Population size estimate and level of associations of striped dolphin (Stenella coeruleoalba) in the gulf of Corinth (Ionian sea, Greece

Striped dolphin is considered Vulnerable by IUCN at the Mediterranean level but, despite it, is the most abundant cetacean species in the basin, few studies has been carried out to analyse population size and social structure in the Greek waters. This study aimed to fill this gap focusing on the population living in the Gulf of Corinth using mark-recapture method. Data were collected during the summer period between 2009 and 2011. 78 surveys consisting in 186.41 monitoring hours, along 2114.66 km were carried out, 233 individuals have been photo-identified, 35% of which were sighted more than once. Cormak-Jolly-Seber (CJS) open population model in program MARK was used to estimate population size of marked individuals Nm = 611.81, with an apparent average survival rate of 0.85 (SD = 0.06) and average capture probability of 0.22 (SD = 0.22). Associations among individuals were considered at the pairs and group levels, of photo-identified individuals during 2010 and 2011. 69 couples were sighted twice and 4 couples three times. 14% of couples were observed with annual latence. During this period, 7 groups (composed at least of three individuals) were sighted twice. 46 individuals were sight both years. This study shows a high density of striped dolphins in the area and a complex social structure. Analysis of population trends and further researches on social structure will be useful to assess association stability and residency in coming years.