Pacific White-sided Dolphin

Description

The Pacific white-sided dolphin is a strikingly marked, medium-sized dolphin renowned for its energetic and acrobatic behavior. Adults measure 5.5 to 8 feet in length and weigh between 150 and 400 pounds, with males slightly larger than females. Their coloration is a complex three-toned pattern: the back is dark gray to black, the sides feature a large light gray or pale gray patch that sweeps upward from the belly toward the tail creating a distinctive “suspenders” pattern, and the belly is white. A pair of thin light gray stripes run from above the eye along the upper flanks toward the tail. The dorsal fin is bicolored, with the leading edge dark and the trailing portion light gray, and is tall and strongly falcate (hooked). The beak is short and rounded, giving the face a blunt appearance. Pacific white-sided dolphins have 23 to 32 small, rounded, slightly curved teeth in each row of both jaws. Their overall build is robust and powerful, adapted for fast swimming and leaping.

Habitat & Range

Pacific white-sided dolphins are endemic to the temperate North Pacific Ocean, ranging from the Gulf of Alaska and the Kuril Islands in the north to Baja California and Taiwan in the south. In the eastern North Pacific, they are found primarily in offshore and continental shelf-edge waters, though they frequently come closer to shore in areas where deep water is accessible near the coast. Two stocks are recognized in U.S. Pacific waters: a northern stock off Oregon and Washington, and a southern stock from central California southward. The Southern California Bight, including the waters off Orange County, falls within the range of the southern stock, estimated at approximately 26,000 individuals. Pacific white-sided dolphins are more commonly observed off Orange County during the cooler months from fall through spring, when cooler water temperatures and associated prey shift their distribution closer to the Southern California coast. During warmer summer months, they tend to move further north or offshore following preferred water temperatures.

Diet

Pacific white-sided dolphins are generalist predators that feed on a wide variety of small schooling fish, squid, and occasionally crustaceans. Common prey species in Southern California waters include anchovies, sardines, Pacific saury, market squid, lanternfish, and hake. They are active, cooperative hunters that often forage in coordinated groups, herding fish into dense concentrations before feeding. Pacific white-sided dolphins frequently feed in association with other marine species, including common dolphins, sea lions, seabirds, and baleen whales, participating in multi-species feeding aggregations around dense bait balls. They are capable of diving to at least 200 meters in pursuit of prey, though much of their feeding occurs in shallower waters. Like other dolphins, they use echolocation to detect and track prey, producing rapid series of clicks that provide detailed information about the size, shape, distance, and movement of potential food items. An adult Pacific white-sided dolphin is estimated to consume approximately 15 to 20 pounds of food per day.

Behavior

Pacific white-sided dolphins are among the most gregarious and acrobatic of all cetacean species. They are typically found in groups ranging from a few dozen to several hundred individuals, and super-groups of more than 1,000 have been documented. Their aerial displays are spectacular, including high, spinning leaps, somersaults, belly flops, and rapid porpoising across the surface. They are enthusiastic bow-riders and wake-riders, frequently approaching boats at high speed and riding the pressure waves. Pacific white-sided dolphins are fast swimmers, capable of sustained speeds of 20 miles per hour and short bursts exceeding 25 miles per hour. They are highly vocal, producing a rich repertoire of whistles, clicks, and burst-pulse sounds. Their social structure appears to be fluid, with individuals moving between groups over time. They are known to associate with a variety of other cetacean species, including common dolphins, Risso’s dolphins, and northern right whale dolphins, sometimes forming mixed-species groups. Females give birth to a single calf after a gestation period of approximately 11 to 12 months, with calves weaned at about 8 to 10 months. The estimated lifespan is approximately 40 to 45 years.

Conservation Status

The Pacific white-sided dolphin is listed as Least Concern on the IUCN Red List, reflecting its large population and wide distribution. In the United States, the species is protected under the Marine Mammal Protection Act. The total eastern North Pacific population is estimated at approximately 26,000 individuals for the southern stock and 21,000 for the northern stock. Historically, Pacific white-sided dolphins suffered significant mortality as bycatch in the Japanese and Korean drift-net fisheries for squid and tuna in the North Pacific, with tens of thousands killed annually before the United Nations imposed a moratorium on high-seas drift-net fishing in 1993. This ban dramatically reduced bycatch mortality for the species. Current threats include entanglement in fishing gear, particularly drift gillnets and purse seines, ocean pollution, marine debris ingestion, and underwater noise. In California, the drift gillnet fishery for swordfish and sharks has been identified as a source of Pacific white-sided dolphin bycatch, contributing to ongoing regulatory scrutiny of the fishery. Climate change may affect the distribution and abundance of their prey, potentially altering the seasonal patterns of dolphin occurrence off Southern California.