The case for regarding Mediterranean Common Dolphins as a distinct subpopulation is not perfect, and admittedly rests upon a somewhat complicated chain of inference. Genetic studies indicate a significant level of divergence between Mediterranean and Atlantic populations (Natoli et al. in press). Differences in contaminant levels between dolphins from the Alboràn Sea (northwestern Mediterranean) and Atlantic Ocean also suggest a certain degree of isolation. Organochlorine concentrations in Alboràn Sea dolphins were about double those typical of dolphins in neighboring North Atlantic waters and showed a completely different profile (proportions between PCB congeners, the DDE/tDDT ratio, etc.) (Borrell et al. 2001). Genetic exchange between Common Dolphins from the Mediterranean Sea and Atlantic Ocean, to the extent that it occurs, appears to involve only animals from the Alboràn Sea (Natoli et al. in press), possibly due to oceanographic features such as the Almería-Orán thermohaline front.
At the eastern end of the Mediterranean, there is little indication of movement by Common Dolphins through the narrow Dardanelles Strait between the Aegean and the Marmara and Black Seas, where Common Dolphins are known to occur (Öztürk and Öztürk 1997, Frantzis et al. submitted). A preliminary study of skull morphometrics (Amaha 1994) suggested differences between Black Sea and Mediterranean Common Dolphins. In contrast, a genetic comparison of relatively small samples (8 Black Sea, 20 central Mediterranean) revealed no significant differences (Natoli et al. in press). Clearly, further work based on larger samples is needed to assess and characterize the relationship between Black Sea and Mediterranean Common Dolphins. It is acknowledged that some genetic exchange might occur in portions of the Aegean Sea where favorable habitat still exists (e.g., in the Thracian Sea; Frantzis et al. submitted). However, what remains between the Aegean and Alboràn sectors of the Mediterranean seems to be only isolated, remnant groups (possibly indicative of further population substructure). The once-large aggregate Mediterranean subpopulation is now a small fraction of what it was as recently as the middle of the twentieth century (Bearzi et al. 2003). One note of caution is that there has been relatively little survey coverage of waters along the North African coast.
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