Cornwall Mammal Group
Recognition:
The warmer waters around Cornwall mean that we have had more striped dolphins recorded from our coastline than any other UK county. Striped dolphins regularly strand alive in Cornwall and have been found to be suffering from a form of Brucellosis specific to cetaceans. One animal was diagnosed as the first confirmed case of Brucella species-associated meningoencephalitis in England.
The striped dolphin is one of our smaller dolphins, similar in appearance tothe common dolphin. Adults are 1.8 to 2.5 m long and weigh up to 130 kg, whilst calves are about 1 metre long and 7 to 10 kg at birth.The striped dolphin has a slender, streamlined body; long, narrow beak; centrally-placed sickle shaped dorsal fin; pointed pectoral fins; and a tapering tail
stock leading to the flukes. This species gets its name from the patterns and colouration on its body, which are key to confirming identity. The back is dark grey, the flanks paling until reaching the underside where it turns to a creamy white colour. Overlaid on this are thin dark grey stripes, usually one starting from the eye that curves gently towards the underside of the tail stock and another starting below the eye and running to the base of the pectoral fin. Sometimes a third stripe extends a short way back from the eye, tapering to an abrupt end between the pectoral and dorsal fins. Behaviour can also be helpful in identification, as striped dolphins tend to roam offshore in large pods. The pods can be spotted from long distances due to the dolphins’ very active behaviour at the surface, with spectacularly high breaches of over 6 m, sometimes accompanied by somersaults and tail spins. Bow-riding with boats is also common in some areas.
Striped dolphins generally inhabit deeper waters, so they are rarely seen inshore. Seasonal movements have been reported in the northern Pacific population, possibly linked to warm ocean currents, but are currently unknown in the rest of their distribution around the world. Females are thought to give birth to a single calf every 3 or 4 years, following a gestation period of a year. The calf remains with the mother for up to 18 months. Males mature some time between the ages of 7 and 15 years, females 2 years earlier. Life expectancy has been recorded up to 58 years, although 30 to 35 years is probably more usual. A highly gregarious and social species, the striped dolphin is regularly observed in large groups, with pod numbers in the hundreds not uncommon, although most UK sightings seem to be in groups of around 30 individuals. UK strandings indicate that pods may be composed of single sex groups. Social interaction with other cetacean species, particularly with common dolphins, is also recorded regularly.
Striped dolphins feed on small fish such as sprat and anchovy, squid, and krill and it would appear that they feed actively throughout the water column, since dives of upto 700 m have been recorded.
Orcas and the larger shark species are natural predators of this species. Disease could also be a significant factor, as a mass die-off of over 1000 animals occurred in the Mediterranean Sea population between 1990-92, though it is suspected that higher concentrations of pollutants and contaminants in the region’s food webs may have made the dolphins more susceptible. As one of the more abundant of the cetacean species, hunting by humans in some areas poses a high risk to local populations. Very intensive drive hunts by Japanese fishermen in the last 50 years has caused the population to fall to very low levels, perhaps even to the
point that they are locally extinct in certain areas. Another notable threat is bycatch, which occurs throughout their range. Twelve hundred animals, perhaps 1.6% of the population, die in the Bay of Biscay each year through conflict with the drift nets used by tuna fishermen.
Survey Methods:
Land-based surveys are not generally suitable for this pelagic (open-ocean) species. Boat-based surveys, such as those by Marinelife and Organisation Cetacea (ORCA) around Europe, are much more appropriate for this species.
© JM Bampar - Sea Watch Foundation
© Jan Loveridge
Conservation:
The striped dolphin is one of the more abundant cetacean species and although no overall world population figure exists, estimates of some regional populations include approximately 2,000,000 in the eastern tropical Pacific, 500,000 off Japan, 62,000 in the western Atlantic and 100,000 in the Mediterranean Sea. In the UK they are protected against intentional harm and disturbance under the Countryside and Wildlife Act 1981. They are listed as ‘least concern’ in the IUCN Red List.
Did you know?:
The striped dolphin is also known as the euphrosyne dolphin, whitebelly, blue-white dolphin, Meyen’s dolphin, Gray’s dolphin and streaker porpoise. As they are frequently observed socialising with the very similar common dolphins, there is the possibility that striped/common dolphin hybrids could occur and there have been occasional reports of individuals bearing mixed colouration and markings of both species.
Distribution:
Rarely sighted in
Cornwall, there have
been a scattering of
sightings and strandings,
mainly on the south
west coast. Most records around the UK are of live stranded or dead individuals washed up on the shore. The UK isat the northern limit of their range in the Atlantic. This cosmopolitan species occurs in most areas of the world, except polar and cold temperate regions, an oceanic species that is rarely seen close to the shore.
Records:
2007-2012: 16
2002-2007: 18
Pre-2001: 51
Total: 85
