Bottlenose dolphin abundance in the Northern Evoikos Gulf, Greece

The Mediterranean subpopulation of the common bottlenose dolphin Tursiops truncatus has been classified as Vulnerable in the IUCN Red List. However, population size data are only available for a few Mediterranean subareas, hampering understanding of bottlenose dolphin status and trends. To contribute baseline information, we assessed bottlenose dolphin abundance in the Northern Evoikos Gulf, a 960 km2 semi-enclosed basin in central Greece exposed to high heavy-industry and fishing impacts. Boat-based surveys covering the entire Gulf were conducted in October 2010 and March-April 2011, totalling 3340 km of navigation over 39 survey days. A total of 54 dolphin groups were encountered (mean size 10.0 animals, SD=5.90, range 1–30, based on 84 group size samples recorded at 60 min intervals). Dolphin group follows averaged 94 min (SD=74.3, n=54, range 2–282 min), totalling 84 h 20 min of individual photo-identification effort across 457 km. Of 5890 digital photos taken (at 18 megapixel resolution), 3141 high quality images portraying single dorsal fins were selected. The dorsal fins of 95 individuals had natural marks allowing for long-term identification. Based on the number of photos portraying marked vs. unmarked individuals, we estimated a marked proportion of 0.92. The hypothesis of population closure was rejected by the Stanley and Burnham closure test (χ2=31.34181, df=8, p=0.0012). We therefore used an open capture-recapture model (POPAN package) in program MARK 7.1 to estimate bottlenose dolphin abundance in the study area. The model yielded a population of 100 marked animals (95% C.I.=95–106), resulting in a total population of 109 (95% C.I.=101–117). We recommend continued photo-identification effort in the Northern Evoikos Gulf and contiguous waters, for longer-term status and trends analyses.