From coordinated to individualized responses: familiarity and onset abruptness shape reactions to boat noise in European sea bass (Dicentrarchus labrax)

Anthropogenic noise generated by vessel traffic is a pervasive source of disturbance in marine environments, overlapping with the hearing-sensitive range of many fish species, including the European sea bass (Dicentrarchus labrax). Although behavioural responses to the specific nature of the noise have been documented, the influence of stimulus familiarity and the temporal dynamics of noise onset on these responses remains poorly explored. This study investigated how familiarity with the stimulus and onset abruptness shape acute behavioural responses to noise in European sea bass. Groups of five fishes were exposed to one of different acoustic conditions combining familiarity (familiar vs. unfamiliar boat noise) and onset profile (progressive vs. sudden), or to background noise only. Behavioural data were collected on videos across five time-intervals. Results showed that progressive familiar noise produced minimal behavioural disruption, consistent with habituation to stimuli encountered at the rearing facility. Sudden onset consistently elicited stronger acute reactions than progressive onset, and familiarity moderated both the magnitude of those reactions and the speed of recovery. Familiarity also influenced the quality of the response: sudden familiar noise triggered organized, coordinated group behaviour with rapid return to baseline, while sudden unfamiliar noise led to persistent disruption of social synchronicity, pointing to a shift toward more individualistic, less predictable responses. These findings demonstrate that familiarity and onset abruptness interact to determine both the magnitude and the quality of behavioural responses, shifting patterns from organized, group-level reactions to individualized, startle-like, disorganized responses as novelty and suddenness increase. Condition-specific fluctuations were also observed in side-tilting, suggesting a potential role of this behaviour as a displacement activity and as a component of welfare assessment in farmed fish. Taken together, these results indicate that temporal dynamics and prior acoustic experience are critical determinants of the impact of noise. The findings of this study can help define practical strategies to minimize noise-related distress and promote welfare in aquaculture settings.