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RISSO'S DOLPHIN
Grampus griseus (Cuvier 1812)

Order Cetacea : Family Delphinidae

DESCRIPTION. This is a medium-sized dolphin that averages 3 m in length and 300 kg in weight. Maximum size is about 4.3 m in length and 500 kg in weight. Body form is stocky from the dorsal fin forward, but the tailstock is slender. The head is blunt, beakless, and divided medially by a heavy crease. Coloration is dark gray with lighter gray patches ventrally. In older individuals, the face and area just forward of the dorsal fin is also light gray. They are often heavily scarred by parasites and by wounds inflicted by other Risso's dolphins. The blunt, creased head and extensive scarring are noticeable as the animals ride boat bow waves, or spyhop, and are probably the best field characters for distinguishing these dolphins. Stranded specimens are distinguished by the unique head shape and crease and by the teeth. There are up to seven teeth in each side of the lower jaw and none in the upper jaw.

Grampus griseus

DISTRIBUTION. Risso's dolphins are widely distributed in temperate and tropical waters worldwide. There are many records of the species in the northern Gulf of Mexico where they are frequently found on the steep sections of the upper slope. There is only one stranding from Texas, and a group of nine Risso's dolphins was seen off the South Texas coast in November 1980. Data from the most recent stranding period (2002–2014) include three strandings of these dolphins from Nueces, Cameron, and Galveston counties.

SUBSPECIES. Monotypic species.

HABITS. Risso's dolphins have been observed in large groups of several hundred, but smaller groups of 3–30 are more common. They feed primarily on squid but also will eat fish and crustaceans. They often associate with other cetacean species, and hybrids between Risso's and common bottlenose dolphins have been reported in captivity and in nature. Their reproductive habits are poorly known. There appears to be a summer calving peak in the North Atlantic. Newborns are about 1.4 m long at birth, and these dolphins may live as long as 24 years. A well-known Risso's dolphin called Pelorus Jack was sighted in a New Zealand harbor for over 20 years.

POPULATION STATUS. Common; strandings and observations. Risso's dolphin is another offshore, deepwater species that rarely strands. They appear to be rare near the coast where the continental shelf is broad and the nearshore waters are relatively shallow. Aerial and shipboard sightings are common in the deeper waters of the Gulf. Pelagic sightings of Risso's dolphins in the Gulf of Mexico show that these dolphins appear to prefer deep offshore waters; they have been sighted at ocean depths of 200–1,530 m (656–5,020 ft.). This dolphin is apparently a year-round resident of the Gulf and common along continental slope areas. During the GulfCet surveys, these dolphins were sighted in all seasons. Abundance has been estimated as high as about 4,000 animals in the northern Gulf.

CONSERVATION STATUS. Risso's dolphin is not listed as a species of concern by either the USFWS or TPWD; its status with IUCN is least concern. This is a deepwater species that is unlikely to be sighted or strand along the Texas coast, but it is common in offshore waters and does not appear to have any serious conservation threats.

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From The Mammals of Texas, Seventh Edition by David J. Schmidly and Robert D. Bradley, copyright © 1994, 2004, 2016.  Courtesy of the University of Texas Press.

Natural Science Research Laboratory