SPINNER DOLPHINS

Dolphin Research Project

The first day on Midway Island the researcher explained the goals of the spinner dolphin research project was to determine habitat use, the abundance and distribution of dolphins,  group size and composition, social structure of the dolphin schools, social affiliation patterns between individual dolphins, to develop a photo-identification file for the dolphin population and develop year-round baseline activity patterns.

Such information will impact the development of boat usage in the atoll. The aim is to disturb the dolphins as little as possible in order to keep them in the area and not to disturb them enough to leave. Is the population  transient or residential? Do sub-groups exist and if so how stable they are.

No research/documentation had been done yet on dolphins in an atoll habitat. Midway is a perfect place for such a study as the shallow clear waters of the lagoon allow for detailed observation below the water as well as above it.

Spinner dolphins, Stenella longirostris, receive their name from the frequent activity of leaping from the water and spinning in the air. Apparently spinning is done solely for fun but, it may also serve as a signal that feeding is about to begin. It is significant that spinning activity peaks in mid-afternoon before the group spreads out at dusk to forage for dinner. Aerial activity is used in socialization and is believed also to be used in communication. Some scientists speculate that aerial activity also serves to remove ectoparasites from the dolphin bodies.   Spinners can reach a height of about 3 meters when they spin and can make a maximum of four revolutions. Spinner dolphins swim mostly in deep waters, but occur in regions where there are shallow platforms or islands.

The schools of dolphins on Midway are larger than those around the other Hawaiian Islands, very likely because there are fewer sheltered land and resting areas. There is a large shark population in the area so the lagoon provides a safe and quiet place to rest. They typically live in tropical waters, but may range into temperate water with a worldwide distribution.

The average spinner is 5 ½ -6 ½ feet long. Calves at birth are generally 30 inches long weighing 33 pounds. Gestation is 10 ½ months and females will lactate for a period of 1-2 years. Typically, males are bigger than females. Female spinners sexually mature between 4-7 years and tend to have one ovulation  period each year. Males mature later, usually between 7-10 years. Mating is promiscuous with peak activity during summer and fall. Lifespan for spinners is about 20 years. Only 1% of females have been documented to reach post-productive status. Although spinner dolphin society is highly promiscuous, family units are apparent.

Contact is an essential ingredient in spinner dolphin society where touching, nuzzling, and stroking occur frequently among the animals in the group even though the composition of the group changes. Many dolphins are day feeders, but spinners feed at night in deep waters on fish, squid, and shrimp. They have been documented to dive to depths of 200-300 meters while feeding.

For most dolphin species the dorsal fin is used for identification because it can be photographed from the surface of the water. Nicks and marks in the trailing edge of the fin are most often used for identification because they are relatively permanent and change little over time. 

We used a 23 foot pontoon boat for water surveys which were conducted at random in five transects of the lagoon. It was great fun to watch the dolphins bow surfing with us. The schools sometimes contained some small animals.

This was a fun 10 days to say nothing of being educational and interesting.