Common Dolphins are found in both coastal and offshore oceanic waters (Bearzi et al. 2003, Cañadas and Hammond 2008, Hammond et al. 2013, Murphy et al. 2013, Bearzi et al. 2016, Rogan et al. 2018, Bearzi and Genov 2021, Hammond et al. 2021).

Diet appears relatively variable and key prey species appear to vary across the distribution range. In the North-East Atlantic, the diet includes a wide variety of fish and squid species, with dominant species varying with region and season (reviewed in Murphy et al. 2013). Common Dolphins caught in French drift nets mainly fed on myctophids Notoscopelus kroyeri, Benthosema glaciale and Myctophum punctatum; the sternoptychid Maurolicus muelleri; and cephalopods Ancistroteuthis lichtensteinii, Gonatus steenstrupi, Brachioteuthis riisei and Teuthowenia megalops, with dominating Notoscopelus kroyeri the diet (Pusineri et al. 2007). Common Dolphins off the south-west coast of the United Kingdom were found to consume a variety of specues, with sardine (Sardina pilchardus), mackerel (Scomber scombrus), horse mackerel (Trachurus trachurus), Norway pout (Trisopterus esmarkii), other clupeids and various squid species representing primary prey (Murphy et al. 2013). Stomach contents and stable isotope analyses off Galicia, Spain, identified blue whiting (Micromesistius poutassou) and sardines as the most important prey species (Mèndez-Fernandez et al. 2012, Murphy et al. 2013). Likewise, sardines were found to be the most important prey in Common Dolphins stranded in Portugal (Marçalo et al. 2018). In the eastern Mediterranean Sea, stomach contents analyses and fish scales sampled during surface feeding suggest that epipelagic and mesopelagic fish such as Clupeidae and Myctophidae are the preferred prey (Bearzi et al. 2003, Bearzi et al. 2006, Milani et al. 2018), whereas in the western Mediterranean Sea, stomach contents and stable isotope analyses have shown that their diet consists predominantly of Myctophidae and Sparidae, as well as a small proportion of cephalopods (Giménez et al. 2017, Giménez et al. 2018).

Common Dolphins often occur in mixed-species groups with striped dolphins in several European areas (Forcada and Hammond 1998, Frantzis and Herzing 2002, Bearzi et al. 2016), which sometimes makes species identification in these groups difficult (Rogan et al. 2018, ACCOBAMS 2021, Hammond et al. 2021).

Common Dolphins are widely distributed throughout most of European waters, including the North Atlantic Ocean, the North Sea, Mediterranean Sea, and the Macaronesian archipelagos. Common Dolphins in the European North Atlantic are distributed from coastal waters all the way to the mid-Atlantic ridge, and from around 70°N west of Norway in the north to Canary Islands in the south, including around the Azores and Madeira archipelagos (Jaber et al. 2006, Quérouil et al. 2010, Murphy et al. 2013). Within the Mediterranean Sea the Common Dolphin has experienced a major reduction in geographic range (Bearzi et al. 2003). Although historical distribution and abundance remain unknown, literature and osteological collections show that Common Dolphins were widespread and abundant in much of the basin until the 1960–1970s, but declined substantially in the following decades (Bearzi et al. 2003, Bearzi et al. 2004). The species is relatively abundant in the Alboran Sea (Cañadas and Hammond 2008, Cañadas and Vázquez 2017), but its occurrence declines steeply east of the Almería-Orán thermohaline front (Bearzi et al. 2021). Aerial surveys conducted in the summer of 2018 (ACCOBAMS 2021) yielded no records of Common Dolphins throughout most of the western Mediterranean basin situated east of this front (including the Gulf of Vera, the Balearic Sea, the Gulf of Lion, the Pelagos Sanctuary and the Sea of Sardinia), in the Adriatic Sea, and in the easternmost sector of the Mediterranean (Bearzi et al. 2021). Common Dolphins from the northern Aegean Sea may move across the Turkish Straits System, but little information exists about potential mixing with Black Sea Common Dolphins, which are recognized as a separate subspecies D. delphis ponticus.

Common Dolphins occur in both inshore and offshore waters (Bearzi et al. 2003, Hammond et al. 2013, Murphy et al. 2013, ACCOBAMS 2021, Bearzi and Genov 2021, Hammond et al. 2021). In the Mediterranean Sea, they have been recorded in nearshore habitats in several areas (Frantzis et al. 2003, Bearzi et al. 2005, Cañadas and Hammond 2008, Pietroluongo et al. 2020, Genov et al. 2021, Mussi et al. 2021), but also regularly found offshore or in deeper waters (Forcada and Hammond 1998, Cañadas and Hammond 2008, Bonizzoni et al. 2019). In the Eastern North Atlantic, the species is found from nearshore habitats all the way to the mid-Atlantic ridge (Murphy et al. 2013, Laran et al. 2017, Rogan et al. 2018, Hammond et al. 2021).

Existing conservation actions
The Common Dolphin in Europe is protected under a number of treaties, including the EU Habitats Directive, ACCOBAMS, ASCOBANS, CITES Appendix II, CMS Appendix I (North, Baltic, Mediterranean and Black Seas populations) and II (species level) and Barcelona Convention SPA/BD protocol Annex II.

Conservation actions needed to mitigate major threats
This species: (a) is commonly found in coastal areas and over the continental shelf), (b) its distribution range overlaps substantially with areas subjected to very high levels of anthropogenic activities, particularly fisheries.

The conservation recommendations for the NE Atlantic population were recently provided by Murphy et al. (2021) and are relevant in the context of the assessment here (see Murphy et al. 2018 for details on each of these items):

1. Implementation of a species action plan for common dolphins and an associated steering committee
2. Assessment of management unit boundaries
3. Finalize a bycatch management framework
4. Assessment of the bycatch level
5. Mitigation of bycatch
6. Monitoring of abundance and distribution
7. Monitor health and nutritional status, reproductive parameters, pollutant burdens and causes of mortality
8. Investigate the effects of anthropogenic sound
9. Evaluate the functional role of common dolphins in the ecosystem
10. Studies of cumulative impacts of pressures