Pacific white-sided dolphins, like all members of the genus Lagenorhynchus, are stocky animals with very short, thick snouts. The large flippers have slightly rounded tips. The dorsal fin, the species’ most diagnostic feature, is prominent, strongly-recurved, and bi-colored.
The dark gray back and sides are distinctly set-off from the white belly by a black border. Light gray streaks beginning on the sides of the melon sweep downwards gradually past the dorsal fin and finally expand into large gray flank patches; gray "suspender stripes," which start above the eyes, widen to bands on the sides of the tail stock. The lips are black. Each tooth row contains 23-36 pairs of relatively fine, sharply pointed teeth.
Adults of this species reach 2.5 in length, with males slightly larger than females. Maximum weight is about 180 kg. Length at birth is estimated to be 92-100 cm in the central Pacific.
Can be Confused With
Pacific white-sided dolphins are most likely to be confused with common dolphins, because both species are found in large schools and have large light-colored flank patches. Beak length and specifics of the color pattern are the best keys to distinguishing them.
Pacific white-sided dolphins inhabit temperate waters of the North Pacific and some adjacent seas. Although they are widely distributed in deep offshore waters, they also extend onto the continental shelf and very near shore in some areas. They reach their southern limits at the mouth of the Gulf of California (and occasionally venture northward in the Gulf to or beyond La Paz) and southern Japan (records from Taiwan are considered to be misidentifications). On both eastern and western sides of the Pacific, separate stocks have been documented.
Ecology and Behavior
Often seen in large herds of hundreds or even thousands, these highly gregarious dolphins are also commonly seen with other species, especially northern right whale dolphins and Risso’s dolphins. They are highly acrobatic and playful, commonly bowriding, and often leaping, flipping, or somersaulting. Large herds are often observed feeding in an apparently cooperative manner on large schools of fish.
Males reach sexual maturity at about 10 years, and females at 8-11 years of age. Calving in some areas apparently occurs during a protracted summer breeding season, which extends into fall. In the central Pacific, calving takes place in late winter to spring. Length at birth is estimated to be 92-100 cm in the central Pacific.
Feeding and Prey
Pacific white-sided dolphins feed mostly on small schooling fish and squid. There is evidence that these dolphins feed mostly on deep scattering layer (DSL) organisms, possibly using cooperative foraging techniques.
Threats and Status
Main threats include:
• Gillnet and drift net fisheries
Pacific white-sided dolphins have never been primary targets of Japanese drive fisheries, but they have been harpooned in Japanese waters. They are taken in a number of fisheries (mainly gillnet and driftnet fisheries) in the eastern North Pacific, and in the 1980s and early 1990s, several thousand per year were killed in the now-defunct squid driftnet fisheries that were operated in the offshore central Pacific by Japan, Taiwan, and Korea.
Currently, white-sided dolphins are ’Not Listed‘ (IUCN) and ‘Not Listed’ (ESA).
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Van Vaerebeek, K. and B. Würsig. 2002. Pacific white-sided dolphin and dusky dolphin Lagenorhynchus obliquidens and L. obscurus. pp. 859-861 in W.F. Perrin, B. Würsig and J.G.M. Thewissen, eds. Encyclopedia of marine mammals. Academic Press.