Rough-toothed Dolphin (Steno bredanensis)
The Rough-toothed dolphin (Steno bredanensis) is species of
dolphin that can be found in deep warm and tropical waters around the world.
The species was first described by Georges Cuvier in 1823. The genus name Steno, of which this species is the only member, comes from the Greek for 'narrow', referring to the animal's beak - which is a diagnostic characteristic of the species. The specific name honours van Breda, who studied Cuvier's writings. There are no recognised sub species.The rough-toothed dolphin is a relatively large species, with adults ranging from 209 to 283 centimetres (6.9 to 9.3 ft) in length, and weighing between 90 and 155 kilograms (200 and 340 lb); males are larger than females. Its most visible characteristic feature is its conical head and slender nose; other dolphins either have a shorter snout or a more visibly bulging melon on the forehead. As the common name for the species implies, the teeth are also distinctive, having a roughened surface formed by numerous narrow irregular ridges. They have been reported to have between nineteen and twenty-eight teeth in each quarter of the jaw.
The flippers are set back further along the body than in other similar dolphins, although, at sea this dolphin may be confused with spinner, spotted and bottlenose dolphins. The dorsal fin is pronounced, being from 18 to 28 centimetres (7.1 to 11 in) in height. The animal's flanks are a light gray, while the back and dorsal fin are a much darker gray. Older individuals often have distinctive pinkish, yellow, or white markings around the mouth and along the underside.
Population and distribution
The Rough-toothed Dolphin is a tropical to subtropical species, which generally inhabits deep, oceanic waters of all three major oceans, rarely ranging north of 40°N or south of 35°S (Jefferson 2002). However, in some areas (such as off the coast of Brazil and West Africa), rough-toothed dolphins may occur in more shallow coastal waters. They are found in many semi-enclosed bodies of water (such as the Gulf of Thailand, Red Sea, Gulf of Mexico, Caribbean Sea, and Gulf of California), but they are regarded as visitors in the Mediterranean Sea (Watkins et al. 1987; Miyazaki and Perrin 1994; Reeves and Notarbartolo di Sciara 2006).
Albania; Algeria; American Samoa (American Samoa); Angola (Angola); Anguilla; Antigua and Barbuda; Aruba; Australia; Bahamas; Bahrain; Bangladesh; Barbados; Belize; Benin; Bosnia and Herzegovina; Brazil; British Indian Ocean Territory; Brunei Darussalam; Cambodia; Cameroon; Cape Verde; Cayman Islands; Chile; China; Christmas Island; Cocos (Keeling) Islands; Colombia; Comoros; Congo; Congo, The Democratic Republic of the; Cook Islands; Costa Rica; Côte d'Ivoire; Cuba; Cyprus; Djibouti; Dominica; Dominican Republic; Ecuador; El Salvador; Equatorial Guinea; Fiji; France; French Guiana; French Polynesia; Gabon; Gambia; Ghana; Gibraltar; Greece (Kriti); Grenada; Guadeloupe; Guam; Guatemala; Guinea; Guinea-Bissau; Guyana; Haiti; Honduras; Hong Kong; India; Indonesia; Iran, Islamic Republic of; Iraq; Israel; Italy (Sardegna, Sicilia); Jamaica; Japan; Jordan; Kenya; Kiribati; Kuwait; Liberia; Libya; Macao; Madagascar; Malaysia; Maldives; Malta; Marshall Islands; Martinique; Mauritania; Mauritius; Mayotte; Mexico; Micronesia, Federated States of; Monaco; Montserrat; Morocco; Mozambique; Myanmar; Namibia; Nauru; Netherlands Antilles; New Caledonia; Nicaragua; Nigeria; Niue; Norfolk Island; Northern Mariana Islands; Oman; Pakistan; Palau; Panama; Papua New Guinea; Peru; Philippines; Pitcairn; Portugal; Puerto Rico; Qatar; Réunion; Saint Helena, Ascension and Tristan da Cunha; Saint Kitts and Nevis; Saint Lucia; Saint Vincent and the Grenadines; Samoa; Sao Tomé and Principe; Senegal; Seychelles; Sierra Leone; Singapore; Solomon Islands; Somalia; South Africa; Spain (Baleares, Canary Is.); Sri Lanka; Suriname; Taiwan, Province of China; Tanzania, United Republic of; Thailand; Timor-Leste; Togo; Tonga; Trinidad and Tobago; Turks and Caicos Islands; United Arab Emirates; United States; Venezuela, Bolivarian Republic of; Vietnam; Virgin Islands, British; Virgin Islands, U.S.; Wallis and Futuna; Western Sahara; Yemen
Habitat and Ecology:
Most often, Steno bredanensis is found in deep water far
offshore, usually beyond the continental shelf (Maigret 1994), but may be seen
close inshore in areas of steep bottom relief (Ritter 2002). In the eastern
tropical Pacific, they tend to associate with other cetaceans (especially pilot
whales and Fraser’s dolphins) (Miyazaki and Perrin 1994). Rough-toothed Dolphins
feed on cephalopods and fish, including large fish such as Coryphaena hippurus
(Pitman and Stinchcomb 2002).
