Pacific white-sided dolphin is identified by the unusually large, curved dorsal fin, due to which the animal is sometimes called "hookfin porpoise", although it's not a porpoise. The sides and underneath of the dolphin are white. The animal is dark grey on top with a faint gray band, running along either side, starting above each eye and widening towards the tail. The small beak is dark, and the dolphin exhibits dark rings around the eyes. Pacific white-sided dolphins are also strong and fast swimmers. These playful animals are known to bow-ride, remaining around moving vessels for long periods of time.
This species is endemic to North Pacific, where the animal is typically found in deep, cool to temperate waters. The area of Pacific white-sided dolphin's population extends to the South China Sea in the south and the Baja California Peninsula in the east. These dolphins are also known to occur in the Sea of Japan and the Sea of Okhotsk. In the north, the animals can be found in the Bering Sea. On the eastern side of their range, Pacific white-sided dolphins display somewhat migratory behavior, living in the Southern California Bight in winter and moving to Oregon (Washington) in summer.
These highly social animals are occasionally observed in large schools of more than 1000 individuals. However, schools of 50 individuals are most common. Groups of Pacific white-sided dolphins typically include individuals of both sexes and different ages, generally getting along well with each other. Moreover, these dolphins are known to associate with other dolphin species, often feeding in the same areas without any conflicts. Within a group, it is common to help ill or hurt individuals. These animals prefer feeding by night, when fish shcools come closer to the water surface.
Pacific white-sided dolphins have polygynous mating system. Groups of this species contain individuals of both sexes, but only the dominant male mates with females of the pod. They typically breed in summer-autumn. Females give birth with intervals of 4.5 - 5 years. The gestation period lasts for 11 - 12 months, yielding a single baby, which is nursed by its mother for 8 - 10 months. Even after weaning, the female continues caring for her offspring for 2 - 3 years. Sexual maturity is reached within 5 - 6 years in females and by 8 - 10 years old - in males.
Presently, Pacific white-sided dolphins are often incidentally caught in fisheries like gillnets or trawls. In the North Pacific part of their range, the animals are occasionally captured in purse-seine fisheries. The species is also hunted by Japanese coastal fishermen in the East China and Japan seas.
According to IUCN, the Pacific white-sided dolphin is abundant and widespread throughout its range but no overall population estimate is available. However, there are estimates of its population in following areas: the central North Pacific, which is likely to hold 900,000-1,000,000 individuals; and the U.S. West Coast - about 24,000 dolphins. Currently, Pacific white-sided dolphins are classified as Least Concern (LC) on the IUCN Red List.