Aquatic Contamination and Salmon Behavior: A New Perspective

The increasing presence of substances, both legal and illicit, in global waterways represents an ever-growing environmental challenge. These substances, consumed by humans and subsequently excreted into sewage systems, ultimately contaminate lakes and rivers, where they can be absorbed by wildlife. In a context of rapidly increasing global demand for cocaine, traces of the drug and its main metabolite, benzoylecgonine, have been detected in aquatic environments, raising concerns for sensitive species like Atlantic salmon.

While previous research had already highlighted a link between cocaine exposure and behavioral changes in aquatic species, such studies were conducted exclusively in laboratory settings. A clear understanding of how these contaminants affected fish behavior in their natural habitat was lacking. New research, published in Current Biology, has filled this gap, demonstrating that cocaine and benzoylecgonine can accumulate in the brains of Atlantic salmon, an ecologically and economically important species, and disrupt their movement and space use in the wild.

Study Methodology: Monitoring in Real Environments

To address this knowledge gap, the research team, led by Michael Bertram from the Swedish University of Agricultural Sciences, designed a field experiment. Over a hundred young Atlantic salmon, known as "smolts," raised in a hatchery until they were two years old, were used. These fish were divided into three groups of 35 individuals each, and every specimen was equipped with a slow-release chemical implant and tracking tags.

The first group received an implant containing cocaine, the second an implant with benzoylecgonine, while a third control group received an inert dummy implant. On April 12, 2022, all three groups were released simultaneously at the same site, on the southwestern side of Lake Vättern in Sweden, alongside 200 other smolts not involved in the experiment. For approximately two months, researchers monitored the fish's movements to assess the effects of the contaminants.

Unexpected Results and Behavioral Implications

The study's results revealed a significant impact: the exposed groups showed notably higher movement activity compared to the control group. In particular, the group exposed to benzoylecgonine traveled almost twice the distance per week compared to the unexposed smolts. This finding surprised researchers, as benzoylecgonine is generally considered psychoactively inactive in humans.

Benzoylecgonine is a long-lived byproduct of cocaine, produced by the liver and excreted in urine, and is often present in higher concentrations in natural environments than its parent compound. The discovery that this metabolite can have such a marked impact on fish behavior raises new questions about its biological importance and the possibility that metabolites can be as, if not more, disruptive than the original compound for aquatic wildlife.

Future Perspectives and Ecological Impact

The research team emphasized the need for further investigation to fully understand the mechanisms by which cocaine and its metabolites alter fish behavior and movement in the wild. It will be crucial to test the generality of these effects across other species and systems, and to use higher-resolution tracking systems to determine whether these behavioral changes affect predation risk, migration, reproduction, or population survival.

For example, the current study focused on hatchery-raised smolts released into the wild, but future research could examine the effects of these contaminants on fully wild populations, which possess unique behavioral characteristics. The urgency of unraveling the effects of these human-sourced substances is amplified by the fact that global illicit drug use has increased by approximately 20 percent over the last decade, suggesting that the environmental impact of such substances is likely to grow. Alterations in wildlife behavior and movement can have broader consequences for food webs and population dynamics, representing an additional stressor for already pressured species.