Short-Beaked Common Dolphin
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Habitat and Ecology
Mediterranean common dolphins frequent coastal and upper slope waters (Bearzi et al. 2003). In the Black Sea, common dolphins are distributed mainly offshore and visit shallow coastal waters following seasonal aggregations and regular mass migrations of small pelagic fishes such as anchovy and sprat (Birkun 2006). Black Sea common dolphins avoid waters with low salinity, and this may explain why they do not occur in the Sea of Azov and in the Kerch Strait.
Associations with other marine mammal species are common. Schools in the Eastern Tropical Pacific (ETP) are sometimes associated with yellowfin tuna, and have thus been involved in tuna purse-seine fishing operations (Gerrodette 2002). Mixed-species groups of common, striped and Risso’s dolphins (Grampus griseus) have been observed frequently in the pelagic waters of the Gulf of Corinth, Greece (Frantzis and Herzing 2002). The prey of common dolphins consists largely of small schooling fishes and squids (Perrin 2002).
Despite ongoing threats to local populations, the species is widespread and very abundant (with a total population in excess of four million), and none of these threats is believed to be resulting in a major global population decline.
Incidental capture of common dolphins in European Atlantic fisheries has been well studied in recent years, and as a result of recent EU legislation, on-board observer programs are being carried out in most of the fisheries considered to have a potentially significant bycatch of common dolphins. Northridge (2006) showed that bycatches of common dolphins in European pelagic trawl fisheries probably total around 800 animals per year in UK and French pelagic trawl fisheries for sea bass. Annual catch rates in the UK sector of this fishery have been falling in recent years and the annual average total mortality recently (2000-2006) has been 170 animals in this sector. Other bycatches in the same area are known to occur in gill nets, tangle nets and possibly other fisheries (ICES 2005; Northridge 2006).
In the western North Atlantic, 105 common dolphins are taken on average each year by sink gill nets and bottom trawls (Waring et al. 2006). By-catches have also been reported from other areas (Crespo et al. 2000; Bearzi et al.2003).
The main factors thought to have contributed, singly or in synergy, to the decline of Mediterranean short-beaked common dolphins include: 1) incidental mortality in fishing gear, especially driftnets, 2) reduced availability of prey caused by overfishing and habitat degradation, 3) contamination by xenobiotic chemicals resulting in immunosuppression and reproductive impairment, and 4) environmental changes such as increased water temperatures affecting ecosystem dynamics (Bearzi et al. 2003, 2006). A recent survey focusing on the Moroccan driftnet fishing fleet estimated that about 12,000-15,000 dolphins are killed annually around the Strait of Gibraltar (Tudela et al. 2004).
At least 840,000 dolphins were taken from the Black Sea from 1946 until a ban of small cetacean hunting was declared in Turkey in 1983. The take was certainly much greater because that value did not incorporate catch statistics from Romania (whole period), Turkey (before 1976 and after 1981) and Bulgaria (before 1958) (Birkun 2006).
Reduced prey availability is considered to be a major threat to Black Sea common dolphins (Bushuyev 2000). Of two mass mortality events that killed an unknown but certainly large number of common dolphins in 1990 and 1994 (Krivokhizhin and Birkun 1999), the latter was recognised as the result of a morbillivirus epizootic. However, both die-offs coincided with a drastic (8 to 12 fold) decline in the abundance of the two main common dolphin prey species, anchovy and sprat. Such a reduction was caused by a combination of overfishing, eutrophication and the explosive increase of the introduced ctenophore Mnemiopsis leidyi. Correlation between large die-offs of Black Sea common dolphins and prey scarcity suggests that reduced prey availability increases susceptibility to viral infection (Birkun 2006). Prey depletion caused by overfishing was considered as a main cause for the decline of common dolphins in the eastern Ionian Sea (Bearzi et al. 2006).
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