Assessing the influence of environmental and anthropogenic factors on distribution of bottlenose and striped dolphins in the Ionian Greek waters

Bottlenose and striped dolphin distribution has been investigated in relation to environmental (slope, distance from shore and depth) and anthropogenic (n° of boats) variables, in an area of the Ionian Greek waters interested by intense boat traffic especially in the summer period. In the summers 2008, 2009 and 2010, 480 hours of survey were carried out in an area of over 2600 Km2, navigating 6500 Km. 21 sightings of bottlenose and 9 of striped dolphins were collected. The mean encounter rate (ER=n° of sightings/monitored km) resulted 0.003 for bottlenose dolphins and 0.001 for striped dolphins. The analysis of spatial data elaborated with the software Arcgis 9.3 and Maxent allowed to study the relationships among dolphins and the environmental variables, and to elaborate prediction models of distribution for both species. Statistical analysis (Mann-Whitney Test) showed the slope parameter as the most relevant environmental factor influencing the bottlenose dolphin distribution, while for striped dolphin the depth represents the significant one. For bottlenose dolphin the distribution model confirms that the most relevant parameter is slope (AUC=0.880), while for striped dolphin distance from shore resulted the most important one (AUC=0.772). In the study area boats distribution leads to a high overlapping with the presence of Tursiops truncatus. Overlapping with Stenella coeruleoalba instead is not significantly relevant because the pelagic areas in which the species is more distributed are less interested by commercial and tourist traffic. A preliminary behavioural analysis has been conducted in order to investigate if boat presence influence bottlenose dolphin behaviour. In the 68% of the analysed interactions, the bottlenose dolphins showed a positive response to the boats displaying the following behaviours: inspecting boat, bow riding and surfing. Further behavioural studies are needed to better understand if the highlighted positive interactions could affect negatively the animals in the short or long term.