Dall's porpoise (Phocoenoides dalli) is a species of porpoise endemic to the North Pacific. It is the largest of porpoises and the only member of the genus Phocoenoides. The species is named after American naturalist W. H. Dall.

thumb|[[William Healey Dall's 1873 field notes on Phocoenoides from the Smithsonian Institution's Field Books collection]]

Taxonomy

Dall's porpoise is the only member of the genus Phocoenoides. The dalli and truei types were initially described as separate species in 1911, but later studies determined that the available evidence only supported the existence of one species. Currently, these two color morphs are recognized as distinct subspecies, Dall's porpoise (Phocoenoides dalli dalli) and True's porpoise (Phocoenoides dalli truei).

They are the largest living porpoise species, growing up to in length and weighing . Growth rates are similar at first, but at about two years old males begin to grow faster than females. Sizes vary between populations, but on average females reach a maximum size of 210 cm and males grow to about 220 cm, except in the southern Okhotsk Sea where males can grow as long as 239 cm.

Distribution and habitat

thumb|left|A Dall's porpoise on a calm day in the [[Shelikof Strait]]

Dall's porpoises are limited to the North Pacific: in the east from California to the Bering Sea and Okhotsk Sea, and in the west down to the Sea of Japan. They have been sighted as far south as Scammon's Lagoon in Baja California when water temperature was unseasonably cold. but have been recorded diving to depths of up to 94 m.

Social

Dall's porpoises live in small, fluid groups of two to ten individuals, During the mating season, a male will select a fertile female and guard her to ensure paternity.

Dall's porpoises are prey to transient killer whales. They have, however, been observed in association with resident killer whales, engaging in apparent play behaviors with their calves, and swimming with them. One recognizable Dall's porpoise was observed travelling with the AB pod of resident orca from May through October 1984.

Movement

thumb|left|Dall's porpoise in [[Prince William Sound causing a "rooster tail"]]

Dall's porpoises are highly active swimmers. Rapid swimming at the surface creates a characteristic spray called a "rooster tail". They are commonly seen approaching boats to bowride, and they will also ride on the waves formed at the heads of larger swimming whales. Alaska's population is estimated to be 83,400. Abundance in coastal British Columbia is nearly 5,000 individuals.

Populations in the western North Pacific are divided by both subspecies and migratory patterns. Abundance of the offshore dalli type is about 162,000. It is estimated that there are about 173,000 dalli type that travel between Japan and the southern Okhotsk Sea. The dalli type that migrates to the Okhotsk Sea in the summer is estimated at 111,000. Before the moratorium went into effect, 8,000 Dall's porpoises are estimated to have been bycaught in one year alone (1989–1990). Smaller numbers, from several hundred to a few thousand, are estimated to have been bycaught in Japanese salmon fisheries in US waters and in the Bering Sea from 1981 to 1987. Driftnet and trawl fisheries still operate in some areas throughout their range,

Hunting

thumb|Dall's porpoises at market in Japan

The Dall's porpoise is still harvested for meat in Japan. The number of individuals taken each year increased following the 1980s moratorium on whaling of larger cetacean species. In 1990, after international attention was drawn to the issue, the Japanese government introduced a reduction on take. The hunt of Dall's porpoises has been criticized by scientific committees which question the sustainability of large quotas on regional populations. It has been suggested that the assessment of these target populations is outdated and that there may be a regional population decline given the reported annual catch.

Pollution

Environmental contaminants, including dichlorodiphenyldichloroethylene (DDE) and polychlorinated biphenyls (PCBs), are another threat to Dall's porpoises. Pollutants accumulate in the blubber layer, and in high concentrations can reduce hormone levels, affect the reproductive system, and result in calf death.

Conservation status

Dall's porpoise is listed as Least Concern on the IUCN Red List. of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), and, like all other marine mammal species, is protected in the United States under the Marine Mammal Protection Act (MMPA).

References

To learn more about the Dall's porpoise and other cetacean species visit:

  • Whale & Dolphin Conservation Society (WDCS)
  • NOAA Fisheries
  • Porpoise Conservation Society
  • Voices in the Sea – Sounds of the Dall's Porpoise
  • American Cetacean Society
  • Society for Marine Mammalogy