Abundance and habitat preferences of the short-beaked common dolphin Delphinus delphis in the southwestern Mediterranean: implications for conservation

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 is believed to be the most important remaining Mediterranean habitat for this species, and thus constitutes a vital source of information for the development of conservation measures. We used spatial modelling to estimate the abundance and explore the habitat use 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 19 189 km 2 study area. The point estimate of abundance was 19 428 (95% CI = 15 277 to 22 804) dolphins. Seasonal and geographical variations in abundance were detected, with higher average density in summer than in winter, and in the Western Alborán Sea than in the east Gulf of Vera, which has different physical/environmental characteristics. No overall trend in abundance was observed in the Alborán area. However, a decline was observed in the Gulf of Vera, with a summer density 3-fold lower in the period from 1996 to 2004 than in 1992 to 1995. A potential link of this decline with prey depletion due to the exponential growth of aquaculture in the area is discussed. Clear differences in habitat use were also found when examining the influence of biological factors. In particular, groups with calves and groups that were feeding preferred more coastal waters. This result could have important implications for the development of conservation measures for this species in the Mediterranean.

Understanding the Patterns and Causes of Variability in Distribution, Habitat Use, Abundance, Survival and Reproductive Rates of Three Species of Cetacean in the Alboran Sea, Western Mediterranean

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The question of how environmental variability affects populations of marine top predators is an important one because of their role within ecosystems and their potential to influence community structure and biodiversity (Heithaus et al 2008). An understanding of the patterns of distribution and abundance and particularly the causes of that variation is critical to making informed assessments of the importance of anthropogenic activities to marine mammal populations.

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Modelling sperm whale habitat preference: a novel approach combining transect and follow data

“…Benoit-Bird & Au 2003, Hastie et al 2004, Frederiksen et al 2006, Friedlaender et al 2006. Other potentially important factors affecting habitat choice include behavioural state, the presence of calves, inter specific relationships (Cañadas & Hammond 2008), predation risk (e.g. Heithaus & Dill 2002), competition (e.g.…”

Habitat preferences among three top predators inhabiting a degraded ecosystem, the Black Sea

“…In the western Atlantic however, most areas of common dolphin distribution coincide with moderate to strong upwelling and common dolphins appear to avoid warm waters (Jefferson et al 2009). Highly productive areas have also been reported in other areas to be the preferred habitat of this species (Reilly 1990, Fiedler and Reilly 1994, Cañadas and Hammond 2008. A review of satellite data in the Black Sea indicates persistent upwelling in summer towards the Turkish coast (see Ozsoy and Unluata 1997), where our prediction map for the species in summer shows the higher probability of occurrence (Fig.…”