Spinner Dolphin

Dwarf spinner dolphins, a subspecies, even occur exclusively close to shore in coral reefs off Thailand. Spinner dolphins have an up to 2.35m long, slender body, a noticeable long beak and a big dorsal fin (fin on the back of the animal) which is either triangular or crescent shaped, depending on age, sex and geographic region. Additionally spinner dolphins and Clymene dolphins are the only two species which jump high in the air while spinning around their longitudinal axis. That is where the name spinner dolphin comes from. Since the distribution of both species does not overlap spinner dolphins can be identified by this behaviour as well. During one of those breaches the dolphins spin up to seven times until they enter the water again with a big splash which can be seen from the distance. Spinner dolphins often approach boats to ride on their bow waves.

Group composition of spinner dolphins can be quite different since it depends on habitat. In Midway (Central Pacific) for example groups are big and the animals exhibit a strong side and group fidelity. There is no sign for exchange of animals with other dolphin schools. In most parts of the Hawaiian Archipelago however, one spinner dolphin population consists of many small groups which interact a lot. These groups either consist of adult females with their offspring or of adult males. Both live around different islands which are close together. The Hawaiian spinner dolphins spend the day in coastal shallow waters where they rest and socialize, and feed in open waters during the night. That is when the groups meet and dolphins change from one school to another. In Midway associations between dolphins are usually stable over long time periods and consist of animals of both sexes and all age classes. The distance between the islands is probably too big to allow interactions with other schools. It would be quite risky to cross large channels of open, deep water since there occur predators like sharks which are a danger for dolphins.

Spinner dolphins feed on different fish and squid species. To search and hunt for prey, they dive up to more than 600m deep. Often spinner dolphins can be seen together with pantropical spotted dolphins. Both species are frequently associated with big tuna schools and therefore were caught as catch in purse sein tuna fishing nets in large numbers which depleted several spinner and pantropical dolphin populations. Since spinner dolphins usually use the same resting area for several years the use of those coastal waters by humans may negatively influence spinner dolphins as well. So e.g. intensive boat traffic in these waters would be a disturbance for them.

Breeding of spinner dolphins is more or less seasonal, depending on geographic area. Spinner dolphins give birth to usually one calf after a gestation period of 10 months. Since nursing lasts 1-2 years the calving interval is about three years. Females get sexually mature with 4-7, males with 7-10 years of age.