Exposure to the Oestrogenic Pollutant 17α‐Ethinyl Estradiol ( <scp>EE2</scp> ) Does Not Alter the Behaviour of a Coastal Marine Fish, <i>Cristiceps australis</i>

ABSTRACT Escalating production and consumption of medications globally has led to a sharp rise in environmental contamination with pharmaceuticals—including 17α‐ethinyl estradiol (EE2), a synthetic oestrogen used in the contraceptive pill. EE2 is now commonly detected in various ecosystems and poses risks to wildlife. Although its effects on behavioural traits are well‐documented, available data are restricted to a handful of model organisms. Therefore, the generality of such effects has not been widely considered, particularly for marine fishes. Accordingly, we investigated the effects of environmentally relevant EE2 concentrations on boldness and anxiety in a marine fish, the southern crested weedfish ( Cristiceps australis ). Weedfishes inhabit shallow coastal waters, which are often contaminated with EE2, and rely on vegetation for predator avoidance. Therefore, investigating the impact of EE2 on their refuging behaviour is critical to understanding threats to their survival. Fish were exposed for 14 days to one of three treatments: control (no added EE2), low‐dose EE2 (33 ng/L), or high‐dose EE2 (70 ng/L). Following exposure, a standard refuge‐use assay was used to assess boldness and anxiety. On the basis of observations in model species, we hypothesised that EE2 exposure would alter boldness and anxiety. Contrary to our predictions, EE2 exposure had no effect on behaviour. These findings contrast with behavioural effects reported at similar concentrations in freshwater fishes, possibly because of species‐specific physiological differences or marine environmental factors that influence the drug's uptake. Possible interspecies variability underscores the need to investigate a broader range of taxa beyond traditional ecotoxicology models to better understand EE2's ecological impact.