Delphinus delphis
SHORT-BEAKED COMMON DOLPHINS
IUCN Red List Category & Criteria: Endangered A2abc
Geographic Range
The Short-beaked Common Dolphin (hereafter referred to as the Common Dolphin) is a small cetacean species with a wide distribution. Like most other cetaceans, however, it is not panmictic and occurs as a series of geographically separate subpopulations (e.g., Jefferson and Van Waerebeek 2002). Once one of the commonest species in the Mediterranean Sea, the Common Dolphin has experienced a generalized and major decrease in this region during the last 30–40 years (Bearzi et al. 2003). Coastal groups in western Greece seem to exhibit relatively high levels of site fidelity (Politi 1998), but little is known about the movements and ranging patterns of animals living offshore.
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.
Population
Literature and osteological collections unambiguously confirm that Common Dolphins were widespread and abundant in much of the Mediterranean Sea until the late 1960s, and that their decline occurred relatively quickly (Bearzi et al. 2003; and see references contained therein). Today, Common Dolphins remain relatively abundant in the westernmost portion of the basin, the Alboràn Sea. There are sparse records off the coast of Algeria where, however, survey coverage has been limited. Possibly isolated groups are present around Sardinia and Corsica, particularly off their western coasts (Bearzi et al. 2003). Common Dolphins are seen in the early summer in the south-eastern Tyrrhenian Sea off the island of Ischia (Mussi et al. in press). The species is also present in the Sicily Channel, with larger groups being observed around Malta (Vella in press). Common Dolphins can be found in portions of the eastern Ionian Sea, particularly around the island of Kalamos (Politi and Bearzi in press), and in the Gulf of Corinth (Frantzis and Herzing 2002). Sighting and stranding data indicate a regular presence of Common Dolphins in the Aegean Sea, particularly in the Thracian Sea, Northern Sporades, the southern Evvoikos Gulf, the Saronic Gulf, and the Dodekanese (Frantzis et al. submitted). Otherwise, these dolphins are rare in, or completely absent from, Mediterranean areas where information is available (Bearzi et al. 2003). Mediterranean regions where Common Dolphins have apparently vanished include the Adriatic Sea, Balearic Sea, Provençal basin, and Ligurian Sea.
There is no basin-wide estimate of abundance for Common Dolphins in the Mediterranean Sea. Line-transect ship surveys of the Alboràn Sea in 1991-1992 produced an estimate of 14,736 (CV=0.38; 95% CI=6,923-31,366), with a density of 0.16 dolphins per km², but no estimates were made for this species elsewhere in the western Mediterranean due to the low number of sightings (Forcada and Hammond 1998). Vella (in press) combined data from ship and aerial surveys conducted between 1997-2002, and obtained a density estimate of 0.135 dolphins per km² (CV=0.28; 95% CI=0.066-0.290) in the area around the Maltese islands. Around the island of Kalamos in the eastern Ionian Sea, the mean sighting frequency was 0.016 groups per km (or 0.11 dolphins per km) in the years 1993-2000, but in 2001-2002 there was a significant decrease to 0.007 groups per km (or 0.04 dolphins per km) (Student's t=4.88, p<0.001). The number of individuals encountered in this area has decreased continually, and many individuals that used to be seen regularly until 1996 have disappeared (Bearzi et al. 2003).
Habitat and Ecology
In the Mediterranean, Common Dolphins are found in both pelagic and neritic environments, occasionally sharing the former with Striped Dolphins (Stenella coeruleoalba) and the latter with Common Bottlenose Dolphins (Tursiops truncates) (Bearzi et al. 2003). Mixed-species groups of Common, Striped and Risso's Dolphins (Grampus griseus) have been consistently observed in the pelagic waters of the Gulf of Corinth, Greece (Frantzis and Herzing 2002). Mediterranean Common Dolphins are typically found in groups of 50–70 animals, with larger aggregations occasionally recorded. In the eastern Ionian Sea coastal waters, however, groups rarely include more than 15 individuals, and groups greater than 40 have not been observed (Bearzi et al. 2003).
Major Threat(s)
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.
Conservation Actions:
A large Marine Sanctuary for cetaceans in the Corso-Ligurian Basin has been declared by the Governments of Italy, France and Monaco. Other smaller marine protected areas exist or have been proposed throughout the Mediterranean Sea (Bearzi et al. 2003). In 1999, the Spanish Ministry for the Environment included the Common Dolphin in its National Endangered Species Act as "vulnerable". The following year, a program was initiated to identify important areas for the conservation of cetaceans in the Spanish Mediterranean with the aim of implementing the European Union's "Habitats" Directive, the Barcelona Convention and the Bonn Convention (Convention on Migratory Species, or CMS) through the creation of marine protected areas. Based on the presence of a relict group of Common Dolphins, the eastern Ionian area around the island of Kalamos has been included by the Greek Ministry of the Environment in the Natura 2000 network ("Site of Community Importance") under the 9243 EEC "Habitats" Directive. While these types of designations may benefit Common Dolphins at least indirectly, measures to provide direct benefits, e.g., area-, season-, or fishery-specific reductions in fishing effort, curtailment of inputs of particular pollutants, etc., remain to be identified and implemented. The Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS 2002) considers the Mediterranean Common Dolphin as an endangered population. It is expected that efforts to increase understanding of ongoing threats, monitor status, and provide needed protective measures on behalf of the dolphins and their habitat will be organized and implemented through ACCOBAMS.