Differences in spatial distribution of two small delphinids (Delphinus delphis and Stenella frontalis) in two islands of the Azores Archipelago

Reports of the International Whaling Commission.The archipelago of the Azores represents a special area of cetacean ocurrence, mostly due to its oceanic nature. In this area two small delphinids, the common dolphin and the Atlantic spotted dolphin, the former seeming to be present all year around, while the latter seems to be seasonal (summer). In this study we analyse their respective distribution in two different islands, Pico and São Miguel. Sighting records from 2005 to 2008 in Pico, and from 2008 in São Miguel, were pooled together, analyzed and incorporated into a Geographic Information System (ArcGis 9.3). In order to understand the relations between these species and the ecogeographical variables, a statistical treatment (Kruskal-Wallis) and an Ecological Niche Analisis (using Biomapper 4) were perfomed. While common dolphins and Atlantic spotted dolphins are statistically related with depth and slope in Pico, they are just statiscally related with depth in São Miguel. Moreover there are statiscal differences between their distribution in the two islands, suggesting differences on the ecological niche of the species depending on the island, probably related with differences on bathymetry features. There are also differences in distribution between common dolphins and Atlantic spotted dolphins in São Miguel, while in Pico they seem to cohabit in the same area, possibly indicating a niche overlap. In São Miguel, statiscal differences in the distribution between the two species (in depth and in slope) are present, atlantic spotted dolphins showing more tolerance to deep waters. These results suggest the existence of differences on these two delphind populations’ distributions depending on the island, probably due to different bathymetric features. Further work is being conducted in order to better understand their distribution and interactions