Cephalorhynchus commersonii
Commerson's dolphin

Commerson's dolphins are primarily found in the coastal waters of the southwest Atlantic Ocean near Patagonia, Tierra del Fuego, Cape Horn and the Falklands Islands. They are most commonly seen along the eastern coast of South American between 41˚30' S and 55˚ S latitude, though they have been found as far north as 31˚ S latitude in some areas.

There is also a disjunct population of Commerson's dolphins in the south Indian Ocean near Kerguelen Island. In this area, they range from 48˚30' S to 49˚45' S latitude and are most common around the Golfe du Morbihan and around Heard Island. They are the only common cetacean in these coastal waters. ("The Commerson's Dolphin Story", 2004; "The Ocean Biogeographic Information System", 2009; "Whales, Dolphins, and Other Marine Mammals of The World", 2006; Goodall, et al., 1988; Goodall, 1994; Leatherwood and Reeves, 1983)

Commerson's dolphins inhabit shallow, inshore waters along coastlines, harbors, bays, and river mouths. The live in cold shallow waters, with temperatures ranging from 1˚ C to 16˚ C. They are rarely found at depths greater than 200 m. Commerson's dolphins prefer a neritic environment and are seldom found far offshore. Most sightings occur in the coastal regions near the mouths of bays and estuaries or over the wide shallow continental shelf where the tidal range is great. Commerson's dolphins move towards the shore with the tide. In some areas, dolphins prefer areas with the strongest currents - up to or greater than 15 km/hr. They are also frequently found in kelp beds and in narrow passages like those found in the Strait of Magellan. It is thought that most dolphins seasonally move away from the shore, following fish which move offshore during the winter. Commerson's dolphins in Kerguelen follow this trend and are less common inshore between June and December. In Kerguelen, they are most commonly observed over the Kerguelen shelf, but they are also found in open waters, kelp-ringed coastlines, and protected areas between islets. ("The Commerson's Dolphin Story", 2004; "Whales, Dolphins, and Other Marine Mammals of The World", 2006; Clapman, et al., 2002; Goodall, et al., 1988; Goodall, 1994; Leatherwood, et al., 1988)

Commerson's dolphins are mostly white and have black marked faces and bodies. This black and white pattern varies with geographic location as well as age and sex. These colors are more muted in coastal waters. The black color on the head extends behind the blowhole and down the sides of the body, including the flippers. Black also covers the dorsal fin and runs back to encircle the tailstock behind the anus and flukes. The black color on the chest ends in a posterior-facing point. Large black genital patches are oval or heart-shaped, with the narrow end pointing posteriorly in males. In females the narrow end is anterior, and may or may not include "ears" outside the mammary slits. The throat is generally white, as is the rest of the body. Individuals can be recognized by varying shape of the black "widow's peak" behind the blowhole as well as pigmentation on the side of the tailstock. Calves are born dark grey and black with vertical creasing as a result of fetal folding. These folds disappear after a week, and the dark grey portions become paler in the first few months and white within 4 to 6 months. ("Whale Watcher", 2006; Goodall, et al., 1988)

Coloration of Commerson's dolphins near Kerguelen Island is similiar to that of juveniles from the south Atlantic Ocean. Near Kerguelen, the surface in front of the dorsal fin is grey, as are the sides. The grey behind the blowhole is streaked with black and their "widow's peak" is not as well defined. The white throat patch is more asymmetrical than in individuals from the south Atlantic. Commerson's dolphins near Kerguelen generally have a narrow white line in the center of their chest, a feature seldom found in their counterparts in the south Atlantic Ocean. (Goodall, et al., 1988; Goodall, 1994)

Commerson's dolphins have a dark rounded dorsal fin that rises at a shallow angle and flippers with rounded tips. Their head is blunt and has a sloped forehead and little or no beak. A narrow cap extends on the rear half of body from dorsal fin to flukes. The tail flukes are slightly round tipped and notched, and the flippers are small and rounded. The flukes have concave edges with a slight hatch in the middle with dark coloring above and below. Commerson's dolphins have approximately 29 to 30 pairs of pointed teeth in the upper and lower jaws. Newborns range from 0.5 to 0.75 m in length and weigh 4.5 to 7.3 kg. Adults range from 1.2 to 1.5 meters in length and usually weigh 35 to 65 kg. Females are generally larger than males, and Commerson's dolphins from the Indian Ocean tend to be larger than those in the South Atlantic. Also, although Commerson's dolphins from both areas do not have well defined snouts, there is a distinct rostral depression in Kerguelen animals which individuals in the south Atlantic lack. ("Whale Watcher", 2006; Goodall, et al., 1988; Leatherwood and Reeves, 1983; Lockyer, et al., 1988)

