Striped Dolphin (Stenella coeruleoalba (Meyen, 1833))

Order: Cetacea
Suborder: Odontoceti. Family: Delphinidae

Identification: The striped dolphin has a slender body with a long dark beak. It's narrow forehead and melon are smoothly sloping and there is a distinct crease separating the forehead from the beak. It is boldly patterned in bluish grey and white, although under certain light conditions it may appear brown and white. A lateral stripe and spinal blaze are characteristic, and distinguish the striped dolphin from other white-bellied oceanic dolphins, but these markings vary significantly between individuals and probably geographically. There are 1 or 2 dark bands between the flipper and the eye, and a dark streak behind the eye (which often has a black patch around it). The sides are pale grey, as are the tail flukes. The dorsal fin is falcate and the flippers are small and slender with pointed tips.

Striped dolphins are fast swimmers (travelling up to 15 km hour); when travelling at speed up to a third of the members of a school will be above the surface at any one time. They are highly active, breaching to heights of 7 m above the surface, and are capable of amazing acrobatics including somersaults, tail spins and backward flips.

When swimming, 2 main patterns of surfacing can be observed: the first involves dolphins rolling at the surface, emerging rostrum first and re-entering the water with the tail stock in a flexed position as the last bit of the body to submerge; the second involves animals re-entering the water with the tail extended, the fluke disappearing last - and tends to occur at higher swimming velocities. Leaping is frequently observed in this species whilst swimming at speed; the dolphins often breathing as they leap. Dives usually last from 5 to 10 minutes, and striped dolphins may dive to depths of 200 m when feeding.

Distribution & Habitat: The striped dolphin is an oceanic species, usually found close to shore only in places where the water is deep. It is mainly found in warm waters, although its range may extend into warmer temperate seas, tending to favour waters with large seasonal changes in surface temperature and thermocline depth, with seasonal upwelling.

In the North Atlantic, striped dolphins are found mainly in water depths of 1,000 m or more, beyond the continental slope, but the species has been recordedin shallower waters off othe Scottish mainland and in mid-Atlantic, west of the Faroes. In other parts of the world, the species ranges from southern Africa, southern India and New Zealand to the South and North-west Pacific, central north America on both the east and west coasts, central eastern South America, the Azores, Spain and Portugal and the eastern Atlantic. Records range from the Bering Sea to southern California, and the species is common in Japanese waters. No well-defined migration patterns have been documented for striped dolphins, but in some regions the striped dolphin is encountered in all seasons, while in other areas it appears to be associated with the fronts of warm oceanic currents which move seasonally and result in sporadic warm water intrusions.

Natural History & Ecology: Mature striped dolphins may range in length from 180 cm to 256 cm and 110 to 160 kg in weight. Average length at sexual maturity has been found to be 195 to 220 cm for both males and females of the species at an age of 9 to 10 years old. Breeding is seasonal, with calving peaks in summer and winter. Gestation lasts approximately 12 months. The estimated length of lactation is 15 months, but this may be between 8 and 20 months, according to population density (in response to fishing pressure).

Striped dolphins feed mainly on small, mid-water fish and squid. In the Mediterranean, investigations of stranded striped dolphins have found that cephalopods dominate the stomach contents. Off Japan and South Africa, myctophid fishes predominate. Prey may range in size from 60 to 300 mm and include a wide range of species, including some crustaceans such as shrimp. In captivity, the mean feeding rate of a striped dolphin is 3.45 kg of fish a day.
Striped dolphins may dive to depths of 200 m when foraging. In the eastern tropical Pacific, these dolphins are occasionally found with yellow fin tuna in possible foraging aggregations.

Social behaviour: Schools of striped dolphin vary in size and composition. Examination of school structure off Japan (where whole schools captured in hunting operations have been studied) has shown that most schools contain less than 500 individuals, although schools of several thousand dolphins may occur. In the eastern North Atlantic, schools most commonly comprise 10 to 30 individuals, and rarely reach the 100's.

In the western North Pacific, three types of schools have been identified: juvenile, adult and mixed. Adult and mixed schools are divided further into breeding and non-breeding schools.schools. Calves remain in adult schools until 1 or 2 years old. After weaning, they then leave to join juvenile schools; which migrate closer to the coast than adult and mixed schools. Most sub-adult females rejoin non-breeding adult schools, but some join breeding schools directly. Males rejoin adult schools after reaching sexual maturity, about equal numbers joining breeding and non-breeding schools. Breeding schools contain sub-schools of fully adult females and of socially adult males, which may leave the school after most of the females have been mated with. The breeding school thus evolves into a non-breeding adult school and, after the birth of the calves, a mixed non-breeding school. The breeding system is most likely to be promiscuous, although some degree of polyandry (where a female may reproduce only with several specific males) may occur.

Breaching may have a communicative function.