Commerson's Dolphins are found in cold inshore waters along open coasts, in sheltered fjords, bays, harbours and river mouths, and occasionally in the lower reaches of rivers. Within the Strait of Magellan, they prefer the areas with strong currents, such as the Primera and Segunda Angostura (First and Second Narrows), where the current can reach or exceed 15 km/hr (Goodall 1994). At the Falklands (Malvinas) Islands this species was found to have a highly coastal distribution, with the majority of records from partially enclosed waters and within 10 km of the coast, with no records further than 25 km offshore (White et al. 2002). Off mainland South America, Commerson’s Dolphins were observed entirely in shelf waters < 200 m deep and most frequently close to shore (less than 60 km) (Dellabianca et al. 2016). Commerson’s Dolphins were recorded 24 km upriver in Deseado (Iñíguez and Tossenberger 2007).

Off the South American mainland, Commerson's Dolphins appear to prefer areas where the continental shelf is wide and flat, the tidal range is great, and temperatures are influenced by the cool Malvinas Current. In coastal Patagonia, they are found principally in areas with continental runoff such as at the mouths of rivers in Chubut and Santa Cruz, including the Deseado, Santa Cruz, Coyle, and Gallegos. Pedraza (2008) concluded that coastal fronts affect the density and abundance of their food resources and determine the distribution pattern of the species. Highest densities in her study were observed in northern Santa Cruz province and Tierra del Fuego. Around the Falklands (Malvinas) and Kerguelen Islands, as well as off mainland Argentina, Commerson’s Dolphins are often seen swimming in or at the edges of kelp beds. They sometimes move very close to shore, even inside the breakers. However, they are also observed occasionally offshore in waters deeper than 50 m.

South American Commerson's Dolphins appear to be opportunistic, feeding on various pelagic and bentho-pelagic species of fish, cephalopods, crustaceans, and benthic invertebrates in coastal waters but also on pelagic schooling fish in more open areas (Riccialdelli et al. 2013). In the Kerguelen Islands, they seem to have a more restricted diet, consisting mostly of semi-pelagic fishes (e.g. Champsocephalus gunnari) and to some extent benthic fishes (Robineau and Duhamel 1984). Feeding behaviour and group size depend on the habitat type. Group size may reach one hundred individuals when the dolphins are feeding on schooling pelagic fish. When they are feeding in tidal areas in front of small rivers, in kelp forests and in waters influenced by river discharge, they are more typically alone or in small groups (Iñíguez and Tossenberger 2007, Coscarella et al. 2010, Loizaga de Castro et al. 2013).

The two disjunct subspecies of Commerson’s Dolphins are separated by 130° of longitude and about 8,500 km distance.

Cephalorhynchus commersonii commersonii
Occurs in the coastal waters of southern South America and the Falkland (Malvinas) Islands. On the Atlantic coast of South America the northern limit of Commerson’s Dolphins is at approximately River Negro mouth (40°S) (Bastida and Rodríguez 2003). The range extends southward to near Cape Horn (56ºS) including the central and eastern Strait of Magellan and the Falkland (Malvinas) Islands (Goodall et al. 1988, White et al. 2002). Single dolphins and groups of up to hundreds were sighted in the late 1980s and early 1990s along the northern coast of Tierra del Fuego (Goodall 1994). Although sightings in the northern parts of the range often are of small groups or solitary individuals, overall numbers and group sizes increase to the south. In Chile the species is found mostly in Strait of Magellan (Aguayo-L. 1975, Venegas and Sielfeld 1978, Sielfeld 1983, Venegas and Atalah 1987, Thielke 1984, Goodall et al. 1988, Leatherwood et al. 1988, Hucke-Gaete and Vallejos 1997, Aguayo-L. et al. 1998, Gibbons et al. 2000), and nearby Seno Skyring, Fitz Roy channel, Seno Otway, and Seno Almirantazgo (Sielfeld and Venegas 1978, Gibbons et al. 2000).

