Boat traffic influences fish biophony in protected Mediterranean seagrass meadows: evidence supporting the need for acoustic refugia.

Recreational boating represents a primary source of underwater noise in coastal regions, however, its impacts on fish communities within critical habitats, such as seagrass meadows, remain poorly understood. In the present study, the soundscape of Posidonia oceanica meadows was recorded in 16 sites spanning two Mediterranean Marine Protected Areas (MPA Capo Caccia-Isola Piana and MPA Capo Gallo-Isola Delle Femmine) and analysed to evaluate how boat traffic influences fish biophony. Data were collected by recording for 15 min per hour over three days during the local low and high tourist seasons of 2024. To assess the effects of boat disturbance on community-level biophony, generalized linear mixed models were employed, using the number of fish sounds (acoustic abundance) and the number of different fish sound types (acoustic richness) as response variables, while accounting for spatial and temporal factors (MPA and tourist season). Furthermore, species-level responses were evaluated by analysing shifts in the acoustic features of Sciaena umbra and Scorpaena spp. sounds using generalized linear models. Results indicate that the increasing persistence of boat presence significantly reduced both acoustic abundance and richness. Moreover, signal structure was found to be altered by boat traffic: in S. umbra, peak frequency, bandwidth, and duration increased; conversely, in Scorpaena spp., bandwidth and duration increased while peak frequency decreased. These findings suggest that recreational boats reshape seagrass soundscapes by reducing biological acoustic output and inducing plastic changes in acoustic signals associated with key ecological functions. Consequently, to address protection effectiveness, in each MPA the zonation should be implemented with temporal boat restrictions and the integration of passive acoustic monitoring (PAM) into conservation management strategies.