The population size is unknown. Region-wide estimates based on strip transect surveys in the USSR (1967-1974; Zemsky and Yablokov 1974) and Turkey (1987; Çelikkale et al. 1989) have been shown to be fundamentally flawed for a number of methodological analytical reasons so their use as indicators of absolute abundance is unwarranted (e.g., IWC 1992, Buckland et al. 1992). Nevertheless, it is generally recognized that for almost the first two-thirds of the 20th century, the abundance of common dolphins in the Black Sea was far higher than that of bottlenose dolphins, Tursiops truncatus ponticus, and harbour porpoises, Phocoena phocoena relicta (Tzalkin 1940, Kleinenberg 1956, Geptner et al. 1976).

By the mid 1960s, the population was depleted due to long-running overexploitation, which involved the killed of many 100,000s of common dolphins in the mid-20th century (IWC 1983; see “Threats”). The directed takes continued until 1983 when cetacean hunting finally ceased in Turkey. The numbers of animals taken were not recorded systematically or reliably, and therefore total removals have to be estimated indirectly. It can be inferred that the number of common dolphins was much reduced by the directed kills. It might be assumed that during the period from 1983-2005, the population was increasing. However, this may not be the case in view of mass mortality events (in 1990 and 1994) and the pronounced depletion of these dolphins’ primary prey species during the same period. The population has not fully recovered, and in fact it may have recovered only very little, from the depletion caused by hunting. Further decline seems likely if degradation of the Black Sea environment continues.

Last century, the population was depleted because of directed takes. The total number of animals killed is unknown, but it was estimated that before the mid 1950s common dolphins comprised 94.8% of the total number of Black Sea cetaceans killed and processed in the former Soviet Union (Tzalkin 1940, Kleinenberg 1956). Based on this value, it was calculated that during the last five years of the cetacean fishery in the USSR and Bulgaria (1962-66), these two countries landed 121,395 common dolphins, while during the preceding 31 years (1931-61) a further 1,449,304 had been landed mainly by the USSR (Zemsky 1996). Between 1976 and 1981, D. d. ponticus was believed to account for 15-16% (or 37,500-40,000 individuals) of the Turkish catch, estimated for that period as 250,000 animals of all three species (IWC 1983).

Reduced prey availability has been considered an ongoing major threat to D. d. ponticus since the late 1980s (Bushuyev 2000). Of two mass mortality events that killed unknown but certainly large numbers of common dolphins in winter–spring 1990 and summer–autumn 1994 (Krivokhizhin and Birkun 1999), the latter was recognised as being the result of a morbillivirus epizootic (Birkun et al. 1999). However, both die-offs coincided with a drastic decline in the abundance of both principal prey species, anchovy and sprat, which has been blamed on overfishing, eutrophication (e.g. water hypoxia) and the explosive increase of the introduced ctenophore Mnemiopsis leidyi (Zaitsev and Mamaev 1997). The total commercial catch of anchovies declined by 12-fold (from 468,800 tonnes in the 1987-88 fishing season to 39,100 tonnes in 1990-91), while landings of sprat fell by a factor of nearly eight (from 105,200 tonnes in 1989 to 13,800 tonnes in 1993) (Prodanov et al. 1997). This correlation between large die-offs of Black Sea common dolphins and prey scarcity could signify that reduced prey availability compromised the health of the dolphins and increased their susceptibility to viral infection.

The species D. delphis is listed globally as Least Concern by IUCN. At the same time, the Mediterranean population is listed as Endangered (see Bearzi et al. 2003), and concerns regarding the Black Sea population were expressed in the IUCN/SSC 2002-2010 Conservation Action Plan (Reeves et al. 2003).

Commercial killing of Black Sea common dolphins, as well as other Black Sea cetaceans, was banned in 1966 in the former USSR, Bulgaria and Romania, and in 1983 in Turkey. Black Sea states assumed certain international obligations to protect cetaceans as contracting parties to the Convention on Biological Diversity (CBD), Convention on the Conservation of Migratory Species of Wild Animals (CMS), Convention on the Conservation of European Wildlife and Natural Habitats (Berne Convention), Convention on the Protection of the Black Sea Against Pollution (Bucharest Convention), Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), and the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS). The common dolphin is included in EC Directive No.92/43/EEC on the conservation of natural habitats of wild fauna and flora; D. delphis is listed in its Annex IV (Animal and Plant Species of Community Interest in Need of Strict Protection). In 2003 the IWC Scientific Committee’s Sub-Committee on Small Cetaceans recommended that the Black Sea population should be managed for conservation as a distinct entity (IWC 2004).

The Strategic Action Plan for the Rehabilitation and Protection of the Black Sea (1996) envisages some special cetacean-oriented conservation and research actions. The common dolphin was included as Data Deficient in the Black Sea Red Data Book (1999). Nevertheless, it is listed as Endangered in the Provisional List of Species of Black Sea Importance, an annex to the Black Sea Biodiversity and Landscape Conservation Protocol (2002) of the Bucharest Convention. The regional Conservation Plan for Black Sea Cetaceans prepared in accordance with the ACCOBAMS International Implementation Priorities for 2002-2006 (Notarbartolo di Sciara 2002) was adopted by the 3rd Meeting of Parties to ACCOBAMS (Dubrovnik, Croatia, 22-25 October 2007).