Chronic boat noise exposure elevates boldness and anxiety-like behaviour, while impairing learning in juvenile cichlids Maylandia zebra

Underwater anthropogenic noise is a pervasive environmental stressor, yet its impact on the ontogeny of cognitive processes and behavioural coping styles in fish remains poorly understood. This study investigates how chronic exposure to boat noise during early development affects the behavioural phenotype and associative learning in juvenile cichlids ( Maylandia zebra ). Using a split-brood design to control for genetic variation, siblings were reared under either control (lab-silent) or intermittent boat noise conditions for 12 weeks. Subjects were subsequently tested to assess boldness, anxiety-like responses, social aggression, and aversive learning performance. Results revealed that chronic noise exposure induced a complex shift in behavioural coping styles: noise-reared juveniles exhibited faster engagement with novel objects (indicating increased risk-taking) but simultaneously displayed heightened anxiety-like avoidance in light/dark preference tests. Strikingly, noise-reared fish showed significantly impaired performance in a colour discrimination learning task conducted under silent conditions. This demonstrates that the cognitive deficit was not a result of acute attentional distraction, but rather a persistent consequence of developmental stress. Conversely, social aggression was unaffected by rearing history and decreased only during acute noise playback. These findings suggest that early-life acoustic stress reshapes the behavioural phenotype towards a high-anxiety, reactive state and imposes lasting costs on cognitive integrity, potentially compromising fitness in noise-polluted ecosystems.