| Summary: | International audience In the Anthropocene, the current biodiversity losses caused by human activities have various features, from individuals to populations, species, and ecosystems. Chemical pollution is one of the major current threats, especially because Wastewater Treatment Plants do not reduce or treat psychiatric drugs leading to the chemical pollution of aquatic environments. Most of the studies in ecotoxicology focus on the effects on survival or development, losing sight of the potential effects on biodiversity. Individual behavior and the related intraspecific diversity are the main components of it, even though overlooked.Here, we assessed the effects of a psychiatric drug on the European glass eels’ migratory behavior. Glass eels are eel juveniles migrating from the sea towards rivers crossing estuaries and WWTP effluents to colonize hydro-systems. Usually, individuals colonize and distribute themselves from estuaries to upstream river systems leading to changes in local density, which drive sexratio and individual growth. Since we used an anxiolytic, we hypothesized that contaminated individuals would be bolder and less active than controlindividuals. We put 40 control individuals and 40 individuals contaminated with diazepam at an environmentally relevant concentration through 7-day chronic contamination in a behavioral monitoring tank that mimics a river. This tank is like an annular tank enabling individuals to swim continuously either against the current (i.e., upstream movement) or with the current (downstream movement) and us to study migratory behavior in experimental facilities. We video-recorded around 1-min sequences over 7 days since the depuration of diazepam by glass eels takes 7 days. We also video-recorded the first 15 minutes since individuals hide directly under the gravel at the tank bottom as soon as we placed them in the tank. The time it took them to get out of their shelter is a proxy for their boldness.We found the anxiolytic contamination led individuals to be bolder but ...
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