Phocoenoides

Dall's Porpoise (Phocoenoides dalli) is a species of porpoise that came to worldwide attention in the 1970s when it was disclosed for the first time to the public that salmon fishing trawls were killing thousands of Dall's Porpoise and other cetaceans eachyear by accidentally capturing them in their nets.

The Dall's Porpoise is the only member of the Phocoenoides genus. It was named after the famousAmerican 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 an Orca - the main body of the porpoise is verydark grey to black with very demarced white patches on the flank that begin some way behind the flippers, which are small andclose to the head. The dorsal fin is set just back from the middle of the backand 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 anotherdistinguishing feature. It is larger than other porpoises - growing upto 230cm in length and weighing between 130 and 200kg.Dall's Porpoise live for upto 15 years.

Population and distribution

Two consistent and well-defined colour morphs - the dalli-type morph and the truei-type morph have beenidentified. The dalli-type is more widespread ranging across the north Pacific Ocean from southern California to southern Japan (including the Sea of Japan in thesouth up to the Bering Sea in the north. The truei type is morerestricted - occupying a smaller area of the north west Pacific north and east of Japan. The species differs from other porpoisesin that it prefers deep water and the open ocean. It may come closer to land but if it does typically stays close to deep watercanyons. The population is believed to be common in most of its range and although no precise estimates exists (surveying ishampered by the porpoises' propensity for approaching boats) the global population numbers in at least the hundreds of thousands(but see also 'conservation' below). The greatest concentration is perhaps in the Sea of Okhotsk .

Behaviour

Dall's Porpoises are hugely active creatures. They will often zig-zag around at great speed on or just below the surface ofthe water creating a spray called a "rooster tail". They may appear and disappear quite suddenly. The fastest of all smallcetaceans, and coming close to matching the mighty Killer Whale for speed,Dall's Porpoises can swim at up to 55km/h. The porpoises will approach boats and will bow- and stern-ride but is liable to loseinterest unless the boat is travelling quickly.

Dall's Porpoises appear in small groups (about 2-10 in number). The groups appear not to be tight knit and may aggregatetogether at good feeding grounds. On very rare occasions a thousand or more individual may be seen at the same time. They feed ona variety of fish and cephalopods . Shoalsof fish such as herring , anchovies and mackerel are common meals.

Hybrids

Baird et al (1998) determined through DNA sequencing that a fetus found in BritishColumbia was a intergeneric hybrid of a Dall's Porpoise and a Harbour Porpoise . This hybrid may not be rare - it may describe the originsof some atypically coloured individuals that otherwise appear to be Dall's Porpoises spotted off the coast of Vancouver Island .

Conservation

Many Dall Porpoise's are killed each year in fishing nets. However this accidental kill is probably not sufficient to depletethe productive populations in most areas. A serious cause of concern for conservationists is the hunting of Dall's Porpoises byharpoon by Japanese hunters. The number of porpoise killed each year rose dramatically following the moratoria on hunting largercetaceans introduced in the mid-1980s. 1988 saw the greatest number - 40,000 - killed. A quota of 18,000 individuals per year isnow in operation. However concern remains that this is sufficient to deplete populations in the western Pacific.

References

1. Dall's Porpoise in the Encyclopedia of Marine Mammals Thomas A. Jefferson, 1998. ISBN 0125513402
2. National Audubon Society Guide to Marine Mammals of the World Reeves et al, 2002. ISBN 0375411410 .
3. Whales, Dolphins and Porpoises Carwardine, 1995. ISBN 0751327816
4. An intergenetic breed in the family Phocoenoidae, Canadian Journal of Zoology, Baird, Willis, Guenther,Wilson and White 1998. Vol 76 pages 198-204.
5. Adescription and image of Dall's Porpoise at phocoena.org