Commerson's Dolphin (Cephalorhynchus commersonii) is
one of four dolphins in the Cephalorhynchus genus. The species has also the
common names Skunk Dolphin and Piebald Dolphin. The dolphin is found in two
geographical distinct areas - around the southern tip of South America around
Tierra del Fuego and the Falkland Islands and near the Kerguelen Islands in
the southern part of the Indian Ocean.
Physical description:
Commerson's Dolphin has a very distinctive patterning. It has a black head,
dorsal fin and fluke and white throat and body. The demarcation between the
two colours is very clear-cut. In shape and size - the creature is stocky
and grows to around 150cm - resembles that of that a porpoise but its conspicious
behaviour (see behaviour below) is typical of a dolphin. The dorsal fin has
a long, straight leading edge leading to a curved tip. The trailing is typically
concave but not falcate. The fluke has a notch in the middle. The dolphin
has no beak.
Females reach breeding age at six to nine years. Males reach sexual maturation at about the same age. Mating occurs in the spring and summer and calving occurs after a gestation period of 11 months. The oldest known Commerson's Dolphin died at age 18.
Population and distribution:
The species is distributed in two locations. The larger population is found
inshore in various inlets in Argentina, in the Strait of Magellan and near
the Falkland Islands. The second population is resident close to the Kerguelen
Islands, 8000km to the east of their nearest special cousins. They prefer
shallow waters. Global populations are unknown, but the species is accepted
to be locally common. A survey in 1984 estimated there to be 3400 individuals
in the Strait of Magellan.
Behaviour:
Commerson's Dolphin is very active. It is often seen swimming rapidly on the
surface and leaping from it. It also spins and twists as it swims and may
surf on breaking waves when very close to the shore. It will bow-ride and
swim behind fast-moving boats.
The dolphin feeds on a mix of coastal and pelagic fish and squid. Those in the South American sub-population supplement their diet with crustaceans.
Conservation:
The IUCN lists Commerson's Dolphin as data deficient in its Red List of Threatened
Species. The proximity of the dolphin to the shore makes accidental killing
in gillnets a common occurrence. The dolphin was killed for use as crab bait
by some Argentianian and Chilean fishermen in the 1970s and 80s. This practice
has now tailed off.
References:
National Audubon Society: Guide to Marine Mammals of the World .
Encyclopedia of Marine Mammals .