Cornwall Mammal Group
Conservation:
A UK BAP species, the common dolphin is protected under the Wildlife and Countryside Act 1981. It is included in Annex A of EU Council Regulation 338/97 and therefore treated by the EU as if the animals are on CITES Appendix I, thus
prohibiting their commercial trade. The UK is a member of the ASCOBANS agreement. Short-beaked common dolphins
are currently listed as ‘least concern’ in the IUCN Red List.
Recognition:
common dolphin-(Delphinus delphis)
Order: Cetacea, Cornish Name: Morhogh
In June 2008 a mass stranding of about 70 common dolphins took place in Falmouth Bay, near St Mawes. 24 individuals died and a further 2 were euthanised, while 8
were successfully refloated and herded back to the open sea together with 40 others from the Percuil River. In a normal year less than 3 strandings are reported. Reports by the UK Cetacean Strandings Investigation Programme and the Cornwall Wildlife Trust Marine Strandings Network concluded that the stranding may have been connected with naval exercises in the area.
The species of common dolphin seen in our coastal waters is the short-beaked form. Further south in equatorial waters, the
long-beaked form (Delphinus capensis) predominates. Until 1994 the two forms were considered to be a single species.
The common dolphin has a slender, streamlined body. Adults usually measure between 1.6 and 2.6 m in length with newborns measuring 0.8 to 0.85 m. Common dolphins have a tan or yellowish-tan hourglass pattern on the lower flanks.
Dorsal fins in this species are centrally placed, slender and sickle shaped. The forehead has a distinct dorsal groove along the slender beak. Sexual dimorphism in cetacean species is subtle and the common dolphin is no exception. Females have mammary slits either side of the genital slit on the underside
of the body; these are absent in the males. However, it is normally impossible to determine the sexes of animals in the water. Common dolphins can be confused with striped and bottlenose dolphin, however the distinctive hourglass pattern on the flank quite clearly distinguishes this species from others when observed at close proximity.
Common dolphins are often found in very large groups, or superpods, of thousands of animals, but in UK coastal waters the largest groups sighted contain hundreds, rather than thousands, of individuals. Common dolphins are distributed from oceanic to coastal waters, but always in full salinity waters and they are rarely seen entering smaller estuaries. Males and females school together but adolescent males often form their own pods.
Little is known about dolphin reproduction, but in common with other cetaceans the common dolphin typically produces a single calf. Birth usually occurs in periods of high productivity in the food chain between April and September, to sustain the lactating mother. Females usually lactate for 10 months after the birth of the calf. Mothers have been observed using their beaks to nudge their newborn calves to the surface for their first couple of breaths.
Common dolphins may live for up to 25 years (this has been estimated by sectioning teeth with have annual growth rings of dentine, so this is much like counting tree rings). Common dolphins feed on squid and fish and often display coordinated hunting activity schooling prey into ‘bait balls’.
The only natural predator of common dolphins in UK waters is the orca. However, there is evidence to show deaths by attacks of bottlenose dolphins are becoming more prevalent, though the reasons for this are unknown. Human impacts include pollution, reduction in prey stock due to over fishing, entanglement in fishing nets (bycatch) and increased noise in the marine environment. In coastal waters bycatch is of major concern and several mitigation measures are being tested throughout the country.
Survey Methods:
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Incidental sightings records.
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Surveys from vessels or land observation points.
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Acoustic monitoring to identify areas of high activity around the Cornish coast.
© David Chapman
© JM Bampar - Sea Watch Foundation
© G Pesante - Sea Watch Foundation
Did you know?:
Common dolphins may feed at depths of
260m for up to 8 minutes at a time.
Records:
2007-2012: 977
2002-2007: 1009
Pre-2001: 1364
Total: 3350
Distribution:
Sightings of common
dolphins are
concentrated around
Land’s End and the
Lizard. There were
more sightings between 2002 and 2006 off Gribba Point, Land’s End, Lizard Point, Manacle Point and Rosemullion Head than during the 1997-2001 period. Sightings of common
dolphins further up the north coast were sparse. They occur around the western coasts of the UK and are widely distributed in the eastern North Atlantic.
