Cephalorhynchus commersonii
Commerson's dolphin

There are two populations of Commerson's dolphins; the groups are separated by about 8500 km (Goodall et al, 1994). The northern limit of the South American population is the Rio Negro at 40 deg S. Commerson's dolphins are most common at the western South Atlantic along the eastern coast of South America from 41deg 30'S to 55deg S, at the Strait of La Maire (Goodall et al, 1988). The overall range in the South American region is along the Patagonian coast south to Cape Horn, east to central Strait of Magellan and the Falkland Islands. Several sightings have also been noted in Drake Passage and just north of the South Shetlands at 61deg 50'S 63deg 17'W (Goodall, 1994).

There is a disjunct population of C. commersonii in the area of Kerguelen Island in the South Indian Ocean (Leatherwood and Reeves, 1983). The range extends from 48deg 30'S to 49deg 45'S. Commerson's dolphin is the only cetacean that can be called common in Kerguelen coastal waters (Goodall, 1994).

Both populations prefer a neritic environment and are seldom found far offshore. SOUTH AMERICAN POPULATION

Most sightings are 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 waters that range from 4deg C in winter to 9-12deg C in summer (annual mean 6deg C-12deg C). In some areas, dolphins prefer the areas with the strongest currents - up to or greater than 15 km/hr, and in others, they frequent kelp beds (Goodall et al., 1988a).

KERGUELEN

All sightings of C. commersonii have been over the Kerguelen shelf. Water temperatures range from 1-2deg C in winter to 6-8deg C in summer (annual mean 4deg C). Sightings are most common within the Golfe du Morbihan, where there are observation programs in operation. The dolphins are found in areas with open waters, kelp-ringed coast lines, and protected areas between islets (Goodall, 1994).

SOUTH AMERICAN POPULATION The total lengths of 150 measured animals ranged from 64.0 to 146.5 cm. Maximum lengths were 140.5 cm (male) and 146.5 cm (female). The weights of the animals ranged from 26 to 44.5 kg. One pregnant female (near term) weighed 66 kg (Cornell et al., 1988). Females are larger than males at all ages (Lockyer et al., 1988).

The color pattern of the South American population of C. commersonii is black and white. The black color on the head extends to behind the blowhole and down the sides to include 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 rest of the body, including the tear-drop patch, is pure white. Individuals can be recognized by the varying shape of the "widow's peak" behind the blowhole and pigmentation on the side of the tailstock. The fetus is dark grey on the surfaces that are white in the adult. This dark grey fades to a paler color in the first few month and then to white within four to six months, at approximately a 117 cm length (Goodall et al., 1988a).

KERGUELEN ISLANDS

Specimens from the Kerguelen Islands are larger than those from the South American population at all ages. The total lengths range from 124.6 to 174 cm. Maximum lengths are 167 cm (male) and 174 cm (female). The weights range from 50 to 86 kg. Males were 50 to 78 kg and females weigh 72 to 86 kg.

The coloring of C. commersonii adults in the Kerguelen Islands is similar to that of South American juveniles. The surface in front of the dorsal fin is grey, as are the sides. The grey behind the blowhole is streaked with black and the "widow's peak" is not as well defined. The white throat patch is more asymmetrical than in South American animals. The chest always has a narrow white line in the center, a feature seldom found in South American animals (Goodall, 1994).

In both populations, there are approximately 29-30 pairs of pointed teeth in the upper and lower jaws (Leatherwood and Reeves, 1983).

The external shape of both groups is very similar. Girths in South American animals range from 50% to 77% of total length, and 60-67% in those from Kerguelen. Neither has a well defined snout, although there is a distinct rostral depression in Kerguelen animals that is not present in the South American population. The dorsal fin is rounded and undercut on the posterior edge and the flukes are 16-33% of the total length. Both flukes and flippers also have rounded tips (Goodall, 1994). There are frequently (68% of 32 individuals) saw-tooth serrations on the leading edges of left flippers, and rarely on right flippers. This appears to be a genetic development, but the function of the serrations is not known (Goodall et al., 1988c).

SOUTH AMERICAN POPULATION Herds are usually made up of one to three individuals. There have been sightings of solitary individuals as well as reports of groups of 100 or more individuals, which may be seasonal, feeding, or breeding congregations. It is speculation whether these large herds remain together for a long time or are short term congregations of smaller herds (Goodall et al., 1988a).

One group in captivity was controlled by one dominant male; other less agressive males often went over to an adjacent pool. A calf 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. Currently, observations of wild behavior are underway (Goodall, 1994).

There are some dolphins throughout the year in most areas. Fishermen claim that most dolphins move out of the shore area during the winter to return in November. It has been speculated that they follow the fish ,which move offshore in the winter (Goodall et al., 1988). A larger number of C. commersonii are observed in the summer in the Strait of Magellan than in the late autumn (Leatherwood et al., 1988a)

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 associate most commonly with Peale's dolphins, Lagenorhynchus australis, and southern sea lions, Otaria flavescens. With Peale's dolphins, C. commersonii often swim synchronously for long periods, ride the waves of the same vessels or feed in the same areas (Goodall et al., 1988a).