Little information is available regarding the mating systems of Commerson's dolphins. They have been observed copulating in a vertical, belly to belly position. (Goodall, et al., 1988)

Commerson's dolphins breed between the months of September and February. After a gestation period of at most 12 months, females give birth to one individual in the winter. Calves are born tail first and are grey in color. Newborns range from 0.5 to 0.75 m in length and weigh 4.5 to 7.3 kg. Their dorsal fin and tail flukes are pliable at birth and gradually stiffen as they mature. The length of the nursing period in the wild is unknown. However, in captivity calves begin eating solid food by 2 months of age and take whole fish by 4 months. Males typically reach sexual maturity between 6 and 9 years of age, and females between 5 and 9 years. Individuals from the the south Atlantic Ocean, however, tend to reach sexual maturity at a younger age than populations from Kerguelen. ("The Commerson's Dolphin Story", 2004; Goodall, et al., 1988; Repetto and Wurtz, 2003)

Mother Commerson's dolphins nurse their calves through abdominal mammary slits for about 9 months. Mothers are attentive to their calves, which swim close to their mother. Calves learn to swim by following in their mother's slip stream. Females also appear defensive of their young. ("The Commerson's Dolphin Story", 2004; Repetto and Wurtz, 2003)

Commerson's dolphins generally do not live more than 10 years in the wild. In captivity, they commonly live to 18 years of age, and one individual at SeaWorld San Diego lived to be 25.8 years of age. ("The Commerson's Dolphin Story", 2004; "Whale Watcher", 2006; "Whales, Dolphins, and Other Marine Mammals of The World", 2006; Clapman, et al., 2002; de Magalhaes and Costa, 2009)

Commerson's dolphins are usually found in groups of only 1 or 3 individuals, but may socialize in groups as large as 100. One group in captivity was controlled by a dominant male, and other less aggressive males often resided in an adjacent pool. A calf in captivity became synchronized to its mother's swimming immeditaely after birth. When another female tried to "adopt" the calf, the mother appeared defensive of her young. ("The Commerson's Dolphin Story", 2004; Clapman, et al., 2002; Goodall, 1994)

Commerson's dolphins are sometimes seen feeding alone along boundaries of adjacent currents, but they are more commonly observed cooperatively feeding. Two main types of herding are used. In one type, a group of 15 or fewer dolphins form a half circle and drive a school of fish against the shore. Sometimes, a dolphin is temporarily stranded on shore as well, but stranded dolphins are usually able to return to the sea on their own. The other type of herding does not involve the use of the shore as a barrier. In groups of 2 to 6, Commerson's dolphins circle around a group of fish and take turns passing through the center of the circle, feeding and then returning to the perimeter of the circle. (Goodall, et al., 1988)

Commerson's dolphins are known for their high-speed swimming, routinely reaching speeds of 11 to 13 kph. They ride all four types of waves described by Hertel in 1969: wind waves at sea, breaking shore waves, bow waves of vessels and other waves of vessels, including stern wakes. There are various accounts of aerial acrobatics, such as vertical leaps in Commerson's dolphins. They often swim on their back, and they spin underwater on their longitudinal axes behind the pressure waves of vessels. Commerson's dolphins are considered playful swimmers. On a number of occasions, they have been observed pushing objects, such as inflated inner tubes, around a bay and onto the beach. They have also been observed surfacing under and nudging birds off of a rig. ("The Commerson's Dolphin Story", 2004; Clapman, et al., 2002; Goodall, et al., 1988; Leatherwood, et al., 1988)

In South America, Commerson's dolphins have been seen swimming near and interacting with numerous birds and other marine mammals. Like other dolphins, their presence is often announced by flocks of birds overhead (usually terns). Commerson's dolphins commonly assiciate with Peale's dolphins, swimming synchronously for long periods, feeding in similiar areas, and riding the waves of the same vessels. They also commonly associate with southern sea lions. (Goodall, et al., 1988)

Little information is available regarding the home range of Commerson's dolphins.

