A number of factors may have contributed, singly or in synergy, to the decline of Common Dolphins in the Mediterranean (Bearzi et al. 2003). Mediterranean biodiversity is undergoing rapid alteration under the combined pressure of human impact and climate change (Bianchi and Morri 2000), and it is difficult to discriminate between the effects of environmental shifts due to climate change, whether "natural" or a result of the greenhouse effect, and other factors that may be affecting the availability of dolphin prey, such as overfishing and habitat degradation. In all Mediterranean areas where Common Dolphins have been studied consistently, namely the Alboràn Sea, southeastern Tyrrhenian Sea, and eastern Ionian Sea, competition with fisheries is a source of concern (Notarbartolo di Sciara et al. 2002, Bearzi et al. 2003) although cause-effect relationships and ecosystem dynamics remain poorly characterized. The role of xenobiotic contamination is controversial but likely significant. High levels of PCBs in Mediterranean dolphins, compared to levels in dolphins from other areas (Fossi et al. 2000, Aguilar et al. 2002), represent a major concern because of the possibilities of immune suppression and reproductive impairment. The high PCB levels in Common Dolphins from the Alboràn Sea are close to the range at which adverse effects could be expected, based on extrapolation from other species (Borrell et al. 2001). Fossi et al. (2000, in press) found a significant correlation between mixed-function oxidase activity and organochlorine levels in Common Dolphin skin biopsies, suggestive of exposure to endocrine-disrupting chemicals and potential for transgenerational effects. The cumulative importance of these threats and other factors, including incidental mortality in fishing gear (below), is poorly understood.
Fishery bycatch is a major threat to many cetacean populations, and it could well have played a role in the decline of Common Dolphins in at least some Mediterranean areas (IWC 1994). In the Alboràn Sea, for example, drift gillnets are known to have caught a few hundred Common Dolphins per year (Silvani et al. 1999). This fishery has stopped, but it operated for many years and undoubtedly had some impact on the population. If drift nets were taking Common Dolphins in the Alboràn Sea, it is reasonable to assume that they were (and are) doing so in other parts of the Mediterranean where drift net fishing and Common Dolphin occurrence overlap. Bearzi et al. (2003) suggest that bycatch alone is unlikely to be the factor most responsible for the decline of Common Dolphins in the Mediterranean, but it may have played a significant role at certain times and in certain areas.
The possibility that the Striped Dolphin has been increasing in the Mediterranean and has begun to occupy the ecological niche of the Common Dolphin has been discussed in the literature (Viale 1985, Aguilar 2000, Bearzi et al. 2003). Such a hypothesis is extremely difficult to prove or disprove, particularly if invoked as a causal factor in the Common Dolphin's decline. Even if it were true that Striped Dolphins have been extending their range to inshore waters traditionally inhabited by Common Dolphins, it would be unclear whether this process was being driven by competitive exclusion, or was instead a secondary outcome of the Common Dolphin's disappearance for some other reason. In any event, competition would not be an issue in areas such as the northern Adriatic Sea, where the Common Dolphin has disappeared while the Striped Dolphin rarely occurs.
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