Play behavior has been observed a number of times. C. commersonii has been seen pushing objects, such as inflated inner tubes, around the bay and onto the beach. They have also been observed surfacing under and nudging birds off of a rig for 30 minutes. There are various accounts of aerial acrobatics, such as vertical leaps (Goodall, et al., 1988a). Sometimes, C. commersonii swims on its back (Leatherwood et al., 1988b). Commerson's dolphins also engage in wave riding. They ride all four types described by Hertel in 1969 : wind waves at sea, breaking shore waves, bow waves of vessels and other waves of vessels, including stern wakes (Goodall et al., 1988a).

KERGUELEN POPULATION

Groups are usually composed of two to three individuals. On rare occasions, groups of 20-30 and of 100s have been reported. The majority of sightings occured when the dolphins approached boats (Goodall, 1994).

Some dolphins do stay in the area through the year, most move out of the Golfe du Morbihan from June to December (winter and spring). Dolphins were rarely noted over the adjacent continental shelf, it has been thought that they may move to other parts of the archipelago (Goodall, 1994).

No associations with other species have been reported for C. commersonii in the Kerguelen Islands.

Commerson's dolphins have also been observed at play at the Kerguelen Islands. There have been reports of dolphins spinning underwater on their longitudinal axis as they ride the pressure waves from vessels. Frequently, these dolphins are seen swimming on their backs (Goodall, 1988a).

The sound production of both populations has been studied. The South American population has a narrow band of 5-10 kHz and high frequency (125-135 kHz) pulses of short duration (0.1-1.2 msec) at low levels. This may lead to entanglement in some fishing nets because the sound does not reflect well off the nets. The composit clicks of Kerguelen Island Commerson's dolphins have a broader band spectrum (Goodall, 1994).

SOUTH AMERICAN POPULATION By analyzing the stomachs of C. commersonii caught in fishing nets, it has been determined that the Commerson's dolphins of Tierra del Fuego are opportunistic coastal feeders. Cephalorynchus commersonii eat mysid shrimp (22.5% of total diet), three species of fish (20.4%), squid (14.1%), algae, isopods and other benthic invertebrates. A total of 25 food items were found in 53 stomachs, as well as miscellaneous plant remains, seeds, sand, and pebbles in the stomachs of the C. commersonii specimens (Bastida et al., 1988).

In areas of high tides, Commerson's dolphins seem to feed in the shallow areas, in or just beyond the advancing tide breakers. Apparently they are taking fish such as sardines and anchovies, which are also feeding in the area, or other organisms which are dislodged by the the turbulent water.

In other areas, C. commersonii remain for long periods and feed in kelp beds, in open waters, around submarine banks, and near artificial structures such as piers and oil rigs.

Commerson's dolphins are sometimes seen feeding singly along boundaries of adjacent currents, but it is more common to find them feeding cooperatively by herding fish. 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, the dolphins are temporarily stranded on shore as well, but they 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 two to six, C. commersonii 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., 1988a).

KERGUELEN POPULATION

The diet of this population was primarily found to be 15-25 cm semipelagic chaennichthyid fish, as well as pelagic and benthic crustaceans (Goodall, 1994).

In general, both populations seem to utilize a large variety of good resources.

In the South American population, Commerson's dolphins were hunted for meat and oil during the first half of this century. More recently, they have been harpooned for crab bait; fisherman claim that the meat does not deteriorate in salt water and that it attracts crabs.

There have been at least six episodes of live capture of C. commersonii for public display. At this time, Commerson's dolphins can be found in aquariums in Germany, Japan and the United States (Goodall et al., 1988a).

SOUTH AMERICAN POPULATION In the first half of this century, Commerson's dolphin was hunted for its meat and oil. More recently, C. commersonii was harpooned for crab bait in the Strain of Magellan in Chile. As a result, the dolphins are no longer common in this area.

Commerson's dolphin is the cetacean species most frequently taken in fishing nets off the coast of southern South America. In Tierra del Fuego alone at least 5-30 dolphins die each year as by-catch in nets set perpendicular to the shore.

KERGUELEN POPULATION

There are no recorded incidents of dolphins being taken for bait or any dolphin mortality through fishing activities. Low levels of chlorinated hydrocarbons (DDT, PCB and HCB) were found in the blubber of Commerson's dolphins in this area. This confirmed the presence of pollutants in oceans far away from their main source. The levels of contaminants were 10-100 times that of cetaceans in the North Atlantic. Contamination varied with age and sex; as males levels rise with age, whereas in females the levels decrease, perhaps through transfer to the fetus (Goodall, 1994).