Lagenorhynchus obliquidens Gill 1865

5.

Pacific White-sided Dolphin

Lagenorhynchus obliquidens

French: Dauphin de Gill / German: Pazifik-Wei 3seitendelfin / Spanish: Delfin de flancos blancos del Pacifico

Other common names: Hook-finned Porpoise, Pacific Striped Porpoise, Pacific White-striped Dolphin

Taxonomy. Lagenorhynchus obliquidens Gill, 1865,

“obtained at San Francisco, California,” USA.

Taxonomy of Lagenorhynchus is currently in dispute; recent molecular analyses have revealed that the genus is not monophyletic. L. obliquidens and L. obscurus may form a sister lineage to Cephalorhynchus. The genus Sagmatias has been suggested as an alternative classification. Geographical forms may be distinguished by body length and skull characteristics; there are two forms in the eastern North Pacific Ocean and two forms in the western Pacific Ocean. Coastal populations seem to be genetically and morphologically distinct from offshore populations. Monotypic.

Distribution. Cool temperate waters of the N Pacific, primarily between 38° N and 47° N, including the Yellow Sea, Sea ofJapan, S Sea of Okhotsk, S Bering Sea, and S Gulf of California.

Descriptive notes. Total length 240-250 cm (males) and 230-240 cm (females); weight c.198 kg. Neonates are 92-100 cm long. The Pacific White-sided Dolphin has stocky body-shape with short, well-defined beak. Flippers are large and falcate with slightly rounded tips. Dorsal fin is also large and falcate, and in older males, it may be very hooked at tip, which is slightly rounded. Belly, lowerjaw and throat are white and bordered by a black band. Sides and back are black with several pale-gray patches. These patches include streaks around lower melon,streakjust above beak and eyes that may merge with larger gray patches on lower anterior flanks, wide pale-gray patches along posterior margin of dorsal fin, and wide lateral gray streak along tailstock that may diverge into one or two wispy stripes along upper mid-body below dorsal fin. Beak, flippers, and flukes are dark gray to black, but flippers may have paler patches on upper surfaces. Young Pacific White-sided Dolphins have muted color pattern, often with slight yellow-orange tinge. Variations on the basic color morph are common and include melanistic individuals, individuals with rust-colored patches, and mostly white individuals. There are 23-36 pairs of small, conical teeth in each jaw.

Habitat. Most abundant in deep offshore waters, but also over continental shelves in some areas and even inshore waters where deeper waters approach closer to shore. Pacific White-sided Dolphins are found in such places off British Columbia (Canada) and Washington (USA) and in submarine canyons near Monterey Bay, California (USA).

Food and Feeding. The Pacific White-sided Dolphin prefers small schooling fish and cephalopods, butthis preference may vary between inshore and offshore populations. It is an opportunistic feeder and preys on a wide variety of species (60 fish species and 20 cephalopod species). Foraging occurs both day and night. The Pacific White-sided Dolphins off the west coast of North America prefer epipelagic species such as Atlantic herring (Clupea harengus), Pacific hake (Merluccius productus), salmon (Oncorhynchus spp.), cod (Gadidae), capelin (Mallotus villosus), and various shrimps and squid. Offshore populations prefer more mesopelagic species. In the western Pacific Ocean, the Pacific White-sided Dolphin prefers myctophids. In the North Pacific Ocean, preferred prey species include northern anchovy (Engraulis mordax), Pacific hake, Pacific saury (Cololabis saira), redfish (Sebastes spp.), Pacific jack mackerel (Trachurus symmetricus), and California market squid (Doryteuthis opalescens). Groups of the Pacific Whitesided Dolphin may cooperate to herd schools offish for easier capture.

Breeding. Breeding of the Pacific White-sided Dolphin peaks in May-September. Gestation lasts 11-12 months. Females reach sexual maturity at 8-11 years and males at 9-12 years. Males may live to 42 years and females to 46 years. Male testes are known to grow in size during the breeding season from midto late summer, which suggests that mating is promiscuous and may involve sperm competition.

