Alaska Marine Mammals: Dall's Porpoise

Marine Mammals: Dall's Porpoise

Alaska's State Flower is the "Forget-Me-Not. Forget me not flowers are very fragrant in the evening and night time, though there is little or no scent in the daytime. They can be annual or perennial plants. Their seeds are found in small, tulip shaped pods along the stem to the flower.

The Dall's porpoise is easily identified by it's very unique black and white markings and is named for the naturalist who first discovered them, W.H. Dall. 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 coloration is rather like that of a killer whale(orca); the main body of the porpoise is very dark grey to black creeper, with very demarcated white patches on the flank and belly. The dorsal fin 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 7 ft 7 inches in length and weighing between 130 and 200 kg. There is also sexual dimorphism in the species, with males being larger, having a deeper caudal peduncle and a pronounced hump behind the anus. Young Dall's have a grayish tint and dark-colored flukes.

Dall’s porpoises eat a variety of small fishes and cephalopods.Schooling fish, such as herrings, anchovies, pilchards, mackerels, hake and sauries are common prey. 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 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 behaviors 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. The calving season for all studied populations is mostly in the summer. Porpoise gestation lasts 10 to 11 months. Females in their prime can give birth annually. Dall's porpoises live for up to 15 years.

These fun-loving playful porpoises can only be found in the North Pacific Ocean where they prefer cooler water temperatures between 36 and 60 degrees (2C-15C). Many are year round residents of their region. These regions range from up north in the Bering Sea and Alaska to as far south at Baja California and as far east as the Sea of Japan.

Their small, narrow mouth has about 20 small teeth in each side of the upper and about 22 teeth on each side of the lower jaw. Protruding, rigid growths separate each tooth often called “gum teeth”. These small teeth and horny growths help the porpoise grasp slippery food such as squid. The Dall porpoise feeds mostly at night eating about 30 pounds of food each day. Their diet varies depending on what's available, such as squid and small schooling fish like herring, capelin and sardines. They have also been seen feeding in the deep waters of the Pacific ocean on deep-sea smelt and lantern fish.

Estimates of total population size for the whole Northern Pacific in 1983 ranged from 790,000 to 2,300,000 animals. There are an estimated 83,400 Dall’s porpoise in the Alaskan stock, though due to difficulties in gathering reliable population counts and lack of direct effort, these are not exact numbers.

Killer whales and sharks are the natural predators of the Dall’s porpoise; though because of their large body size, speed and agility, it s believed they generally escape predation. They also are susceptible to certain parasites. The trematode fluke Nastitrema, an internal parasite, is known to cause death and stranding of the porpoises.

There is no Dall’s porpoise fishery in the United States, but it is estimated that about 30 animals per year are killed as a result of being caught in fishing nets in Alaska. There is still an active Dall’s porpoise fishery in Japan.

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