Seasonal variability in cetacean presence in the Pelagos Sanctuary: implication for conservation purposes

A large scale cetacean monitoring program using four ferry routes has been realized within the PELAGOS Sanctuary by a network of research organisms. The sampled routes were quite regularly distributed over the Sanctuary: Tolon-Ajaccio (TA) in the western part, Nice-Calvi (NC) and Savona-Bastia (SB) in the central area and Leghorn(Livorno)-Bastia (LB) in the eastern part. All routes were monitored once a week all year round on TA and SB (February 2011-March 2012) and during the summer on NC and SB. Results confirm the high spatio-temporal variability occurring within the Pelagos Sanctuary both in species presence and relative abundance. The four frequently sighted species were Stenella coeruleoalba (Sc), Balaenoptera physalus (Bp), Tursiops truncatus (Tt), Physeter macrocephalus (Pm). Grampus griseus, Delphinus delphis, Globicephala melas, Ziphius cavirostris where sighted at least on one of the central routes (NC and SB). The effort-weighted species richness varied a lot on inter- and intra-line levels and by seasons. During summer, the higher Encounter Rates (ER=sightings/100 km on-effort) per species were recorded in the western part of the sanctuary (ERNC=6,284±0,674; ERTA=3,617±0,306). Dolphin species distributions reflect respective habitat preferences: the higher ER of Sc was recorded during summer on NC and the higher ER of Tt was obtained on LB. ER of Bp showed inverse seasonal line-trends on the two borders of the Pelagos Sanctuary: the higher ER on TA was obtained during the summer and it corresponded to the lower ER recorded on SB. This trend may point out different uses of habitat. Pm showed a clear preference for the western area (TA) during summer season. The study highlights how species ER and species richness vary a lot according to seasons and areas. All large-scale and inter-season results give clear inputs for species conservation. These results show how the monitoring with regular sampling using ferries is valuable.