often early and sensitive indicators that something in the environment is affecting wildlife. Dr. Marcus Michelangeli, a study co-author, emphasized this point in a press release, stating that where fish go determines what they eat, what eats them, and how populations are structured, and if pollution is changing these patterns, it has the potential to affect ecosystems in ways we are only beginning to understand.
Related evidence supports these concerns, as a 2024 study from Brazil found cocaine in the muscles and liver of wild sharks caught off the coast of Rio de Janeiro, according to research. This indicates that cocaine pollution is not limited to freshwater systems and can accumulate in marine predators, raising questions about broader ecological impacts.
Despite these findings, several unknowns remain regarding the long-term ecological consequences of altered salmon behavior due to cocaine pollution. Researchers are uncertain how increased movement and dispersal might affect population dynamics, predator-prey relationships, and overall ecosystem stability in Lake Vättern and similar environments. Additionally, the specific mechanisms that cause benzoylecgonine to have a greater impact on fish behavior than cocaine itself are not fully understood, requiring further investigation into biochemical pathways and neurological effects.
Another key unknown involves the limitations of current wastewater treatment methods in removing cocaine and its metabolites. While sewage systems are a primary source of contamination, effective removal techniques are not widely implemented, and research is needed to develop better filtration and degradation processes. Moreover, the broader wildlife impacts beyond studied species like salmon and sharks are unclear, as other aquatic and terrestrial animals may also be vulnerable to cocaine pollution in their habitats.
Implications of this research underscore the need for enhanced monitoring and proactive measures to address pharmaceutical and illicit drug pollution in aquatic environments. Scientists advocate for integrating behavioral studies into environmental risk assessments to detect early warning signs of ecological disruption. Policymakers and water management authorities must consider upgrading wastewater treatment infrastructure to mitigate contamination sources. Public awareness campaigns could also help reduce drug disposal practices that contribute to pollution, fostering a collaborative approach to safeguarding water quality and biodiversity.