Risso's Dolphin (Grampus griseus)

 

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

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.

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. Length is typically 10 feet (3.0 m) although specimens may reach 14.1 feet (4.3 m). 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".

Range and habitat

They are found worldwide in temperate and tropical waters, usually in deep waters rather than close to land. 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.Range Description: This is a widely-distributed species, inhabiting primarily deep waters of the continental slope and outer shelf (especially with steep bottom topography), from the tropics through the temperate regions in both hemispheres (Kruse et al. 1999). It also occurs in some oceanic areas, beyond the continental slope, such as in the eastern tropical Pacific. It is found from Newfoundland, Norway, the Kamchatka Peninsula, and Gulf of Alaska in the north to the tips of South America and South Africa, southern Australia, and southern New Zealand in the south. Its range includes many semi-enclosed bodies of water, such as the Gulf of Mexico, Gulf of California, Red Sea, Persian Gulf, Sea of Japan, and Mediterranean Sea.
Countries: Native:
Algeria; American Samoa (American Samoa); Anguilla; Antigua and Barbuda; Argentina; Aruba; Australia; Bahamas; Bangladesh; Barbados; Belgium; Belize; Benin; Bermuda; Brazil; British Indian Ocean Territory; Brunei Darussalam; Cambodia; Cameroon; Canada; Cape Verde; Cayman Islands; Chile; China; Cocos (Keeling) Islands; Colombia; Comoros; Congo; Congo, The Democratic Republic of the; Cook Islands; Costa Rica; Côte d'Ivoire; Croatia; Cuba; Denmark; Djibouti; Dominica; Dominican Republic; Ecuador; Egypt; El Salvador; Equatorial Guinea; Estonia; Fiji; France; French Guiana; French Polynesia; Gabon; Gambia; Germany; Ghana; Greece; Greenland; Grenada; Guadeloupe; Guam; Guatemala; Guernsey; Guinea; Guinea-Bissau; Guyana; Haiti; Honduras; Hong Kong; India; Indonesia; Iran, Islamic Republic of; Iraq; Ireland; Isle of Man; Israel; Italy; Jamaica; Japan; Jersey; Jordan; Kenya; Kiribati; Kuwait; Lebanon; Liberia; Madagascar; Malaysia; Maldives; Malta; Marshall Islands; Martinique; Mauritania; Mayotte; Mexico; Monaco; Morocco; Mozambique; Myanmar; Namibia; Netherlands; Netherlands Antilles; New Caledonia; New Zealand; Nicaragua; Nigeria; Niue; Northern Mariana Islands; Norway; Oman; Pakistan; Palau; Panama; Papua New Guinea; Peru; Philippines; Pitcairn; Portugal; Puerto Rico; Qatar; Russian Federation; Saint Kitts and Nevis; Saint Lucia; Saint Pierre and Miquelon; Saint Vincent and the Grenadines; Samoa; Sao Tomé and Principe; Saudi Arabia; Senegal; Serbia (Serbia); Sierra Leone; Singapore; Slovenia; Solomon Islands; Somalia; South Africa; Spain; Sri Lanka; Sudan; Suriname; Sweden; Syrian Arab Republic; Tanzania,; Thailand; Timor-Leste; Togo; Tonga; Trinidad and Tobago; Turkey; Turks and Caicos Islands; United Arab Emirates; United Kingdom; United States; United States Minor Outlying Islands; Uruguay; Vanuatu; Venezuela, Bolivarian Republic of; Viet Nam; Virgin Islands, British; Virgin Islands, U.S.; Wallis and Futuna; Western Sahara; Yemen
Vagrant:
Finland; Lithuania; Poland

Population:

There are no estimates of global abundance, but there are some estimates for specific areas. Forney and Barlow (1998) observed that the estimated abundance of Risso's dolphins off California was almost an order of magnitude higher in winter (N= 32,376) than in summer (N= 3,980). However, the California, Oregon, Washington subpopulation is now estimated at only 16,066 (CV=28%) whales (Barlow 2003). Hawaiian waters are estimated to contain 2,351 (CV=65%) Risso’s dolphins (Barlow 2006). Abundance estimates off Sri Lanka ranged from 5,500 to 13,000 animals (Kruse et al. 1999). In the eastern Sulu Sea, Dolar et al. (2006) estimated the abundance at 1514 (CV=55%) individuals. There are an estimated 20,479 (CV=59%) Risso’s dolphins off the eastern United States (Waring et al. 2006), 2,169 (CV=32%) in the northern Gulf of Mexico (Mullin and Fulling 2004), 83,300 (CV=17%) in three areas of concentrated occurrence off Japan (Miyashita 1993), and 175,000 in the eastern tropical Pacific (Wade and Gerrodette 1993).  In relative terms, there are several examples of long term changes in abundance and distribution, e.g. in the Southern California Bight (Kruse et al. 1999). In the late 1950s, Risso's dolphins were rarely encountered in this area, and between 1975 and 1978 they were still considered to be a minor constituent of the cetacean fauna of the Bight, representing only 3% of the cetaceans observed. After the El Niño of 1982/83, however, numbers of Risso's dolphins increased, especially around Santa Catalina Island where they came to be considered common (Shane 1995). There is no information on global trends in the abundance.
Population Trend: Unknown

