Whale of the Month - Striped Dolphin

Species fact file

Common names: striped dolphin, streakers

Icelandic name: rákahöfrungur

Scientific name: Stenella coeruleoalba

Family: Delphiniidae (dolphins)

Max length: 2.6m (males), 2.4m (females)

Distribution: Tropical and subtropical waters globally

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Striped dolphins are easily distinguished from other species by the striking and distinctive colour pattern that gives them their name.

This species is found mainly in tropical and warm-temperate regions, throughout the Atlantic, Pacific, and Indian Oceans. They are not a common sighting in Iceland, preferring waters further to the south around the coasts of the UK and Europe, but there are a few records of pods of striped dolphins off the south coast of Iceland as well as some strandings.

Striped dolphins are highly social. They live in units of around 10-30 individuals, but can form groups of as many as one thousand dolphins! In the western Pacific, three distinct types of social group have been identified: groups of juveniles, groups of non-breeding adults, and groups of breeding adults.

Their large group sizes and conspicuous behaviours at the surface make striped dolphins easy to spot while at sea. They are very fast swimmers, capable of maintaining travelling speeds of up to 11 km/h. In the Pacific they have earned the nickname “streakers” because they swim away from boats at top speed. Striped dolphins are also highly acrobatic, known to make leaps as high as 7 metres in the air, and perform backward flips and tail spins.

In the North Atlantic, striped dolphins feed mainly on lanternfish – a kind of deep-water fish. Lanternfish undergo diel vertical migration: this means that they move between deep water and shallow water every day. During the daylight hours, lanternfish can be found between 300-1500 metres underwater, but as the sun begins to go down they rise into shallower waters less than 100 metres below the surface. So many marine species – from tiny plankton to large fish and squid – migrate like this that diel vertical migration is the largest movement of living creatures on Earth. You can read more about this incredible phenomenon here. Striped dolphins are capable of diving down several hundred metres to reach their deep-sea prey, and their dives normally last between 5 and 10 minutes.

The striped dolphin is currently hunted for food in some parts of the world. However, they are not endangered, and with an estimated global population of over 2 million dolphins this species is listed as least concern by the IUCN. Striped dolphins are the most abundant species of cetacean in the Mediterranean, but they are considered vulnerable in this region because of the numerous threats they face and the isolated nature of the population. There is strong evidence that the Mediterranean population is genetically distinct from the population in the North Atlantic. Genetic studies have found little to no gene flow across the Strait of Gibraltar, and striped dolphins in the Mediterranean have smaller body and skull sizes compared to the dolphins in the Atlantic.

Striped dolphins will frequently form groups with other species of dolphin. In the Mediterranean, they have been observed associating with groups of common and Risso’s dolphins, and in the waters around Greece there have been several cases of hybridisation between common and striped dolphins. These hybrids appear to be fertile and are capable of producing offspring with other hybrids as well as either parent species.

These mixed-species groups are not unique to the Mediterranean: striped dolphins readily associate with other species in the Pacific and Atlantic as well. In fact, in the tropical Atlantic, a new species called the Clymene dolphin arose as a result of extensive cross-breeding between spinner and striped dolphins.

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Mixed-species groups of striped dolphins with common and Risso’s dolphins in the Mediterranean

Vocal communications between striped dolphins while socialising in large groups