Whales, Dolphins, and Porpoises of the Western North Atlantic Part 30

DORSAL FIN

Small, slender, slightly Taller dorsal fin, broader at falcate, and pointed on top. base.

Distribution

Although Fraser's dolphins have yet to be described for the western North Atlantic Ocean, they are included here as "possibles" because of the recent discovery that their range is far more extensive than previously known. Records to date have been limited to offsh.o.r.e tropical waters.

The species was first described in 1956 from the remains of a beach-washed specimen from Sarawak in the South China Sea. Since that time specimens have been collected from the eastern tropical Pacific, and others have stranded in such widely divergent localities as Australia, South Africa, and j.a.pan. Recent summaries have added sighting records from the Central Pacific, near the Phoenix Island, from northwest of the Galapagos Islands, and from South African waters.

Stranded Specimens

Stranded Fraser's dolphins should be readily identifiable by 1) distinctive coloration of the body; 2) short, indistinctive beak; and 3) robust form. The only other species of small dolphins with beaks of similar length and general appearance are the Atlantic white-sided and white-beaked dolphins (p. 123 and 126); these dolphins, both with far more northerly ranges, have 30-40 and 22-28 teeth in each side of each jaw, respectively, while Fraser's dolphins have from 40 to 44 teeth in the upper jaw and from 39 to 44 in the lower jaw.

[Ill.u.s.tration: Figure 135.--Fraser's dolphins, like these photographed off the Phoenix Islands in the Pacific (top) and this one off the Philippines (bottom), are definitely identifiable in their tropical range by the short snout, the dark flank stripe and the small dorsal fin and flippers. They may reach 8 feet (2.4 m), or more, in length and occur in herds of at least 500 animals, sometimes with spinner dolphins or Atlantic spotted dolphins. (_Photos by K. C. Balcomb (top) and T.

Hammond (bottom)._)]

[Ill.u.s.tration: Figure 136.--Fraser's dolphins from the eastern tropical Pacific: adult (top and inset) and calf (bottom). Note the distinctive black lateral strip and the extremely small flippers and dorsal fin.

(_Photos by R. Garvie (top and inset) and S. Leatherwood (bottom)._)]

[Ill.u.s.tration]

ATLANTIC WHITE-SIDED DOLPHIN (T)

_Lagenorhynchus acutus_ (Gray 1828)

Other Common Names

Atlantic white-sided porpoise, jumper (Newfoundland).

Description

Atlantic white-sided dolphins reach about 9 feet (2.7 m) in maximum length and are robust in form with a small but distinct beak (less than 2 inches [5.1 cm] long).

The dorsal fin is tall, distinctly back curved, and pointed on the tip.

The tail stock is extremely thick and does not narrow laterally until very near the tail flukes.

The back is distinctly black, the belly white. The sides have zones of gray, tan, and white.

The single most distinctive feature of Atlantic white-sided dolphins is an elongated oval zone of white and yellowish white along the sides from just below the dorsal fin to the area above the a.n.u.s. These patches of lighter coloration, clearly demarcated from each other and from the surrounding coloration, are frequently visible simultaneously with the dorsal fin as the animals roll at the surface to breathe. Even alone this feature permits positive identification of the species. The dorsal fin is often part gray, part black. The beak is all black.

Natural History Notes

Atlantic white-sided dolphins are known to congregate in herds of perhaps a thousand animals, though smaller herds are far more common.

The species is often wary of s.h.i.+ps and does not ordinarily ride the bow wave. Like a number of other species, white-sided dolphins have been reported in a.s.sociation with Atlantic pilot whales.

May Be Confused With

At sea, Atlantic white-sided dolphins are most likely to be confused with the white-beaked dolphins, with which they overlap in distribution.

Though they are very similar in general appearance, the two can be distinguished in the following ways:

ATLANTIC WHITE-SIDED DOLPHIN WHITE-BEAKED DOLPHIN

COLORATION

Elongated band of yellowish Two grayish areas--one in white and white along side, front, the other below and visible behind and below behind dorsal fin, visible as dorsal fin as animal rolls. animal rolls.

BEAK

All black. Sometimes white in parts of range though western Atlantic animals are usually dark.

MAXIMUM SIZE

To 9 feet (2.7 m). To 10 feet (3.1 m).

DORSAL FIN

Often part black, part lighter Uniformly dark.

gray.

Distribution

Atlantic white-sided dolphins are distributed, primarily offsh.o.r.e, in the cool waters between the Gulf Stream and the Labrador Current. They have been observed from Hudson Canyon, off New York City, north to southern Greenland and perhaps Davis Straits. Their normal range shares a southern boundary with the white-beaked dolphin but does not extend as far to the north.

Stranded Specimens

In addition to the features described above for living animals at sea, stranded Atlantic white-sided dolphins can be distinguished from white-beaked dolphins by the following:

ATLANTIC WHITE-SIDED DOLPHIN WHITE-BEAKED DOLPHIN

NUMBER OF TEETH

30-40 per side per jaw, 22-28 per side per jaw; have sometimes more in upper. larger individual teeth--to 6 mm in diameter.

FLIPPERS

Lower portion of forward Lower portion of forward margin more curved. margin less acutely curved.