Dall's Porpoise
Overview
Dall's porpoise is a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of porpoise
Porpoise
Porpoises are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. They are distinct from dolphins, although the word "porpoise" has been used to refer to any small dolphin, especially by sailors and fishermen...

 found on the North Pacific. It came to worldwide attention in the 1970s when it was disclosed for the first time to the public that salmon
Salmon
Salmon is the common name for several species of fish in the family Salmonidae. Several other fish in the same family are called trout; the difference is often said to be that salmon migrate and trout are resident, but this distinction does not strictly hold true...

 fishing trawls were killing a lot, thousands of Dall's porpoises and other cetaceans each year by accidentally capturing them in their nets. The Dall's porpoise is the only member of the Phocoenoides genus. It was named after American naturalist W.
Encyclopedia
Dall's porpoise is a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of porpoise
Porpoise
Porpoises are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. They are distinct from dolphins, although the word "porpoise" has been used to refer to any small dolphin, especially by sailors and fishermen...

 found on the North Pacific. It came to worldwide attention in the 1970s when it was disclosed for the first time to the public that salmon
Salmon
Salmon is the common name for several species of fish in the family Salmonidae. Several other fish in the same family are called trout; the difference is often said to be that salmon migrate and trout are resident, but this distinction does not strictly hold true...

 fishing trawls were killing a lot, thousands of Dall's porpoises and other cetaceans each year by accidentally capturing them in their nets. The Dall's porpoise is the only member of the Phocoenoides genus. It was named after American naturalist W. H. Dall.

Physical description

The unique body shape of the Dall's porpoise makes it easily distinguishable from other cetacean species. The animal has a very thick body and a small head. The colouration is rather like that of a killer whale(orca); the main body of the porpoise
Porpoise
Porpoises are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. They are distinct from dolphins, although the word "porpoise" has been used to refer to any small dolphin, especially by sailors and fishermen...

 is very dark grey to black creeper, with very demarcated white patches on the flank and belly. The dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

 is set just back from the middle of the back and sits up erect. The upper part of the dorsal fin has a white to light grey "frosting".

The fluke has a similar frosting. The adult fluke curves back towards the body of the animal, which is another distinguishing feature. It is larger than other porpoises, growing up to 2.3 metres (7.5 ft) in length and weighing between 130 and 200 kg. There is also sexual dimorphism
Sexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...

 in the species, with males being larger, having a deeper caudal peduncle and a pronounced hump behind the anus. Young Dall's have a greyish tint and dark-colored flukes.

Population and distribution

The Dall’s porpoise ranges through much of the North Pacific and adjacent seas, such as the Bering and Okhotsk Seas and the Sea of Japan. The southernmost part of its range is southern Japan in the west and southern California in the east, while it is northernmost range is the central Bering Sea. They do enter Scammon's Lagoon in Baja California, though, when the waters are unseasonably cold. They also travel up to the Chukchi Sea, though very rarely.

The Dall’s porpoise prefers cold waters more than 180 metres (590.6 ft) deep . It is found over the continental shelf adjacent to the slopes and oceanic waters. While it mostly lives in offshore waters, it does occur in deep nearshore and inshore waters on the west coast of North America. There, it typically stays close to deep-water canyons.

Two consistent and well-defined colour morphs, the dalli-type and the truei-type, have been identified. The dalli-type is present throughout the porpoise’s range, while the truei-type is limited to the western Pacific between 35° N and 54° N. These morphs have been considered different in colour pattern (the truei-type having a more extensive belly patch) and not separate subspecies. However, some genetic evidences suggests that there statistically significant genetic differentiation was found between them. They are believed to number around 104,000 off Japan, 554,000 in the Okhotsk Sea, 83,000 off Alaska and 100,000 off the continental US.

Ecology and lifestyle

Dall’s porpoises eat a variety of small fishes and cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

s. Schooling fish, such as herring
Herring
Herring is an oily fish of the genus Clupea, found in the shallow, temperate waters of the North Pacific and the North Atlantic oceans, including the Baltic Sea. Three species of Clupea are recognized. The main taxa, the Atlantic herring and the Pacific herring may each be divided into subspecies...

s, anchovies, pilchards, mackerel
Mackerel
Mackerel is a common name applied to a number of different species of fish, mostly, but not exclusively, from the family Scombridae. They may be found in all tropical and temperate seas. Most live offshore in the oceanic environment but a few, like the Spanish mackerel , enter bays and can be...

s, hake
Hake
The term hake refers to fish in either of:* family Phycidae of the northern oceans* family Merlucciidae of the southern oceans-Hake fish:...

 and sauries are common prey, as well as mesopelagic
Mesopelagic
The mesopelagic is that part of the pelagic zone that extends from a depth of 200 to 1000 metres below the ocean surface. It lies between the photic epipelagic above and the aphotic bathypelagic below, where there is no light at all...

 fish such as myctophids and deep sea smelts. They may also consume krill, but these are probably not important in their diet. Dall’s porpoise are also deep divers. They have been recorded to dive to 94m. Dall’s porpoise are prey to killer whales and white sharks. They also are susceptible to certain parasites. The trematode fluke Nastitrema, an internal parasite, is known to cause death and stranding of the porpoises. External parasites of the porpoise are whale lice.

