Small recreational vessels reshape the seasonal coastal soundscape of the Cabo de San Antonio Marine Reserve.

Seasonal recreational boating activity can strongly modify underwater soundscapes in shallow coastal environments, although these sources remain poorly represented in conventional monitoring frameworks largely based on AIS-tracked commercial shipping. This study evaluates vessel-related underwater noise at the boundary of the Cabo de San Antonio Marine Reserve (western Mediterranean Sea) using long-term passive acoustic monitoring conducted between 2021 and 2024. Vessel passages were identified through visual inspection of spectrograms and analyzed using decidecade Sound Pressure Level (SPL) band measurements between 63 Hz and 1 kHz together with event-based exceedance and vessel-occupancy related metrics. Results revealed a marked seasonal increase in anthropogenic acoustic pressure during summer months, with August presenting the highest SPL, exceedance and Hourly Vessel Occupancy (HVO) values. Daytime HVO reached median values close to 60 min h-1, indicating an almost continuous presence of vessel passages during summer daylight hours. Spectral analyses further showed that vessel-noise signatures were frequently dominated by bands above 125 Hz, with exceedance progressively increasing towards higher frequencies. These findings highlight the importance of experimental monitoring in coastal environments dominated by recreational and non-AIS vessels and suggest that complementary higher-frequency indicators may be necessary to adequately characterize underwater-noise conditions in shallow protected coastal areas.