by zteve t evans

Created on: January 11, 2011

This porpoise is named after William Healey Dall who acquired the first specimen in the 19th century off the coast of Alaska.  Dall’s porpoise is also known as the Spray porpoise, White-flanked porpoise and True’s porpoise and its scientific name is Phocoenoides dalli.

It comes in two recognisable forms known as the Dalli-type and the Truei-type.   The Dalli-type has smaller white patches on its flanks and is found throughout the Dall’s porpoise’s range.  The Truei-type has larger patches of white and lives only in waters of the North Pacific.

Although one of the smaller cetaceans it has a muscular, stocky body that give it great speed though the water making it the fastest of the cetaceans.  It will often approach boats and bow-ride though soon appears to lose interest if the boat is not going fast enough.

Physical Characteristics

Males can sometimes reach 6.5 feet (2.29 m) in length and females 7 feet (2.1 m), though the usual length is about 6 feet (1.8 m). The average weight for both males and females is 270 pounds (125 kg), though some individuals weighing up to 350 pounds (160 kg) have been recorded.

Dall’s porpoise are distinguished by its striking black and white coloring to the dorsal fin, sides and underside.   The back is jet black with a dorsal fin that is triangular-shaped, though slightly curved at the point and carries a white patch. 

The head is small in proportion to its body with an indistinct beak.  When it surfaces at speed to breathe, water shoots off its head causing a spray known the ‘rooster tail,’ which is very distinctive and helps with identification.

Diet

The diet of Dall’s porpoise is quite diverse but it will take what ever is available within its range and depending on the season.  It usually eats 28-30 pounds (12.7-13.6 kg) of food a day.    Generally, as well as squid, it preys on schools of small fish such as sardines and herring, though it also hunts deep water fish such as hake and smelt and in the northwest Pacific it preys on lantern fish.

Breeding

Females give birth mostly in the summer after a gestation period of 10-12 months but along the coast of North America it can happen all year round.  Calves are nursed by the mother for about 2 years.

Range

Dall’s porpoises are found in the Sea of Japan, the Bering Sea, the Okhotsk Sea and the North Pacific. Although oceanic it is often found near the shore where there is deep water such as over underwater canyons.

Threats

Dall’s porpoises are thought to live no more than 20 years with the main natural threats coming from sharks and killer whales.   Human activity takes a much larger toll with many thousands killed each year for human consumption by the Japanese and many others drown caught up in drift-nets and other fishing equipment.  

References

http://en.wikipedia.org/wiki/Dall%27s_porpoise

http://www.acsonline.org/factpack/DallsPorpoise.htm

http://marinebio.org/species.asp?id=363

http://www.cms.int/reports/small_cetaceans/data/P_da lli/p_dalli.htm

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