Dall's porpoises are highly active creatures. They will often zigzag around at great speed on or just below the water surface, creating a spray called a "rooster tail". They may appear and disappear quite suddenly. The fastest of all small cetaceans, Dall's porpoises can swim at up to 55 km/h, almost as fast as the killer whale. The porpoises will approach boats and will bow- and stern-ride, but may lose interest, unless the boat is travelling quickly. They will also "snout ride" on bow waves pushed forward by the heads of large whales. When they move more slowly, they roll at the surface in a subdued manner like other porpoises. They rarely do aerial behaviours such as breaching and leaping.

Dall’s porpoises live in small, fluid groups of two to 12. Loosely associated groups can form feeding aggregations involving tens to hundreds, and rarely thousands, of individuals. They have a polygynous mating system in which males will guard females in estrus. During the mating season, a male will closely attend and defend a fertile female from other males to ensure his own paternity. Males that are paired with females have agonistic interactions with other adult males, and associate with extra-pair individuals infrequently. Guarding males tend to take shorter dives than nonguarding males, which suggests a trade-off between diving and gaurding. The calving season for all studied populations is mostly in the summer. Porpoise gestation lasts 10 to 11 months, and the lactation
Lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...

 period lasts at least two months. Females in their prime can give birth annually. Dall's porpoises live for up to 15 years.
One study determined through DNA sequencing
DNA sequencing
DNA sequencing includes several methods and technologies that are used for determining the order of the nucleotide bases—adenine, guanine, cytosine, and thymine—in a molecule of DNA....

 that a fetus found in British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...

 was an intergeneric hybrid of a Dall's porpoise and a harbour porpoise
Harbour Porpoise
The harbour porpoise is one of six species of porpoise. It is one of the smallest marine mammals. As its name implies, it stays close to coastal areas or river estuaries, and as such, is the most familiar porpoise to whale watchers. This porpoise often ventures up rivers, and has been seen...

. This hybrid may not be rare— it may describe the origins of some atypically coloured individuals that otherwise appear to be Dall's porpoises spotted off the coast of Vancouver Island
Vancouver Island
Vancouver Island is a large island in British Columbia, Canada. It is one of several North American locations named after George Vancouver, the British Royal Navy officer who explored the Pacific Northwest coast of North America between 1791 and 1794...

.

Conservation status

Many Dall's porpoises are killed each year as by-catch
By-catch
The term “bycatch” is usually used for fish caught unintentionally in a fishery while intending to catch other fish. It may however also indicate untargeted catch in other forms of animal harvesting or collecting...

 in fishing nets. A serious cause of concern is the hunting of the species by harpoon
Harpoon
A harpoon is a long spear-like instrument used in fishing to catch fish or large marine mammals such as whales. It accomplishes this task by impaling the target animal, allowing the fishermen to use a rope or chain attached to the butt of the projectile to catch the animal...

 by Japanese hunters. The number of porpoise caught each year rose dramatically following the moratorium on hunting larger cetaceans introduced in the mid-1980s. The greatest number were caught in 1988, when more than 40,000 were taken. International attention to the hunt through a 1990 International Whaling Commission
International Whaling Commission
The International Whaling Commission is an international body set up by the terms of the International Convention for the Regulation of Whaling , which was signed in Washington, D.C...

 (IWC) resolution resulted in a reduction in numbers caught; however, around 15,000 animals are still caught each year, making it the largest direct hunt of any cetacean species in the world. The hunt has been repeatedly criticized by the IWC and its Scientific Committee, most recently in 2008. A quota of just over 16,000 individuals per year is now in effect. In addition, unknown numbers of animals are struck and lost or caught as by catch. Despite these threats, the Dall's porpoise remains a fairly common species with productive populations.

The Dall's porpoise is listed on Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS
Bonn Convention
The Convention on the Conservation of Migratory Species of Wild Animals aims to conserve terrestrial, marine and avian migratory species throughout their range...

). It is listed on Appendix II as it has an unfavorable conservation status or would benefit significantly from international co-operation organized by tailored agreements.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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