Hawaiian Spinner Dolphin (Stenella longirostris)

Description

Body color is gray/white with three distinct longitudinal swatches. Rostrum is long and slender. Dorsal fin is triangular. Adults are about 1700-2000 mm (67-70 inches) in length.

Midway Population

A population of approximately 250-300 spinner dolphins (2001 count) inhabit the lagoon and nearshore waters at Midway Atoll. Spinner dolphin research is currently being conducted by Oceanic Society researchers and volunteers, and Texas A&M University.

Feeding

Feed actively at night. Move long distances as a coherent group to prey on small fish, cephalopods (squid) and shrimp. Dives to 200-300 meters.

Breeding

Little is known about the breeding pattern of spinner dolphins. Females probably mate with several males from their community and paternity differs for their consecutive calves. Females give birth to one calf. Newborns are closely monitored by their mother and several other adults. Babysitting is frequent. Nursing females with calves and several other grown females and large males form nursery groups, which increase the safety of young calves. These groups provide conditions where calves can learn skills important to their adult life.

Calving

Eleven month gestation period. Calving occurs throughout the year, with a peak during June-August months. At birth, calves are approximately 66 cm (2 ft) long, have a pinkish tint and have been seen trying to spin.

Behavior

Aerial activity occurs in dolphins which swim in groups. Spins, leaps, tail over head leaps, back and head slaps are common. The spin for which the dolphin is named is a well defined body movement, and can be as high as 14 spins in a row. It can be vertical or horizontal. When within the Midway lagoon, spinner dolphins are seen predominantly resting and socializing. This low energy activity is characterized by slow and silent movement within a relatively limited area. Frequently, bursts of joyful play, with numerous leaps and spins, can also be seen. An investigation of passing vessels is common.

Predators

Tiiger and cookie-cutter sharks are the spinner dolphin's main predatory threat at Midway. Dolphins best avoid shark predation by staying in groups.