Reported vagrants (northernmost records) along the Atlantic coast of South America include strandings in Buenos Aires province, Argentina (Brownell and Praderi 1985, Iñiguez et al. 2010) and Brazil (Pinedo et al. 2002). To the south, there are a few reports of this species at South Georgia (Brown 1988) and the South Shetland Islands (Aguayo-L. and Torres 1967, Goodall et al. 1988). However, sightings from the vicinity of South Georgia have been rejected by some workers (Jefferson et al. 1993). In the Pacific, a few records have been reported in the Chiloé area (Talcán, near Chaitén, Capella and Gibbons 1991; north of Quellón, J. Capella pers. comm.) and near San Rafael lagoon (estero Elefantes, F. A. Viddi pers. comm.) (http://especies.mma.gob.cl/CNMWeb/Web/WebCiudadana/ficha_indepen.aspx?EspecieId=157&Version=1).

Genetic analyses have revealed significant differentiation among different studied areas (within Tierra del Fuego and Santa Cruz provinces) over small geographic scales, considering these as subpopulations (Pimper et al. 2010, Cipriano et al. 2011). Coastal distribution around the Falkland (Malvinas) Islands and southern South America suggests that there is little mixing, if any, between the populations in these regions (White et al. 2002). Two “ecological stocks” have been identified based on differences in parasite loads and patterns of prey consumption (Berón-Vera et al. 2001). Skull morphology supports recognition of those two stocks (Pedraza 2008).

Cephalorhynchus commersonii kerguelenensis
Occurs in shallow coastal waters around all of the Îles Kerguelen in the southern Indian Ocean (Rice 1998, Robineau et al. 2007). No sightings or specimens have yet been reported from islands between South America and Kerguelen, such as Crozet, Heard, Amsterdam, or St Paul (Goodall 1994). Dolphins of the Kerguelen subspecies are most commonly sighted in the Golfe du Morbihan, on the eastern side of Kerguelen. Sightings have also been reported in other bays and fjords located along the northeastern and southern coasts of Kerguelen where observation effort is much more limited than in Golfe du Morbihan. Sightings from the open ocean in shelf and slope habitats around Kerguelen are scarce.

Recently, a sighting of a single individual south of Cape Town, in South African waters, was reported, although this should be considered extralimital (de Bruyns et al. 2006).

The distribution map shows where the species has been observed, including both subspecies. States for which confirmed records of the species exist are included in the list of native range states.

The species is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES) and also the Convention on Migratory Species (CMS).

Although Commerson’s Dolphins may have been seriously affected by the illegal deliberate take for bait in the Chilean crab fishery, the pressure on them in the southern part of their range apparently was reduced beginning in the late 1980s. However, in various parts of their range, incidental mortality in gillnets and other fishing gear continues and represents an ongoing threat (Dans et al. 2003, Iñíguez et al. 2003). Although the magnitude of this threat is undocumented, the similarity between Commerson’s Dolphins and Hector’s Dolphins (Cephalorhynchus hectori; currently listed as Endangered on the Red List) in their habitat preference and vulnerability to gillnets raises concern about the potential for depletion wherever gillnet fisheries and Commerson’s Dolphins overlap.  Gillnet mortality, in particular, should be monitored and investigated in more detail but even better documentation of the extent of spatial and temporal overlap between fisheries and dolphins would be useful for assessing the degree of threat to the species as a whole, and to demographically independent subpopulations. In 2016, the government of Argentina adopted a National Action Plan to mitigate mortality of marine mammals in fisheries.

Further research is needed to provide current abundance estimates for a larger proportion of the species’ total range, and more up-to-date, quantitative information on human-caused mortality. This latter should include an update on the illegal killing of Commerson’s Dolphins for use in King Crab fisheries in southern Chile. Better information is also urgently needed on the status of the Kerguelen subspecies population of Commerson’s Dolphins at the scale of the whole Kerguelen Plateau.