Cornwall Mammal Group
bottlenose dolphin-(Tursiops truncatus)
Order: Cetacea, Cornish Name: Morhogh
Conservation:
The bottlenose dolphin is a UK BAP species for which there has been a dramatic reduction in sightings, and in the size of groups recorded, since the early 1990s. It is protected under the Wildlife and Countryside Act 1981 and Annex A of EU Council Regulation 338/97 and therefore treated by the EU as if they are on CITES Appendix I, thus prohibiting their commercial trade. The bottlenose dolphin is listed as threatened on the IUCN Red List.
Recognition:
The appearance of the bottlenose dolphin varies depending on individual characteristics and the geographic area in which it is found. There are two main variants: the smaller inshore form; and the more robust offshore form.
The body of the bottlenose dolphin is stout and they are one of the larger dolphin species seen in UK waters with adult males growing up to 3.9 m and females to 3.7m. A new-born calf measures between 1.0 and 1.3 m.
Colouring in this species is variable but generally very plain with a brown or dark grey back, lighter grey lower flanks, grading to white on the belly. Bottlenose dolphins have a distinctive tall, slender, sickle-shaped, centrally placed dorsal fin. The short beak is often white tipped with a characteristic robust bottle-like shape. The melon, the rounded forehead, is very obvious and distinct from the beak.
The visible differences between the sexes are subtle, as in most dolphin species. Females have mammary slits either side of the genital slit on the underside of the body, these are absent in the males. However, due to the nature of sightings and the marine environment it is usually not possible to determine the sex of animals in the water.
They are often seen breaching and bow riding and frequently approach boats. They may be confused with the more agile common dolphin, however the absence of flank markings should aid identification.
Bottlenose dolphins are found worldwide in temperate and tropical coastal, and offshore, waters. Some groups, such as those from Cardigan Bay and the Shannon estuary, seem to be resident, while many of those seen off the Cornish coast are more mobile, possibly as a result of prey availability. Closer to the shore bottlenose dolphins may be found in estuaries and harbours, and even occasionally even in freshwater. They show a particular liking for areas with strong tidal currents.
Bottlenose dolphins generally occur singly or in groups of up to 10 individuals inshore, and up to 25 offshore. However, pods of as many as 500 dolphins have been reported in some offshore areas.
The dolphins are extremely well adapted hunters and feed on squid and fish; they often display coordinated hunting activity, schooling prey into ‘bait balls’. Animals also cooperate to defend against predators and to care for their young. Echolocation is used in hunting and sound is also used for social interaction.
Relatively little is known about cetacean reproduction because of the difficulties of surveying and studying animals in the open ocean. However, there is evidence for competition between males, and a polygamous promiscuous mating system. In common with other cetaceans, bottlenose dolphins typically produce a single young. In UK waters births usually occur in the summer months during periods of high food chain productivity, to sustain the lactating mother. Calves are suckled for 18 to 20 months and they leave their mother when the next calf is born, but they often remain together.
The life span of the bottlenose dolphin is estimated (by sectioning teeth) to be 25yrs, but females on the east coast of the USA may reach 50 years.
There are very few natural predators of bottlenose dolphins in UK waters; the only species to specifically predate on them is the orca. Attacks by other bottlenose dolphins have been documented in Cornwall and it has been suggested that the dolphin’s attacks on harbour porpoises may be connected with aggression between adult dolphins and their own calves.
Human threats include pollution, reduction in fish stocks due to over-fishing, accidental entanglement in fishing nets (bycatch), increased noise in the marine environment, and the impacts of tourism. In coastal waters bycatch is a major concern and several mitigation measures are being tested throughout the country. Cornwall Wildlife Trust has been successfully piloting the use of ‘pingers’ which are attached to fishing nets and emit sound to repel dolphins.
Did you know?:
Work by the Falmouth Marine School has shown that the patterns of scars and markings on the fins and body of bottlenose dolphins can be used to identify individuals and support research into behaviour of the resident Cornish pod.
Survey Methods:
Incidental sighting records. Surveys from vessels or land observation points.
Acoustic monitoring to identify areas of high activity around the Cornish Coast.
© David Chapman
Cornwall is home to one of the UK’s only three identified inshore pods of bottlenose dolphins. These pods may come into conflict with inconsiderate leisure boat owners and in 2013 a calf was killed when a pod was pursued in the Camel estuary by up to 20 boats.
Records:
2007-2012: 768
2002-2007: 1576
Pre-2001: 4726
Total: 7070
Distribution:
Bottlenose dolphins
occur worldwide.
Around Cornwall most
sightings are between
Gwennap Head and
Lamorna Point. They are also seen in
St. Ives Bay, Newquay Bay, Falmouth Bay and Mount’s Bay particularly from Porthleven.
More recently there have been considerably more sightings in St. Ives Bay. A lack of records may reflect a lack of observers, rather than an absence of dolphins.
© David Chapman
