(Cuvier, 1829) - Finless porpoise
Distinctive Characteristics
As the name implies, finless porpoises have no dorsal fin, and this is their most distinctive characteristic. In some ways, they resemble small, slender white whales. The head is beakless; the rounded forehead rises steeply from the snout tip. The body shape, in general, is more slender than in other porpoises. The finless porpoise is soft and mushy, and the neck is very flexible. Instead of a dorsal fin, the finless porpoise has an area of small bumps or tubercles on its back, running from just forward of midback to the tail stock. The trailing edge of the flukes is concave and the flippers are large, ending in rounded tips. Regional differences in body size and morphology have been documented, with Yangtze River animals apparently representing a separate stock.
The common name that was used in the past, “finless black porpoise,” apparently resulted from descriptions of dead animals, after post-mortem darkening. In most areas, finless porpoises are grey in colour, with lighter areas on the throat and around the genitals. Older animals are generally lighter grey than juveniles. In the Yangtze River population, they are very dark grey, nearly black.
Tooth counts range from 13 to 22 in each tooth row.
Can be confused with
The smooth back of the finless porpoise should make it easy to distinguish from other species, such as the Irrawaddy dolphin, baiji, and Ganges River dolphin, which share parts of its range.
Size
Adults of this species reach about 1.9 m in length (males are slightly larger than females). Finless porpoises are apparently about 70 to 80 cm at birth.
Geographical Distribution
Warm, coastal Indo-Pacific waters, both fresh and marine, are home to the finless porpoise. The range runs from northern Japan to the Persian Gulf, including many rivers in the Asian subcontinent (one of the best known populations is in the Yangtze River of China).
Biology and Behaviour
Finless porpoises are generally found as singles, pairs, or in groups of up to 12, although aggregations of up to about 50 have been reported. Like other porpoises, their behaviour tends to be not as energetic and showy as that of dolphins. They do not ride bow waves, and in some areas appear to be shy of boats. Mothers have been seen carrying calves on the denticulated area on their backs. In the Yangtze River, finless porpoises are known to leap from the water and perform “tail stands.”
Reproduction in most areas has not been well studied. Reports indicate that calving in the Yangtze River occurs between February and April, and in Japan it occurs between April and August.
Small fishes, squids, and shrimps form the diet of finless porpoises. They also apparently ingest some plant material, including leaves and rice.
Exploitation
Finless porpoises are known to be taken in various gillnet fisheries throughout their range, including the Yangtze River. They are also incidentally taken in beach seines in India. Direct exploitation with guns, harpoons, and “fish forks” used to occur in China, and previously some incidental catches were sold for human consumption in Japan. Pollution and habitat destruction may also be factors in the status of this species. Some porpoises have been captured live for aquariums in Japan.
IUCN Status
Insufficiently known.