Delphinus delphis

Short-Beaked Common Dolphin

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Short-Beaked Common Dolphin

 

Description

Short-beaked common dolphins hunt in groups of ten to ten thousand animals. They can reach speeds up to 45 kilometers per hour. They are also attracted to ships and like to play in their bow waves. Short-beaked common dolphins live in oceans; they aren't often seen swimming close to shore, usually preferring water no shallower than 200 meters. Nevertheless, they are occasionally seen in the Dutch section of the North Sea. Short-beaked common dolphins are the most common dolphin species in the region of the Mediterranean Sea. And it's been that way for centuries: this is the dolphin that is often depicted on ancient Greek vases.
 

Geography

The short-beaked common dolphin is an oceanic species that is widely distributed in tropical to cool temperate waters of the Atlantic and Pacific Oceans (Perrin 2002), from nearshore waters to thousands of kilometers offshore. They regularly occur in some enclosed seas, such as the Okhotsk Sea and Sea of Japan, and separate subpopulations exist in the Mediterranean and Black seas. Short-beaked common dolphins may occur in parts of the Indian Ocean around southeastern Africa and southern Australia, but previous records of this species in other parts of the Indian Ocean and in waters of Taiwan are now thought to have been of long-beaked common dolphins (D. capensis; Jefferson and Van Waerebeek 2002).
 

Ecosystem

Habitat and Ecology

Short-beaked common dolphins appear to have a preference for upwelling-modified waters, areas with steep sea floor relief, and extensive shelf areas, but they are widespread in warm temperate and tropical waters (Evans 1994). In the eastern tropical Pacific, they prefer equatorial and subtropical waters with a shallow thermocline, relatively large seasonal changes in surface temperature, and seasonal upwelling (Reilly 1990; Fiedler and Reilly 1994). 

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).

Conservation

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.

 

Threats

Major Threats

The common dolphin is one of the most prominent by-catches of pelagic purse-seine, driftnet and trawl fisheries. In the ETP, common dolphins are sometimes found in association with yellowfin tuna, Thunnus albacares, and annual incidental mortality in the tuna purse-seine fishery of this species has been as high as 24,307 (in 1986) (IATTC, 2006). Since the Inter-American Tropical Tuna Commission (IATTC) imposed per-vessel stock limits on the international fleet, the mortality declined to 325 in 2005 (IATTC 2006). Takes have been recorded in other purse-seine fisheries in the Indian Ocean and off the west coast of Africa (Simmons 1968). Short-beaked common dolphins are the most commonly killed cetacean in the U.S. drift gillnet fishery for sharks and swordfish, with about 2,100 estimated killed between 1990 and 2002 (Julian and Beeson 1998; Carretta et al. 2005); measures have been in place to reduce cetacean takes since 1996, and bycatch levels are not a population level concern (Carretta et al. 2006).

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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