Commerson's dolphins communicate using echolocation. They also rely on echolocation to navigate and hunt through dark waters. In captivity, they vocalize at frequencies ranging from 120 to 134 kHz for a duration of 180 to 600 μs. ("The Commerson's Dolphin Story", 2004; Repetto and Wurtz, 2003)

Commerson's dolphins are primarily carnivores but are often opportunistic coastal feeders. They rely on both pelagic and benthic prey. Their diet is mainly composed of mysid shrimp, and small fish like silversides, sardines, and Argentine hake. They are also known to eat squid, octopus, marine worms, tunicates, and even algae. Among 53 Commerson's dolphins in the Tierra del Fuego, 22.5% of their diet was composed of mysid shrimp, 20.4% of 3 species of fish, 14.1% of squid, and the rest of algea, isopods, and other benthic invertebrates. Other plant remains, seeds, sand, and pebbles were also found in their stomachs. Individuals found near the Kerguelen Island eat a high proportion of semipelagic chaennichthyid fish, as well as pelagic and benthic crustaceans. At times, Commerson's dolphins hunt together, as described in the behavior section. In captivity, Commerson's Dolphins consume 3 to 4 kg of Atlantic herring each day. (Bastida, et al., 1988; Clapman, et al., 2002)

In areas of high tides, Commerson's dolphins feed in the shallow areas in or just beyond the advancing tide breakers in order to take fish, such as sardines and anchovies that are also feeding in the area, or other organisms that are dislodged by the turbulent water. Commerson's dolphins also feed for long periods in kelp beds, in open waters, around submarine banks, and near artificial structures such as piers and oil rigs. (Goodall, et al., 1988; Goodall, 1994)

The black and white coloration patterns of Commerson's dolphins breaks up the outline of their body, making them more difficult for predators to spot. Natural predators may include killer whales, sharks, and leopard seals that live within the same geographic range, but such predation has not been documented. Humans actively kill Commerson's dolphins for food, oil and bait and inadvertently through other fishing practices. ("The Commerson's Dolphin Story", 2004; Clapman, et al., 2002)

Commerson's dolphins prey on small fish, mysid shrimp, as well as squid, octopus, marine worms, and tunicates. While natural predators are still unknown, killer whales and leopard seals may prey on them as they are found in the same geological area. Commerson's dolphins also act as hosts for roundworms (Nematoda) and flukes (Trematoda). ("The Commerson's Dolphin Story", 2004; "The Encyclopedia of Earth", 2010)

Humans living in Patagonia and Tierra del Fuego have traditionally harpooned Commerson's dolphins for their meat and oil. Crab fishermen in southern Argentina and Chile use the meat of Commerson's dolphins as bait, as it does not deteriorate in salt water. These practices are now illegal and have steadily declined. Commerson's dolphins can also be found in aquariums in Germany, Japan and the United States. ("The Commerson's Dolphin Story", 2004; "The Encyclopedia of Earth", 2010; Clapman, et al., 2002; Goodall, et al., 1988)

Commerson's dolphins may affect fishermen, as they sometimes become entangled in fishing nets and may also deplete populations of small fish.

Commerson's dolphins were hunted for their meat and oil and more recently for crab bait. Although these practices are now illegal, they are often entangled in gillnets and other fishing gear used in nearshore waters and are occasionally killed in midwater trawl nets used for shrimp. In Tierra del Fuego alone, at least 5 to 30 dolphins die each year as by-catch in nets set perpendicular to the shore.

Near Kerguelen Island, low levels of chlorinated hydrocarbons (DDT, PCB and HCB) were found in the blubber of Commerson's dolphins, confirming the presence of pollutants in oceans far from their main source. The levels of contaminants were 10 to 100 times that of cetaceans in the North Atlantic. ("The Commerson's Dolphin Story", 2004; Clapman, et al., 2002; Goodall, 1994)