Heterogeneity from natural marks in bottlenose dolphin (Tursiops truncatus)

Monitoring trends of animal population is known as an essential part of conservation issues. Although individuals of many cetacean species have distinctive natural variation in appearance which allows identification of individuals through field photography of free-ranging individuals, the use of mark-recapture methods involves several assumptions. The most important assumption is that every animal should have equal probability of being captured within any sampling occasion. In photo-id studies of bottlenose dolphins (Tursiops truncatus) photographs of dorsal fin have been used for matching. Marks on animal were not equally distributed in the population, due to the fact that some individuals are unmarked or with not so visible mark, while others have very large marks that make them easily recognized. The goal of this study was to identify potential heterogeneity in capture probabilities between females and the rest of population. 6550 photographs were taken over four year sampling in both Ligurian and Northern Tyrrhenian Sea. Only high quality pictures were selected from the catalog and used for examination. We analyzed 492 photographs (247 left sides, 245 right sides) making a distinction between adults female, identified as the individuals seen in strong association with a calf, and the other components of the population. We identified different type of natural marks: notch, scar, protuding piece, scrape, back indentation, hole, deformation, white coloured area. Finally the prevalence, abundance and intensity of each mark types have been assessed in order to underline heterogeneity between the two groups analysed.