Effects of vessel traffic on relative abundance and behaviour of cetaceans: the case of the bottlenose dolphins in the Archipelago de La Maddalena, north-western Mediterranean sea

Many studies have shown that vessel traffic has both long- and short-term negative effects on marine mammals. Although there has been a great expansion of recreational vessel traffic in the Mediterranean Sea in recent decades, few studies focused on this problem. Here, Bayesian models were used to explore the influence of vessel traffic on behaviour and relative abundance patterns of bottlenose dolphin in the Archipelago de La Maddalena (Italy), a coastal area included within the Pelagos Sanctuary. Results showed that season, moon phase and presence of calves had an effect on the number of adult dolphins per sighting, and that there were differences in occurrence in the sub-areas. On the contrary, the number of vessels was negatively related to the number of adult dolphins and their mean dive intervals. In particular, when more than three recreational boats were present in the area, dolphins surfaced more frequently per unit time and behaviours such as feeding and socializing were not detected. On the contrary, longer mean dive was found when fishing boats were present. Our results provide additional support for the need to consider disturbance such as vessel traffic in management plans for cetacean conservation.