Dall's porpoise (Phocoenoides dalli) is a species of porpoise endemic to the North Pacific. It is the largest of porpoises and the only member of the genus Phocoenoides. The species is named after American naturalist W. H. Dall.
Taxonomy
Dall’s porpoise is the only member of the genus Phocoenoides. The dalli- and truei-types were initially described as separate species in 1911, but later studies determined that the available evidence only supported the existence of one species. Currently, these two color morphs are recognized as distinct subspecies, Dall's porpoise (Phocoenoides dalli dalli) and True's porpoise (Phocoenoides dalli truei).
Description
Dall's porpoises can be easily distinguished from other porpoises and cetacean species within their range. They have a wide, robust body, a comparatively tiny head, and no distinguished beak. Their flippers are positioned at the front of the body and a triangular dorsal fin sits mid-body. Patterns of coloration are highly variable, but Dall’s porpoises are mostly black, have white to grey patches on the flank and belly, and frosting on the dorsal fin and trailing-edge of the fluke.
They are the largest porpoise species, growing up to 7.5 ft (2.3 m) in length and weighing between 370 and 490 lbs (130 and 220 kg). Sexual dimorphism is apparent in body size and shape, with mature males being larger, developing a deeper caudal peduncle, and having a dorsal fin that’s significantly angled forward in comparison to a female’s. Dall’s porpoise calves have a greyish coloration with no frosting on flippers and flukes. Calves measure about 100 cm at birth. Growth rates are similar at first, but at about 2 years old males begin to grow faster than females. Externally, maturity is measured by length which is usually attained at 3 – 5 years old. Sizes vary between populations, but on average females reach a maximum size of 210 cm and males grow to about 220 cm, except in the southern Okhotsk Sea where males can grow as long as 239 cm.
Two colormorphs have been identified: the dalli-type and truei-type. The truei-type, found only in the western Pacific, has a white belly patch that extends farther forward across the body than that of the dalli-type.
Distribution and habitat
Dall’s porpoises are limited to the North Pacific: in the east from California to the Bering Sea and Okhotsk Sea, and in the west down to the Sea of Japan. They have been sighted as far south as Scammon’s Lagoon in Baja California when water temperature was unseasonably cold.
Dall’s porpoises generally prefer cold waters less than 64 °F (18 °C). Although mostly an offshore species, they do occur in deeper coastal waters, near submarine canyons or in fjords.
Behavior
Foraging
Dall’s porpoises are opportunistic, hunting a variety of surface and mid-water species. Common prey are mesopelagic fish, such as myctophids, and gonatid squid. Stomach content analyses have also found cases of crustacean consumption, including krill and shrimp, but this is abnormal and likely not an important part of their diet. A previous study revealed that tagged Dall’s porpoises spent most of their time within 10 m of the surface, but have been recorded diving to depths of up to 94 m.
Social
Dall's porpoises live in small, fluid groups of 2–10 individuals, but aggregations of hundreds have been reported. They have a polygynous mating system in which males compete for females. During the mating season, a male will select a fertile female and guard her to ensure paternity. While guarding, males may sacrifice opportunities to forage on deep dives. Births usually take place in the summer after a gestation period of 11–12 months. Females generally give birth every 3 years, depending on their condition. Life expectancy is about 15–20 years, but a lot about their mortality is unknown.
Dall’s porpoises are prey to transient killer whales. They have, however, been observed in association with resident killer whales, engaging in apparent play behaviors with their calves, and swimming with them. One recognizable Dall’s porpoise was observed travelling with the AB pod of resident orca from May through October 1984. Great White Sharks are also a known predator, with at least one documented case on the eastern North Pacific Ocean.
Movement
Dall’s porpoises are highly active swimmers. Rapid swimming at the surface creates a characteristic spray called a "rooster tail". They are commonly seen approaching boats to bowride, and they will also ride on the waves formed at the heads of larger swimming whales.