Description & Behavior

Commerson’s dolphins, Cephalorhynchus commersonii (Lacépède, 1804), aka piebald dolphins, are frequently mistaken for porpoises because they lack of beaks and have small heads. They have small stocky bodies with with rounded, paddle-like flippers. Juveniles are gray-black and lighten into their adult white coloring of the anterior body and black head. The dorsal area from the fin back is also black with a black patch on the lighter ventral side. Commerson’s dolphins range in size from 1.2-1.7 m, and can reach up to 86 kg.

The Kerguélen population in the southern Indian Ocean, east-southeast of South Africa, differ markedly from those in South America. They are larger and are black, gray, and white in color unlike the black/white coloration of the South American population.

There are two subspecies of Commerson’s dolphins:

  1. Commerson’s dolphins (S. America), C. commersonii commersonii
  2. Commerson’s dolphins (Kerguélen), C. commersonii subsp

These dolphins tend to be found in small groups of 1-3, although aggregations of 100 or more have been observed. Like other dolphin species, they are quick and agile, and enjoy leaping and riding bow waves. They have also been observed swimming upside down.

World Range & Habitat

Commerson’s dolphins, Cephalorhynchus commersonii, range from the tip of South America, through the Strait of Magellan, around the Falkland Islands and the Kerguélen Islands. Little is known about the migratory patterns of these dolphins, however fishermen report that they disappear from their common areas during the winter. They may follow fish such as róbalo and merluza that migrate offshore in winter. The dolphin population in the Strait of Magellan decreases in late autumn. The South American and Kerguélen populations are 2 distinct populations separated by 130° of longitude, or 8,500 km. In the Kerguélen population, the majority of dolphins migrate out of the Golfe du Morbihan between June-December.

The northernmost limit of the South America population is documented on the Brazilian coast between 31-32°S. Range extends south into Drake Passage (61°50’S) as far as the South Shetland Islands. On the west coast of South America, specimens have been reported from Isla Chiloé, Chile.

Commerson’s dolphins are found in cold inshore waters near open coasts, sheltered fjords, bays, harbors and river mouths. They are occasionally found in rivers. Within the Strait of Magellan, Commerson’s dolphins seem to prefer areas with strong current that reaches or exceeds 15 kph. Off South America, 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. Water temperatures in areas frequented by these dolphins range from 4°C-16°C (South America) and 1°C-8°C (Kerguélen). Around the Falkland/Malvinas and Kerguélen Islands as well as off mainland Argentina, Commerson’s dolphins are often seen swimming in or at the edge of kelp beds.

Sightings are common around the the Golfe du Morbihan where the dolphins inhabit open waters, kelp-ringed coastlines and protected areas between islets.

Feeding Behavior (Ecology)

Commerson’s dolphins, Cephalorhynchus commersonii, appear to be opportunistic feeders. They consume fish, squid, and shrimp. Stomach contents of Commerson’s dolphins taken as bycatch in South America included at least 25 food items: mysid shrimp (22.5% of total diet), three species of small fish (20.4%), squid (14.1%), 17 species of other invertebrates, four species of algae, and miscellaneous plant remains. Studies of the Kerguélen population have shown feeding mainly on 15-25 cm semipelagic chaennichthyid fish (Champsocephalus gunnari) and to a lesser extent on coastal benthic notothenids. Pelagic crustaceans (amphipods, hyperiids and euphausiids), benthic crustaceans (Halicarcinus planatus), and, in one individual, numerous annelid tubes and asciadians were also found. They feed primarily near the sea bed.

Life History

Commerson’s dolphins’ breeding season is in the spring and summer, which runs from September to February. Little else is yet known about their reproductive habits.

Conservation Status & Comments

In spite of negative human impacts on Commerson’s dolphin populations, Cephalorhynchus commersonii are the most abundant member of the genus Cephalorhynchus.

Aerial surveys in the northern Strait of Magellan estimated a minimum population size of 3,221 dolphins in that area. Aerial surveys in the eastern portion of the Strait of Magellan estimated over 900 individuals. Chilean populations may have declined due to hunting pressure and they may have moved to the Strait of Magellan to avoid being hunted. In the late 1980s-early 1990s, both solitary Commerson’s dolphins and aggregations of more than 100 were sighted along the north coast of Tierra del Fuego.

The status of the population at the Kerguélen Islands is unknown, although research on this population of Commerson’s dolphins is increasing. The largest group seen in the area were counted at about 100 dolphins near the edge of the continental shelf.

Various species of small cetaceans such as Commerson’s dolphins and Peale’s dolphins are used as bait for the southern king crab fisheries in Argentina and Chile. Off southern South America, Commerson’s dolphins are the most frequent species caught as bycatch with at least 50 dolphins caught in wide-mesh nets each year. They appear to be able to avoid fine-mesh nets. Dolphins are caught as bycatch in the Argentinean provinces north of Tierra del Fuego and in the eastern Strait of Magellan and Bahia Inútil in Chile. A few are taken by trawlers offshore in northern Patagonia. Commerson’s dolphins are used as bait, therefore fishermen have no reason to avoid dolphin bycatch. The impact of this practice is unknown.

These dolphins are at-risk of environmental contamination as well. Low levels of chlorinated hydrocarbons (DDT, PCB and HCB) were found in the blubber of Kerguélen dolphins, which confirms the presence of serious toxins in waters far from the main sources of pollution. Despite contamination, these levels are 10-100 times less severe than those of cetaceans in the North Atlantic.

Small cetaceans in Argentina and Chile have been protected by regulations since the 1970s. Permits are required for taking these animals, however enforcement is difficult in southern Chile due to geography. No regulations exist in the Falkland/Malvinas Islands but conservation areas have been proposed to protect the habitat. Live-captures are banned in Argentina pending more information on the species. Despite these regulations, some Commerson’s dolphins have been captured live in recent years for aquariums; the species appears to have survived in captivity.

References & Further Research

CMS: Cephalorhynchus commersonii, Commerson’s dolphin
Cephalorhynchus commersonii (Commerson’s dolphin) – OBIS-SEAMAP – Species Profiles
Jefferson, T.A., S. Leatherwood, and M.A. Webber, FAO species identification guide, Marine mammals of the world, Rome, FAO. 1993. 320 p. 587 figs.
Fundacion Cethus – an organisation led by Miguel Iniguez & Vanesa Tossenberger, which studies and campaigns for the protection of Argentina’s whales and dolphins, including Commerson’s dolphins and orcas.

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