Total population size is unknown but may be in the low 10,000s based on observed densities in areas that have been surveyed (Bearzi
et al. 2008). Further subpopulation structure exists and may require future assessments at a finer geographical scale. Further genetic analysis with samples from areas not yet included are strongly recommended.
A declining trend is inferred, refering specifically to the time since the early 1940s (i.e., over the last three generations). It is uncertain whether the subpopulation is still declining and if so, at what rate.
Mediterranean Bottlenose Dolphins are genetically differentiated from those inhabiting the contiguous eastern North Atlantic Ocean and Scottish waters. Based on nuclear and mitochondrial DNA analyses, distinct populations have been identified across the Black Sea and the Mediterranean Sea (Natoli
et al. 2005). The genetic analysis of 145 samples along a continuous distributional range from the Black Sea to the eastern North Atlantic (16 samples from the Black Sea, 74 from the Mediterranean Sea, 35 from the eastern North Atlantic and 20 from Scotland) found population structure with boundaries that coincided with the transitions between different types of habitat. The different zones can be characterized by ocean floor topography and by features such as surface salinity, productivity and temperature. Five populations were identified: Black Sea, eastern Mediterranean, western Mediterranean, eastern North Atlantic and Scottish. The Black Sea population showed the highest differentiation from other populations. Significant genetic differentiation was observed between populations from the eastern and the western Mediterranean. The boundary between the western Mediterranean and the eastern North Atlantic was the weakest observed, although the two populations still showed significant genetic differentiation. Despite the lack of obvious physical barriers, the eastern North Atlantic and the Scottish populations also showed genetic differences. There was genetic evidence of directional emigration of females at the extreme of the range although neither sex showed a strong bias for greater dispersal (Natoli
et al. 2005). Population structure of Bottlenose Dolphins around the Iberian Peninsula was investigated through isotopic signatures and organochlorine pollutant loads in tissues of stranded animals from Catalonia, Valencia and the Balearic Islands and adjacent Atlantic waters (Huelva and Portugal; Borrell
et al. 2006). Significant differences in stable isotopes of carbon (
13C/
12C) and in PCB congener profiles indicated that dolphins from the Atlantic and the Mediterranean do not intermingle. In the Mediterranean, dolphins from Catalonia and Valencia were indistinguishable, suggesting a common distribution area. However, dolphins from the Balearic Islands differed from those of mainland Spain in their DDT/PCB ratio and from all the other sample groups in their PCB congener profiles, suggesting that the deep waters between the Balearic Islands and the Peninsula represent an effective barrier for the species (Borrell
et al. 2006). Evidence of population structure also has been found in other Mediterranean delphinids, including Short-beaked Common Dolphins (
Delphinus delphis) (Natoli
et al. 2008), Striped Dolphins (
Stenella coeruleoalba) (Fossi
et al. 2004, Gaspari
et al. 2007b) and Risso’s Dolphins (
Grampus griseus) (Gaspari
et al. 2007). These findings suggest not only that the more obvious physical boundaries such as the Strait of Gibraltar (minimum width about 45 km and sill depths less than 145 m) and the Turkish Straits system (Dardanelles minimum width about 450 m and sill depths less than 55 m) represent barriers to the movement of individuals, but also that the much wider Sicily Channel (143 km and sill depths less than 200 m), oceanic features such as the Almería-Orán front (Tintoré
et al. 1988) and more generally differences in habitat characteristics tend to limit the movements of Bottlenose Dolphins (Natoli
et al. 2005).
There is no basin-wide estimate of numbers and the most reliable information comes from a few local studies (Bearzi et al. 2008). Useful information on past and present occurrence also comes from stranding records from Algeria, Croatia, France, Greece, Italy, Malta, Morocco, Spain, and Tunisia (however, a stranded carcass is not necessarily indicative of the dead animal’s having lived nearby). Virtually nothing is known for large portions of the south-eastern part of the basin (Bearzi et al. 2008c). Most studies in coastal waters are limited to areas of 400 to 1,000 km² and probably do not even cover the entire ranges of the groups under study. Relatively recent, broad-scale shipboard surveys (4,000–80,000 km²) showed that in some Mediterranean areas Bottlenose Dolphins are present both near shore and offshore, and densities can range between four and 20 animals per 100 km² (Ben Naceur et al. 2004, Forcada et al. 2004, Cañadas and Hammond 2006, Gómez de Segura et al. 2006, Bearzi et al. 2008c). Studies tend to focus on areas of relatively high dolphin density, although study site preferences also may depend on logistical and other considerations. Although the total population size in the Mediterranean remains uncertain, it unquestionably exceeds the threshold level for red listing as Vulnerable under the D criterion.
Indirect but convincing evidence of dolphin abundance in historical times can be found in early accounts describing interactions with fisheries and systematic attempts to exterminate dolphins (including Bottlenose Dolphins) in Mediterranean coastal waters (Bearzi et al. 2008).
The only Mediterranean area with quantitative historical information that can be used to infer population trends over time scales of more than a couple of decades is the northern Adriatic Sea. There, Bottlenose Dolphin numbers likely declined by at least 50% in the second half of the 20th century, largely as a consequence of deliberate killing initially, followed by habitat degradation and overfishing of prey species (Bearzi
et al. 2004). For some other parts of the northern Mediterranean, e.g. Italy and southern France, the available information is less precise but suggests similar trends (Blanco and González 1992, Borrell
et al. 2000). In an area off southern Spain where the species has been studied intensively, abundance estimates have shown variability but no trend since the early 1990s (Cañadas
et al. 2006).