Risso's dolphin

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Risso's Dolphin1
Size compared to an average human
Conservation status
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Cetacea
Family: Delphinidae
Genus: Grampus
Species: G. griseus
Binomial name
Grampus griseus
(G. Cuvier, 1812)
Risso's dolphin range

Risso's dolphin (Grampus griseus) is the only species of dolphin in the genus Grampus.

Contents

Taxonomy

Risso's dolphin is named after Antoine Risso, whose description formed the basis of the first public description of the animal, by Georges Cuvier, in 1812. Another common name for the Risso's dolphin is grampus (also the species' genus), although this common name was more often used for the orca. The etymology of the word grampus is unclear. It may be an agglomeration of the Latin grandis piscis or French grand poisson both meaning big fish. The specific epithet griseus refers to the mottled (almost scarred) grey colour of its body.

Description

Grampus griseus for Edward Drinker Cope.jpg

Risso's have a relatively large anterior body and dorsal fin, while the posterior tapers to a relatively narrow tail. The bulbous head has a vertical crease in front.3

Infants are dorsally gray to brown and ventrally cream-colored, with a white anchor-shaped area between the pectorals and around the mouth. In older calves, the non-white areas darken to nearly black, and then lighten (except for the always dark dorsal fin.) Linear scars mostly from social interaction eventually cover the bulk of the body. Older individuals appear mostly white. Most individuals have 2–7 pairs of teeth, all in the lower jaw.3

Length is typically 10 feet (3.0 m) although specimens may reach 14.1 feet (4.3 m).4 Like most dolphins, males are typically slightly larger than females. This species weighs 300–500 kilograms (660–1,100 lb) making it the largest species called "dolphin".56

Range and habitat

They are found worldwide in temperate and tropical waters, usually in deep waters rather than close to land. (Clarification needed, as statement contradicts range map.) As well as the tropical parts of the Indian, Pacific and Atlantic Oceans, Rissos are also found in the Persian Gulf, Mediterranean and Red Seas, but not the Black Sea. They range as far north as the Gulf of Alaska and southern Greenland and as far south as Tierra del Fuego.3

Their preferred environment is just off the continental shelf on steep banks with water depths varying from 400–1,000 meters (1,300–3,300 ft) and water temperature at least 10 °C (50 °F) and preferably 15–20 °C (59–68 °F).3

The population around the continental shelf of the United States is estimated in excess of 60,000. In the Pacific a census recorded 175,000 individuals in eastern tropical waters and 85,000 in the west. No global estimate exists.

Ecology

Risso's dolphin

They feed almost exclusively on neritic and oceanic squid, mostly nocturnally. Predation does not appear significant. Mass strandings are infrequent.3

These dolphins typically travel in groups of 10–51, but that may reach 400. Smaller, stable subgroups exist within larger groups. They also travel with other cetaceans. They harass and surf the bow waves of gray whales as well as ocean swells.3

Reproduction

See also: Dolphin#Reproduction and sexuality

Gestation requires an estimated 13–14 months, at intervals of 2.4 years. Calving reaches seasonal peaks in the winter in the eastern Pacific and in the summer and fall in the western Pacific. Females mature sexually at ages 8–10, and males at age 10–12. The oldest specimen reached 34.5 years.3

Human interaction

Risso's dolphins generally do not approach boats, but occasionally surf bow waves.3 An exception was one named Pelorus Jack who accompanied boats in Admiralty Bay in New Zealand's Marlborough Sounds for more than 20 years. Hunting of this species has never been particularly widespread though complete pods are often netted and killed in Taiji, Japan.

Risso's have successfully been taken into captivity in the United States and Japan, although not with the regularity of bottlenose dolphins or orca. Hybrid Risso's-bottlenose dolphins have been bred in captivity.

Under the name "grampus", it was one of the royal fish which were traditionally the property of the English Crown.7

Conservation

The Risso's dolphin populations of the North Sea, Baltic Sea and Mediterranean Sea are listed on Appendix II8 of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), since they have an unfavourable conservation status or would benefit significantly from international co-operation organised by tailored agreements.9

In addition, Risso's dolphin is covered by the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS),10 the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS),11 the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region (Pacific Cetaceans MoU)12 and the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia (Western African Aquatic Mammals MoU).13

Risso's dolphin are protected in the United States under the Marine Mammal Protection Act of 1992. The only country known to actively hunt Risso's dolphin is Japan. On October 20, 2012 - volunteer activists live streamed a slaughter of an entire pod of Risso's dolphins in Taiji, Japan. To this day, Japan continues to hunt the Risso's dolphin using the practice of slaughtering entire pods.

Strandings

At least one case report of strandings in Japan's Goto Islands has been associated with parasitic neuropathy of the VIIIth cranial nerve by a trematode in the genus Nasitrema.14

See also

Portal icon Cetaceans portal
Portal icon Marine life portal

References

  1. ^ Mead, J. G.; Brownell, R. L., Jr. (2005). "Order Cetacea". In Wilson, D. E.; Reeder, D. M. Mammal Species of the World (3rd ed.). Johns Hopkins University Press. pp. 723–743. ISBN 978-0-8018-8221-0. OCLC 62265494. 
  2. ^ Taylor, B.L., Baird, R., Barlow, J., Dawson, S.M., Ford, J., Mead, J.G., Notarbartolo di Sciara, G., Wade, P. & Pitman, R.L. (2008). Grampus griseus. In: IUCN 2008. IUCN Red List of Threatened Species. Retrieved 7 October 2008.
  3. ^ a b c d e f g h Baird, Robin W. (2009). In Perrin, William F.; Wursig, Bernd; Thewissen, J. G. M. Encyclopedia of Marine Mammals (2 ed.). Burlington Ma.: Academic Press. p. 975. ISBN 978-0-12-373553-9. 
  4. ^ "Grampus griseusRisso's dolphin". Animal Diversity Web. Retrieved February 2013. 
  5. ^ American Cetacean Society Fact Sheet – Risso's Dolphin
  6. ^ http://www.whale-web.com/dolphins/risso.html
  7. ^ Ball, F. Elrington History of Dublin (1917) Alexander Thom and Co Vol.5 p.49
  8. ^ "Appendix II" of the Convention on the Conservation of Migratory Species of Wild Animals (CMS). As amended by the Conference of the Parties in 1985, 1988, 1991, 1994, 1997, 1999, 2002, 2005 and 2008. Effective: 5 March 2009.
  9. ^ Convention on Migratory Species page on the Risso's dolphin
  10. ^ Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas
  11. ^ Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area
  12. ^ Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region
  13. ^ Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia
  14. ^ See Morimitsu et al. 1992. J Wildl Dis 28:656–658

External links