Cocaine Exposure Alters Wild Salmon Behavior, Study Finds

Cocaine Pollution Disrupts Wild Salmon Behavior

Salmon exposed to cocaine and its primary metabolite, benzoylecgonine, exhibit significant behavioral changes compared to unexposed fish, according to a first-of-its-kind study conducted in the wild. The research, published on Monday, May 13, 2024, in Current Biology, highlights the ecological impact of drug pollution on aquatic species.

How Cocaine Enters Waterways

Many global waterways are contaminated with legal and illegal substances, including cocaine, which enters sewage systems through human consumption and excretion. As cocaine demand rises, traces of the drug—including benzoylecgonine—flow into lakes and rivers, where wildlife like Atlantic salmon absorb it. While prior lab studies linked cocaine exposure to behavioral changes in aquatic species, this is the first to examine the effects on fish in their natural habitat.

Study Design and Findings

The research team, led by Michael Bertram, an associate professor at the Swedish University of Agricultural Sciences, sought to address a critical gap in scientific literature. “Almost everything known about cocaine pollution’s impact on animal behavior comes from laboratory settings,” Bertram explained in an email to 404 Media. “We wanted to determine whether environmentally realistic exposure to cocaine and benzoylecgonine alters fish movement in the wild under real ecological conditions.”

The team divided 105 Atlantic salmon smolts (young fish) into three groups:

• Cocaine group: Received a slow-release cocaine implant.
• Metabolite group: Received a slow-release benzoylecgonine implant.
• Control group: Received a dummy implant with no chemicals.

Each fish was equipped with tracking tags and released simultaneously on April 12, 2022, at Lake Vättern in Sweden, alongside 200 other smolts not involved in the experiment. Over two months, the exposed groups—particularly the metabolite group—traveled significantly farther than the control group. The metabolite group moved 1.9 times farther per week than unexposed smolts.

“We expected an effect of contaminant exposure on the movement of salmon, but the scale of the changes seen still surprised us. The strongest response was close to a two-fold increase in movement, and the most unexpected result was that benzoylecgonine, the main metabolite of cocaine, produced the clearest effect rather than cocaine itself.”

Ecological and Conservation Implications

The study underscores the broader risks of pharmaceutical and illicit drug pollution in aquatic ecosystems. Atlantic salmon, an ecologically and economically vital species, face heightened conservation concerns due to such contaminants. The findings suggest that drug pollution may disrupt natural behaviors, potentially affecting survival and reproduction.

Key Takeaways

• Cocaine and its metabolite benzoylecgonine accumulate in the brains of wild Atlantic salmon.
• Exposed salmon traveled nearly twice as far as unexposed fish over two months.
• Benzoylecgonine, not cocaine itself, produced the strongest behavioral effect.
• This is the first study to observe cocaine’s impact on fish in the wild, not in a lab.