ESR prepress abstract  -  doi: 10.3354/esr00073

ABSTRACT: The Mediterranean sub-population of short-beaked common dolphin is believed to have suffered a steep decline in the Mediterranean in recent years and in 2003 it was listed as endangered in the IUCN Red List of Threatened Species. Effective conservation will depend critically on our understanding of the relationship between the species and its habitats. The Alborán Sea, at the gates of the Mediterranean from the Atlantic, is believed to be the most important remaining habitat for this species in the basin and constitutes, therefore, a vital source of information on its ecology that is essential for the development of conservation measures. We used spatial modelling to estimate the abundance and to explore the habitat preferences of common dolphins in this area, examining regional, seasonal and interannual variations, as well as the influence of biological factors such as presence of calves, interspecific relationships and behaviour. From 1992 to 2004, 37,385 km of non-systematic line transects generated 738 sightings in a 19189 km² study area. The point estimate of abundance for the whole area was 19428 (95%CI = 15277 – 22804) dolphins; an average density of 1 dolphin km^-2. Seasonal and geographical variations in abundance were detected, with higher average density in summer than in winter, and in the west (the area with a strong Atlantic influence) than in the east (the area with more Mediterranean characteristics). The different results in the Alborán Sea and the Gulf of Vera and their different physical/environmental characteristics suggest that exploration of similarities and differences in environmental features elsewhere in the Mediterranean may help us to understand why common dolphins have declined or redistributed to other unstudied areas. No overall trend in abundance was observed in the area of Alborán between 1992 and 2004, which contrasts with the apparent decline of this species in the rest of the Mediterranean Sea, and with the reported high levels of by-catch in Moroccan drift-nets in the southern Alborán Sea. However, a decline was observed in the eastern portion (Gulf of Vera) with a summer density three-fold lower in 1996 to 2004 than in 1992 to1995. A potential link of this decline with prey depletion due to the exponential growth of aquaculture in the area is discussed. Clear differences in the habitat use were also found when examining the influence of biological factors. In particular, groups with calves and groups feeding preferred more coastal waters. This result could have important implications for the development of conservation measures in the Mediterranean.