Opportunistic data: what supply for the knowledge on cetaceans ?

The data of opportunistic sightings compiled in this study range from 1996 to 2016 and come from different categories of actors of the sea. A sorting of these data was carried out, and 56 % of the sightings were considered 'reliable' or 'probable' and were exploited in cartographic analyzes. The objective of this work was to study the contribution of opportunistic data for knowledge on cetaceans. Several elements were analyzed: (i) the spatial and temporal limits of coverage of these data according to distribution and the operating mode at sea of data providers; (ii) the correspondence of the results on the seasonal stands obtained compared with those observed in the scientific literature in the north-western Mediterranean Sea; (iii) the representativeness and/or complementarity of the results of this dataset with the known scientific results about calves and size of groups. The analyzes of this study showed results most often in agreement with what is already known scientifically from animals in the Pelagos Sanctuary. The supply of opportunistic data is interesting from a spatio-temporal point of view only when the suppliers are in small sampled areas and/or outside the summer period.