Activity patterns. The Pacific White-sided Dolphin is known to breach acrobatically (sometimes incorporating somersaults), side-slap, belly-slap, and bow-ride. They tend to travel in large groups that split into smaller groups for foraging. These subgroups are more dispersed while socializing but rejoin for resting. Dives may last more than six minutes but typically last only 24 seconds. Longer dives are generally around three minutes. The Killer Whale (Orcinus orca) may be a natural predator.

Movements, Home range and Social organization. Pacific White-sided Dolphins are gregarious and live in large groups of hundreds to thousands of individuals. Mixedspecies groups include Risso’s Dolphin (Grampus griseus), and there may be consistent associations with certain groups of Northern Right-whale Dolphins (Lussodelphis borealis). Pacific White-sided Dolphins may also associate with the California Sea Lion (Zalophus californianus). Groups tend to be segregated by age and sex classes. There appear to be seasonal movements in some coastal regions where dolphins are more common in nearshore waters during autumn and winter but move offshore during spring and summer, likely following prey. Along the eastern North Pacific Ocean, there may be north-south movement of Pacific White-sided Dolphins; they are more abundant off California in February-April and more abundant off Oregon and Washington in May.

Status and Conservation. CITES Appendix II. Classified as Least Concern on The IUCN Red List. There are currently no concrete estimates of abundance of the Pacific White-sided Dolphin, but 900,000-1,000,000 individuals are estimated in the North Pacific Ocean—perhaps an overestimation biased by attraction to the survey vessel. Off the western USA, abundance is 10,000-41,000 individuals, depending on season and average annual sea-surface temperatures. Population trends are also currently unknown. There is some opportunistic direct catch of the Pacific White-sided Dolphin in Japanese harpoon and drive fisheries, but numbers killed are uncertain. The Pacific White-sided Dolphin is vulnerable to pelagic driftnet fisheries; an estimated 424 individuals were caught incidentally in shark and swordfish fisheries in 1988-2002. Some dolphins are also caught in trammel nets and set nets in coastal California and gillnets off British Columbia and Alaska, but these catches are thought to be below precautionary limits. Incidental catch rate in the US eastern Pacific Ocean is estimated to be very low, only c.1-4 Pacific White-sided Dolphins/year. There are similar estimates for catch rates in near-shore western US waters. In the eastern North Pacific Ocean,fisheries have been required to use “pingers” since 1996 to reduce incidental catch. The Pacific White-sided Dolphin is taken commonly in pelagic squid driftnet fisheries from Japan, Korea, and Taiwan; catch rate for the Japanese squid driftnet fishery was ¢.6100 individuals in 1989. In 1993, the United Nations put a moratorium on high-seas driftnet fisheries, which reduced their impact on the Pacific White-sided Dolphin. Incidental catch in high-seas driftnets was formerly the greatest threat to the Pacific White-sided Dolphin. In the 1970s and 1980s, ¢.100,000 individuals (4000-8000/year) were likely killed in these fisheries, which caused some population depletions in the North Pacific Ocean. A smaller number of Pacific White-sided Dolphins continue to be taken yearly by Japanese seine, set net, and trap net fisheries. Molecular monitoring at markets in South Korea has identified meat of the Pacific Whitesided Dolphin for sale, suggesting opportunistic marketing of incidental catch (given that South Korea has no commercial or scientific whaling programs). Between 1976 and 1980, 466 Pacific White-sided Dolphins were killed in Japanese culls to reduced competition with fisheries. Overall, abundance of the Pacific White-sided Dolphin is high, distribution is widespread, and threats are minimal. They sometimes are taken for captive display, but captive survival rates tend to be low. The Pacific White-sided Dolphin seems to be declining in abundance in the Gulf of California, at its southern distributional boundary, and this may be related to rising sea-surface temperatures as a result of climate change.

Bibliography. Baker et al. (2006), Black (2009), Brownell, Walker & Forney (1999), Dahlheim & Towell (1994), Ferrero et al. (2002), Hammond et al. (2008e), Hayano et al. (2004), Heise (1997a, 1997b), Jefferson et al. (2008), LeDuc et al. (1999), May-Collado & Agnarsson (2006), Salvadeo et al. (2010).