Pharmaceutical contaminants reaching natural aquatic ecosystems can affect fish behaviour, modifying activity patterns, foraging behaviour and antipredator responses. While laboratory-based studies can offer key insights, assessing the ecological relevance of these findings requires field-based approaches. Therefore, we examined the effects of oxazepam, a widely prescribed anxiolytic drug, on the behaviour of a cyprinid fish (the common roach, Rutilus rutilus) in the wild, combining slow-release exposure implants with continuous tracking via acoustic telemetry. To add ecological realism, we created a landscape of fear with an uneven distribution of resources (macrophytes) and exposure to predators (pike, Esox lucius), additionally testing the effects of the drug on roach habitat selection and predator-prey interactions. Fish exposed to the drug showed an increased swimming activity and speed, but exhibited a more constrained spatial distribution in the pond, favouring areas with higher refuge availability. Both exposed and unexposed fish modified their habitat use in the presence of predators. Exposed fish appeared to get closer to the predators when these were caged, but not when predators were free-roaming. Our findings highlight the importance of considering ecological context to understand how pharmaceuticals affect fish behaviour, which is crucial for assessing risks at population and ecosystem levels.

Aquatic Ecology Unit Department of Biology Lund University Lund 223 62 Sweden

Department of Wildlife Fish and Environmental Studies Swedish University of Agricultural Sciences 901 83 Umeå Västerbotten County Sweden

Faculty of Fisheries and Protection of Waters South Bohemian Research Center of Aquaculture and Biodiversity of Hydrocenoses University of South Bohemia in Ceske Budejovice 389 25 Vodnany Czech Republic

National Institute of Aquatic Resources Danish Technical University 8600 Silkeborg Denmark

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