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). 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

They feed almost exclusively on neritic and oceanic squid, mostly nocturnally. Predation does not appear significant. Mass strandings are infrequent. 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.Risso’s dolphins inhabit deep oceanic and continental slope waters, generally 400-1,000 m deep (Baird 2002; Jefferson et al. 1993), mostly occurring seaward of the continental slope. They frequent subsurface seamounts and escarpments, where they are thought to feed on vertically migrant and mesopelagic cephalopods. In Monterey Bay, California, Risso's dolphins are concentrated over areas with steep bottom topography (Kruse 1989). Currents and upwelling causing local increases in marine productivity may enhance feeding opportunities, resulting in the patchy distribution and local abundance of this species worldwide (Kruse et al. 1999). Davis et al. (1998) and Baumgartner (1997) reported that in the Gulf of Mexico, Risso's dolphins were mostly found over deeper bottom depths, concentrating along the upper continental slope, which may reflect squid distribution. Most records of Grampus griseus in Britain and Ireland are within 11 km of the coast. In certain areas, such as in the southwest English Channel, Risso’s dolphins are known to occur seasonally in shallow coastal waters to feed on cuttlefishes Sepia officinalis (Kiszka et al., 2004). Long-term changes in the occurrence of Risso’s dolphins in some areas (e.g., off Catalina Island and in central California) have been linked to oceanographic conditions and movements of spawning squid (Kruse et al. 1999). Risso's dolphins feed on crustaceans and cephalopods, but seem to prefer squid. Squid bites may be the cause of at least some of the scars found on the bodies of these animals. In the few areas where feeding habits have been studied, they appear to feed mainly at night.
Systems: Marine
 

Reproduction

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.

Major Threat(s):

 Occasional direct killing of Risso’s dolphins has occurred. This is generally as a result of the dolphins removing fish from loglines, or in multi-species small cetacean fisheries, such as those that occur in Sri Lanka, the Caribbean, and Indonesia. One regular hunt occurs in Japan, where about 250-500 are taken per year in a drive fishery. Some Risso’s dolphins have been captured for live display in oceanaria, although there are not many of them in oceanaria. In Sri Lanka, Risso's dolphins are apparently the second most commonly taken cetacean in fisheries, providing fish and meat for human consumption and fish bait; subpopulations there may be adversely affected (see Jefferson et al. 1993; Kruse et al. 1991). An estimated 1,300 Risso's dolphins may be landed annually as a result of this fishery, and abundance estimates in these waters range only from 5,500 to 13,000 animals (Kruse et al. 1999). In Japan, Risso's dolphins are taken periodically for food and fertilizer in set nets and as a limited catch in the small-type whaling industry (Kruse et al. 1999), with reported catches in recent years ranging from about 250–500. They are also a major target of artisanal hunting, and are taken often in gillnets and other fishing gear in the Philippines (Dolar 1994, Dolar et al. 1994). Off eastern Taiwan, Risso’s dolphins are also taken by harpoon opportunistically and oceanic large-mesh driftnets for large pelagic fish appear to take considerable numbers incidentally (Wang pers. comm.). There are reports of bycatches from the North Atlantic, the Mediterranean Sea, the southern Caribbean, the Azores, Peru, and the Solomon Islands. They are also a rare bycatch in the US tuna purse seine industry, and are taken occasionally in coastal gill net and squid seining industries off the US coast, or shot by aggravated fishermen (Kruse et al. 1999).  This species, like beaked whales that are also deep-divers that feed on squid, is likely to be vulnerable to loud anthropogenic sounds, such as those generated by navy sonar and seismic exploration (Cox et al. 2006). Predicted impacts of global climate change on the marine environment may affect this species of whale, although the nature of impacts is unclear (Learmonth et al. 2006).

Conservation Actions:

The species is listed in Appendix II of CITES. The North and Baltic Sea subpopulations are included in Appendix II of CMS. This is a circumglobal species, which migrates between summering and wintering grounds. Off California, where these movements are best known, they may cross between US and Mexican waters. Data on abundance, bycatch, and behavior needed in order to develop conservation measures that will enable protection of the natural habitat of the species

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