Microplastics influence size-selected zebrafish behaviour

Abstract Plastic pollution represents a major contemporary threat to aquatic ecosystems, with well-documented consequences for organismal performance and fitness across numerous taxa, including fishes. Importantly, plastic-derived stress does not occur in isolation, but interacts with other anthropogenic pressures such as size-selective harvesting, which can impose strong directional selection on life-history and behavioural traits. In this study, we exposed three experimentally evolved selection lines: large-harvested, small-harvested, and randomly harvested to microplastic contamination and quantified effects on growth and behaviour over a 14-day period. Microplastic exposure reduced boldness and exploratory activity while simultaneously increasing feeding probability and feeding frequency. Prior size-selective harvesting influenced only exploratory behaviour, suggesting that most behavioural responses to microplastics are robust to previous evolutionary history. We detected no effect of microplastics on growth, potentially due to compensatory increases in feeding behaviour. Collectively, these findings demonstrate that microplastic exposure alters key behavioural traits across genetically divergent fish lines and contribute to a broader understanding of how multiple anthropogenic stressors may interact to shape population dynamics in rapidly changing environments.