There are few estimates of abundance for this species. An
estimated 145,900 (CV=32%) Rough-toothed Dolphins inhabit the eastern tropical
Pacific (Wade and Gerrodette 1993), and about 2,746 (CV=36%) occur in the
northern Gulf of Mexico (Waring et al. 2008), including an estimated 1,238
(CV=65%) on the continental shelf (Fulling et al. 2003). The US NMFS has
estimated the regional population around Hawaii to be 19,904 (CV=52%), based on
recent vessel surveys (Carretta et al. 2006).
Population Trend: Unknown
Behaviour and diet
Rough-toothed dolphins are typically social animals, although solitary individuals are also sighted. An average group has between ten and twenty members, but they can vary from as few as two to as many as ninety.Such groups are thought to be temporary assemblages, composed of smaller, more permanent groups of two to eight closely related individuals that occasionally join together with others. They have also been reported to school together with other species of dolphin, and with pilot whales, false killer whales, and humpback whales.
Rough-toothed dolphins have been reported to bow-ride on a number of occasions, although apparently they do not do so as frequently as many other dolphin species. They do, however, commonly "skim", by swimming with their heads and chin above the surface of the water. They are known to be able to dive to at least 50 metres (160 ft) and be able to stay underwater for at least fifteen minutes. Their echolocation clicks are unusually brief, lasting no more than 0.2 seconds, and have a relatively low frequency, ranging from 2.7 to 256 kHz, with a maximum peak frequency of 25 kHz. They also make longer whistles with a frequency between 3 and 12 kHz.
Although details of their diet are sketchy, the stomach contents of stranded dolphins have included such fish such as silversides, sauries, hound fish, smelts, cutlass fish, and various squid and octopuses. Predators on rough-toothed dolphins are thought to include killer whales and sharks.
Rough-toothed dolphins give birth to a single young, after an unknown period of gestation; it is also unknown whether or not they have a distinct breeding season. The young are about 100 centimetres (39 in) long at birth, and grow rapidly for the first five years of life. Females reach sexual maturity somewhere between six and ten years of age, and males between five and ten years.
No fisheries are known to specifically target this species,
but small numbers are taken in drive fisheries at Okinawa in the Ryukyus and
other islands of Japan, the Solomon Islands and Papua New Guinea, by harpoon in
Japan, St. Vincent in the Lesser Antilles, and in West Africa. They were
possibly formerly were taken at St. Helena in the South Atlantic. However, only
23 Rough-toothed Dolphins were captured in Japan (Okinawa) during the period
1976-81 (Miyazaki and Perrin 1994). Recent information suggests catches in
Taiwan (J. Wang pers. comm.).
A few Rough-toothed Dolphins are killed incidentally in tuna purse seines in the eastern tropical Pacific: 21 were estimated killed during the period 1971-75 and 36 died in a single net haul in 1982. Small numbers are also taken as by-catch in gillnet and driftnet fisheries in Sri Lanka, Brazil, the central North Pacific and probably elsewhere around the world in tropical and warm-temperate waters (Miyazaki and Perrin 1994). Monteiro-Neto et al. (2000) reported on fishery-related mortality along the coast of Ceara State, northeast Brazil, commenting on the possible conservation implications for the local subpopulations. Seasonally, incidental catches were more frequent during the austral spring (October-December). Rough-toothed Dolphins are also taken by gill nets, driftnets and pelagic long-lines in Taiwan (J. Wang pers. comm.).
The species is listed in Appendix II of CITES.
The biology, life history, population size, and separation into subpopulations, as well as migratory behaviour are insufficiently known. Research on this species should be encouraged.