Striped Dolphin

Striped Dolphin: Stenella coeruleoalba

IUCN status: least concern

Status and distribution summary:

The estimated world population is thought to be over 2 million, with a pan-global distribution in deep warm to warm-temperate waters offshore. Abundance is patchy, however, and there is at least some migration, moving to warmer waters in winter.

Exploitation:

Up to 20,000 used to be taken every year in drives in Japanese waters, this is now down to about 1,000. Some are also taken around some of the Pacific islands, Sri Lanka and St Vincent in the Caribbean.

Other threats:

Pelagic drift net fisheries have killed thousands of Striped Dolphins in various oceans of the world. In the Mediterranean up to 20,000 are estimated to be killed each year from Morocco, Italy and Turkey. By-catch rates in pelagic fisheries off western Europe is also significant. Estimates of by-catch mortality rate in the Biscay albacore fishery are 1.8% whereas the population is thought only to be able to sustain a mortality rate of up to about 1%.

Pollutants including organochlorines are found in high concentrations in some populations and are thought to immunosuppressive, making the animals vulnerable to disease. meanwhile over-exploitation of their prey species, including the European Anchovy in Biscay could could affect population levels.

Where it is seen:

Occurs predominantly over the deeper waters in the Southern Bay, though occasionally over shelf waters in the Northern Bay.

Frequency of sightings:

Common in the summer, when thousand may be encountered, but appears to leave the area in the winter, presumably for warmer seas.

Recognition:

Small and sleek. Up to 2.5 metres in length.

Dark cool grey upper parts, with blue-grey flanks, and thin dark stripe between flank and white underneath.

The pattern below the dorsal fin is distinctive; the grey of the flank cuts back into the dark upper parts.

Dark falcate dorsal fin set at the midpoint.

Pronounced slender beak.

Behaviour:

Highly social, usually in groups of between 10 and 1000.

Very energetic; swims fast, leaps frequently, and somersaults, back somersaults, and even spins. Sometimes bow rides.

Associates with Common Dolphins regularly, although they have different feeding ecologies so this may be social or for security.

Confusion species:

  • Common Dolphin; is the only other patterned dolphin that regularly occurs in the area. It has warm tones, compared to the cool bluish hue of Striped Dolphins, and the inverted triangle is diagnostic, as is the pattern of Striped Dolphins.
  • White Beaked Dolphin; could be confused, though is more robust and has dark flanks and a stubby beak.
  • Atlantic White-sided Dolphin; is potentially confusable, with broadly similar patterning, but the yellowish patches on the side of the tail stock is diagnostic, also has stubby beak.