LENGTH & WEIGHT: 129cm – 235cm
WEIGHT: 23 – 78kg
GROUP SIZE: 5 – 200. They are sometimes found in larger mixed schools.
DIET: Mainly fish, but also some squid, octopus, krill and other crustaceans.
GESTATION: 10 months.
The spinner dolphin is a small dolphin found in off-shore tropical waters around the world. They are famous for their acrobatic displays. They jump high out of the water and spin around like a spinning top.
The spinner dolphin has a predominantly dark grey slender body, with darker patches in the tail stock, back and throat. They have a 3-toned colour pattern, and the belly often has a creamy white patch. The beak is long and thin, with a dark tip.
Spinners congregate in groups that vary from just a few dolphins to great schools numbering in the thousands. They are consistently acrobatic. When breaching, the spinners hurl themselves high up into the air and spin around on their longitudinal axis up to 7 times in a single jump.
The reason for the animals’ spinning is not known. Some suggest for communication and play. Some individuals have been spotted completing at least 14 spinning jumps in quick succession.
Spinners are also keen bow-riders and will often come to the bow of a boat from far away and stay there for half an hour or even more.
DISTRIBUTION AND HABITAT
Spinner dolphins occur in pelagic (open ocean) tropical waters in all the world’s major oceans. Their greatest population density occurs between the Tropics of Cancer and Capricorn.
These species seem to do most of their hunting at night when the “scattering layer” of marine life (which has spent the day at depths of 3 000 feet) rises toward the surface to feed on microscopic plant materials. This layer is composed of fish, jellyfish, krill, squid, shell-less snails and copepods.
Spinner dolphins form small subgroups and use communication and echolocation to efficiently hunt at night. They rest for most of the daylight hours. Known predators are sharks, orca (killer whales), and possibly false killer whales, pygmy killer whales, and pilot whales.
MATING AND BREEDING
Females reach sexual maturity at about 4 to 7 years, while males mature at about 7 to 10 years. Mating and calving occur all year round, and gestation is similar to that of most dolphins – approximately eleven months. Many males may mate with one female in short, consecutive intervals.
Calving occurs every two to three years. A newborn calf averages 80 cm in length, and may nurse for one to two years. Lactation often takes place for two years, but can also last for only one year. Spinner dolphins live up to a maximum of 20 years.
Spinner dolphins, particularly whitebelly and eastern spinner dolphins in the eastern tropical Pacific, have suffered huge population losses due to entanglement in the nets of tuna fishermen. For a reason not yet known, they swim in herds above schools of yellowfin tuna.
Instead of looking for tuna, the fishermen look for spinner dolphins. When they find them, they encircle the herd with large nets called “purse seine” nets, capturing the dolphins along with the tuna. It is believed that the stock has declined by as much as 80% since the 1960s, when purse seining operations began. Spinners are often found in aquariums because they survive in captivity.