Encounter rate and relative abundance of bottlenose dolphins and distribution modelling of main cetacean species in the North Aegean Sea (Greece)

Cristina B. Milani, Adriana Vella, Pavlos Vidoris, Aristidis Christidis, Emmanouil Koutrakis, Georgos Sylaios, Argyris Kallianiotis

Conservation Biology Research Group, University of Malta, Msida MSD 2080 MALTA
Fisheries Research Institute of Kavala, 64007 Nea Peramos, Kavala, GREECE
Department of Environmental Engineering, Democritus University of Thrace, Xanthi, GREECE


The encounter rate, relative abundance and distribution of bottlenose dolphins were investigated as part of a first detailed study on cetacean populations in the Greek North Aegean Sea. Sighting data were recorded between 2005 and 2013 through dedicated scientific marine surveys, covering 14,701 km. The line-transect sampling method was used to estimate relative abundance, using Distance software 6.0. Bottlenose dolphins presented an overall encounter rate of 0.46 groups/100km (2.5 dolphin/100km) and a mean group size of 3.45 (SE=0.38). The yearly relative abundance in the area was estimated after post-stratification at 377 individuals (95% C.I.=289–465; CV=18.37). Environmental modeling has been proposed for the main species of the study area using GAM and PCA; dolphin sightings were then correlated to eight environmental variables (distance from the coast, depth, slope, gradient of salinity, median salinity, gradient of temperature, median temperature and mean current). Bottlenose dolphin sightings were significantly correlated to depth and median temperature. This research presents the first data on encounter rate, relative abundance for the species and species distribution in relation to environmental variables in study area which are essential for effective conservation and management considerations.

Keywords: DISTANCE sampling analysis, Generalized Additive Modelling (GAM), Marine Protected Areas, North Aegean Sea, Principal Component Analysis (PCA), Tursiops truncatus