Widely detected in global waterways, clobazam – a common drug used to treat anxiety – is altering the migration behavior of wild Atlantic salmon, according to a new study. The findings underscore the far-reaching ecological consequences of pharmaceutical pollutants, revealing how even trace levels of psychoactive drugs can disrupt essential survival behaviors in wildlife. Pharmaceutical pollution – particularly in waterways – is a growing environmental concern that poses a serious threat to biodiversity, ecosystem functioning, and public health. More than 900 active pharmaceutical or pharmaceutical-derived compounds have now been detected in waterbodies across the globe, including Antarctica. These contaminants, designed to target conserved neurobiological pathways and remain effective at low concentrations, persist in the environment, and even trace amounts of psychoactive pharmaceuticals, such as antidepressants and anxiolytics, have been shown to alter animal behavior by acting on neural pathways. While laboratory studies shed light on behavioral effects, they often fail to capture the complexity of natural ecosystems, leaving ecological impacts uncertain. Through laboratory assays and multi-year field experiments, Jack Brand and colleagues investigated the impact of psychoactive pharmaceutical pollutants on the behavior of Atlantic salmon. Brand et al. discovered that the anxiolytic drug clobazam – a common pharmaceutical pollutant – accumulated in the brains of exposed salmon, altering their ability to navigate dam passages and overall river-to-sea migration success. Specifically, the findings revealed that clobazam exposure increased the number of salmon smolts reaching the sea, likely due to heightened risk-taking and reduced shoaling (grouping) behavior. While there were no significant differences in overall migration speed, exposed smolts moved through hydropower dams more quickly, suggesting that increased risk-taking behavior facilitated barrier navigation. However, lab experiments revealed that clobazam also reduced shoaling cohesion, particularly in the presence of predators, which could heighten predation risk in the wild. According to the authors, the findings highlight the complex ecological consequences of pharmaceutical pollution, as behavioral changes induced by psychoactive drugs may both aid migration and increase vulnerability to natural threats.
"Our findings raise important questions about how pharmaceutical pollution alters migration behavior and survival in the wild," said Brand. "Next, we aim to track fine-scale movements of exposed fish using high-resolution animal tracking tools and miniature biologgers – tiny electronic tags that record physiological data such as stress levels or detect predation events – to determine whether behavioral changes from pharmaceutical pollution influences predation risk. Expanding our understanding of how different psychoactive pollutants and their interactions affect migration success will be crucial for predicting the long-term impacts on fish populations. This is especially important in an increasingly polluted world, where evidence-based policies are needed to protect vulnerable species and ecosystems."
