Dall’s porpoises are robust animals, with a wide-based triangular dorsal fin, and small flippers placed near the head. The small head has a short beak, with no demarcation from the melon. From above, the head appears triangular.
These porpoises are strikingly marked, with a black body and bright white flank patches that are continuous ventrally, although young animals have muted color patterns. The flank patches extend from the urogenital area to just in front of the dorsal fin, and up the sides about midway. In addition, there is white to light gray "frosting" on the upper portion of the dorsal fin and the trailing edges of the flukes. There are two commonly-occurring color types, dalli-type (described above) and truei-type (which has a larger flank patch that extends to the level of the flipper). Rarely, intermediate types to occur as well.
Dall’s porpoise has the smallest teeth of any cetacean. There are 23-28 tiny spade-shaped teeth in each tooth row.
The International Whaling Commission currently recognizes eight stocks, based on pollutant loads, parasite faunas, and distribution patterns of cow/calf pairs. Other than color type (dalli-type or truei-type), the stocks can not be distinguished by appearance at sea. Newborn Dall’s porpoises are about 1 m long. Adults are 1.7-2.2 m (females) or 1.8-2.4 m (males). Maximum weight is about 200 kg.
Can be Confused With
Dall’s porpoises are likely to be confused only with harbor porpoises, and even then, only if seen at a great distance. When seen well, the differences in color pattern and dorsal fin shape will be readily apparent.
Dall’s porpoises are found only in the North Pacific Ocean and adjacent seas. They inhabit deep waters of the warm temperate through subarctic zones, between about 30°N and 62°N. There is apparently a single truei-type population that migrates between the Pacific coast of Japan and the Okhotsk Sea; dalli-types predominate in all other areas of the range.
Ecology and Behavior
This may be the fastest swimmer of all small cetaceans, at least for short bursts. When swimming rapidly, Dall’s slice along the surface, producing a characteristic roostertail of spray. At other times, the animals move slowly and roll at the surface, creating little or no disturbance. They are avid bowriders, moving back and forth with jerky movements, and often coming from seemingly nowhere to appear at the bow of a fast-moving vessel. Breaching, porpoising, and other kinds of aerial behavior, are extremely rare in this species. Dall’s porpoises are found mostly in small groups of 2-12, although aggregations of up to several thousand have been reported. Groups appear to be fluid, often forming and breaking up for feeding and playing.
Most Dall’s porpoise calves are born in spring and summer, and there is great geographic variation in age and length at sexual maturity among populations.
Feeding and Prey
Dall’s porpoises are apparently opportunistic feeders, taking a wide range of surface and midwater fish and squid, especially lanternfish and gonatid squid. Occasional krill, decapods, and shrimps found in stomachs are not considered normal prey.
Threats and Status
Dall’s porpoise is one of the primary species taken directly by Japanese fishermen for human consumption. Porpoises are harpooned as they ride the bow waves of catcher boats, and in the recent years the annual catch has been as high as 40,853 (1988). In addition, Dall’s porpoises have been taken incidentally in large numbers in several North Pacific driftenet fisheries, although several of these fisheries are now defunct. Smaller incidental catches occur in several fisheries using gillnets and trawls in US and Canadian west coast waters. Environmental contaminants are also thought to be a threat, and high levels of organochlorines may reduce testosterone levels in males and reduce calf survival, thereby influencing reproduction.
Currently, Dall’s porpoises are listed as ‘Lower Risk/Conservation Dependent’ (IUCN) and ‘Not Listed’ (ESA).
Escorza Trevino, S. and A.E. Dizon. 2000. Phylogeography, intraspecific structure and sex-biased dispersal of Dall’s porpoise, Phocoenoides dalli, revealed by mitochondrial and microsatellite DNA analyses. Molecular Ecology 9:1049-1060.
Ferrero, R.C. and W.A. Walker. 1999. Age, growth, and reproductive patterns of Dall’s porpoise (Phocoenoides dalli) in the central North Pacific Ocean. Marine Mammal Science 15:273-313.
Houck, W.J. and T.A. Jefferson. 1999. Dall’s porpoise Phocoenoides dalli (True, 1885). pp. 443-472 in S.H. Ridgway and R. Harrison, eds. Handbook of marine mammals, Vol. 6 The second book of dolphins and the porpoises. Academic Press.
Jefferson, T.A. 1988. Phocoenoides dalli. Mammalian Species 319:1-7
Jefferson, T.A. 2002. Dalls’ porpoise Phocoenoides dalli. pp. 308-310 in W.F. Perrin, B. Würsig and J.G.M. Thewissen, eds. Encyclopedia of marine mammals. Academic Press.
Miyashita, T. and T. Kasuya. 1988. Distribution and abundance of Dall’s porpoise off Japan. Scientific Reports of the Whales Research Institute 35:107-128.