Recreational boating as a potential stressor of coastal striped dolphins in the northwestern Mediterranean Sea.

Striped dolphin (Stenella coeruleoalba) is the most abundant cetacean species in the western Mediterranean Sea. Coastal populations are locally exposed to intense recreational boating, a growing activity over the last thirty years. Dedicated boat surveys carried out since 1988 (13,896 km of effort), enabled to map relative abundance for two periods, 1988-2003 and 2004-2019, which evidenced a significant decrease of habitat use in the inshore part of study area. Coastal traffic was surveyed from a shore lookout located in Cap d'Antibes (French Riviera) during 47 daily sessions from May 2017 to April 2018: traffic flow often exceeded one boat per minute in summer, with a majority of motorboats. Underwater recordings showed that inshore noise was about 10 dB higher than in the open sea, with much energy being propagated by fast boats, including in the medium to high frequency domain. Ambient noise data collected during spring 2020 lockdown evidenced a clear noise level decrease compared to normal situations. Although other stressors may not be neglected, this study suggested that intense motorboat traffic is a likely contributor to the observed striped dolphin partial habitat loss.