Antidepressant exposure suppresses boldness and behavioural individuality across social contexts in fish

Psychoactive pharmaceuticals have been increasingly detected across the environment worldwide. Alarmingly, these pollutants disrupt ecologically important behaviours in exposed wildlife, both at the population-level and individual-level. Individual-level responses, such as the differences between individuals (i.e. behavioural individuality), are a key facilitator of population resilience and adaptive potential. Nevertheless, the effects of pharmaceutical exposure on behavioural individuality across social contexts are currently poorly understood. To address this, we investigated whether a 21-day exposure to an environmentally realistic level (1 µg l-1) of a ubiquitous pollutant, the antidepressant venlafaxine, caused population- and individual-level behavioural responses in juvenile southern pygmy perch (Nannoperca australis), when tested individually and in the presence of a conspecific. We found that, in the presence of another fish, exposed individuals were less exploratory than controls, but this effect was not observed while in social isolation. Moreover, exposed fish differed less from each other (i.e. lower behavioural individuality) in their activity in the individual trials and their exploration in the paired trials. Lastly, venlafaxine also reduced body condition in exposed fish. Our findings suggest that psychoactive pharmaceuticals can alter body condition and behaviour across ecologically relevant contexts, with potential implications for the adaptive capacity and population resilience of exposed wildlife.