Pacific White-sided Dolphin, Largenorhynchus obliquidens

Pacific White-sided Dolphin, Largenorhynchus obliquidens. Underwater photograph taken in coastal waters off Kona, Hawaii, September 2015. Photography and identification courtesy of Bob Hillis, Ivins, Utah.

Phylogeny:  The Pacific White-sided Dolphin, Sagmatias obliquidens (Gill, 1865), is a member of the Delphinidae family of Oceanic Dolphins. The genus Sagmatias is one of eighteen genera in this family, and there are four species in this genus. They are also known as the Hookfin Porpoise and in Mexico as the Delfín Costado Blanco.

This is a taxonomically confusing species. The American Society of Mammalogists and the World Registry of Marine Species (WoRMS) both state that Sagamatias obliquidens is the currently accepted name for this species. Many other sources state that Lagenorhynchus obliquidens is the currently accepted name. Even WoRMS states that the genus Sagmatias has been replaced by Lagenorhynchus, while at the same time stating that only two species belong under Lagenorhynchus , neither of which are this species.

Morphology: Pacific White-sided Dolphins are robust, but sleek in outline. Their beak is so small as to be indistinguishable. The flippers are relatively large. The dorsal fin is large and falcate (curved like a sickle). The leading edge of the dorsal fin is dark gray and the rear two-thirds of the fin is light gray. The belly is white. The sides, anterior and posterior to the dorsal fin, are light gray. The back and central sides are dark gray. This dark gray back is marked by a whitish line running from above the eye to the posterior light gray side patch. When viewed from above, the white line, on both sides, looks like suspenders. There is a black patch that runs from the lips to the eye, and then extends back, as a black line, to the flippers. Pacific White-sided Dolphins reach a maximum of 2 .5 m (8 feet 2 inches) in length and 200 kg (440 lbs) in weight.

Habitat and Distribution:  Pacific White-sided Dolphins are an open water species, either inshore, or well offshore. They are usually found in groups, ranging from 10 to over 100 individuals. They often associate with whales and other dolphins. Then spend most of their time in shallow water but can dive to depths of 1,000 m (3,281 feet).  They are primarily found in temperate waters of the north Pacific. They venture into the tropics along the coast of Asia and Western Mexico. In Mexican waters, Pacific White-side Dolphins are found along the entire west coast of the Baja Peninsula and throughout the Gulf of California. There are reports of these dolphins have been cited in the Revillagigedos Islands, but Cabo San Lucas is generally considered their southern limit.

Diet:  Pacific White-sided Dolphins are opportunistic predators, primarily eating small, schooling, fish and squid. They hunt using echolocation and cooperative foraging techniques.

Predators: Pacific White-sided Dolphins juveniles are preyed upon by the Great White Sharks, Carcharondon carcharias and Killer Whales, Orcinus orca.

Reproduction:  As mammals Pacific White-sided Dolphins are gonochoric (male or female for life), and reproduce sexually, with internal fertilization. Breeding usually takes place from August to October. The calves are born between  nine and twelve months later, weighing between 13 km (29 pounds) and 22 kg (48 pounds) and are  90 cm (3.0 feet) to  1.5 m (4.9 feet) in length. Females reach sexual maturity between the ages of eight and ten years, and males at ten or eleven years. Mature females give birth at least bi-annually.

Ecosystem Interactions: Pacific White-sided Dolphins are known to host endo-parasitic cestodes, nematodes, and trematodes. They also host ecto-parasitic barnacles (cirripedes)  of the genus Xenobalanus.

Human Interactions:   Pacific White-sided Dolphins are gregarious and active.  They are quick to bow ride and jump in the wake of boats. They appear curious and seem drawn to stopped boats and swimmers in the water. These dolphins are sometimes killed as by-catch in fishing nets and are hunted in Asia. From a conservation perspective the Pacific White-sided Dolphin is currently considered to be of Least Concern with stable, widely distributed populations.

Synonyms:  Delphinus longidens, Lagenorhynchus longidens, Lagenorhynchus obliquidens, and Lagenorhynchus ognevi.