|Last modified: Tuesday September 22nd, 1999|
Dall's porpoisePhocoenoides dalli (True, 1885)
In earlier literature, 2 species have been described: Dall's porpoise (Phocoenoides dalli) and True's porpoise (Phocoenoides truei), based on color variations and differences in the number of teeth. However, it is now generally accepted that there is only one species, with two different forms, referred to as dalli-type and truei-type (Minasian et al (1984), Klinowska (1991)). Occassionally these forms are considered to be subspecies (Evans, 1987).
The Dall's porpoise is a very stocky animal, with small appendages. There is a slight indentation in the head near the blowhole. A dorsal ridge leads from the dorsal fin to the tail flukes. The dorsal fin is pointed and triangular. The flippers are very small and rounded at the tips. The small flukes are almost pointed at the tips. This species is jet-black with a variable white patch on the flanks and the belly. The tips of the dorsal fin and tail flukes are also white. Four varieties are recognized:
The Dall's porpoise grows to 1.8-2.1 m in length and weighs 135-220 kg. They have 19-29 pairs of spatula-shaped teeth in each jaw. They are avid bow-riders and fast swimmers. They create a charateristic "rooster tail" of water when swimming at high speeds (Evans (1987), Minasian et al (1984)).
The Dall's porpoise is found in the Northern Pacific, from north of Honshu, Japan in the west and from 28°N in the east up to the Sea of Okhotsk and the Bering Sea, in summer as far north as the Pribilof islands. The truei-type is found only off Japan and the Kurill Islands, where they overlap with the dalli-type (Klinowksa, 1991). They are usually found in small groups of a dozen or so individuals. In Monterey Bay, group size was on average 5.7 animals with a maximum of 20/group. In this area, mostly subadults were seen. They had a preference for the deeper water near the Monterey Submarine Canyon (Jefferson, 1991).
Calves are born between early spring and early fall. There is a strong peak in births in June, July and August. The gestation period is about 1 year. Fetal growth is about 10 cm/month. At birth the calves are about 1 m long. In the first year they grow at a rate of about 3.75 cm/month (Jefferson, 1989).
In 1987, the estimated number of dalli-type animals in the Sea of Japan/Sea of Okhotsk area was 32,000. The number of truei-type animals in the same area was 56,000-90,000. Estimates for the whole Northern Pacific in 1983 ranged from 790,000 to 2,300,000 animals. (Donovan and Bjørge, 1995). The population in the Western North Pacific (west of 172°W) was estimated to be 141,800 (83,100-241,700) animals in 1984 (Turnock et al, 1995). There was no trend in abundance for the period 1979-1984. For 1984-1986 there was evidence for a decrease in abundance in this area (Turnock and Buckland, 1995)
This species has been extensively hunted in Japanese coastal harpoon fisheries. In 1982, 12,833 animals were reported killed in these fisheries, which marked an increase in catches. In 1988, 40,367 animals were killed, in 1991 17,634. For 1992-1994 the reported catches were 11,403, 14,318 and 15,947 (Donovan and Bjørge (1995), IWC (1996)). This species is also bycaught in driftnet fisheries. Salmon fisheries in the USA 200-mile zone had an incidental take of 4,187 in 1982. In 1987, this number was reduced to 741. (Klinowska, 1991). Reported bycatches for 1993 and 1994 were 20 and 26 animals resp. (IWC (1996); see also Barlow et al (1994))
The Dall's porpoises feeds on a variety of prey species, including squid, other cephalopods, capelin, Pacific hake, jack mackerel, blennies and herring. In the Northwestern Pacific the prey also includes Pacific mackerel, sardines and saury. They are probably deep divers, because deep-water benthic species have been found in their stomachs. A study in the Sea of Okhotsk revealed that the Dall's porpoise in that area, cephalopods comprised about 20% of the diet. The most common fish species eaten were the Japanese pilchard (72% by number, 80% by caloric intake) and the walleye pollock (Walker, 1996)
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