Rissos dolphin

Grampus griseus

Risso's dolphin is the only species of dolphin in the genus ''Grampus''. It is commonly known as the Monk dolphin among Taiwanese fishermen. Some of the closest related species to these dolphins include: pilot whales, pygmy killer whales, melon-headed whales, and false killer whales.
Risso's Dolphin One of a group of about 10 including a couple of young ones.  A 1st for me. Calf of Man,Grampus griseus,Isle of Man,Rissos dolphin

Appearance

Risso's dolphin has 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 grey to brown and ventrally cream-colored, with a white anchor-shaped area between the pectorals and around the mouth. In older calves, the nonwhite areas darken to nearly black, and then lighten . Linear scars mostly from social interaction eventually cover the bulk of the body; scarring is a common feature in toothed whales, but Risso's dolphin tend to be unusually heavily scarred. Older individuals appear mostly white. Most individuals have two to seven pairs of teeth, all in the lower jaw.

Length is typically 10 feet, although specimens may reach 13.12 feet. Like most dolphins, males are typically slightly larger than females. This species weighs 300–500 kilograms, making it the largest species called "dolphin".
Risso's Dolphin A new dolphin species for me off the Calf of Man. Calf of Man,Grampus griseus,Isle of Man,Rissos dolphin

Status

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

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 , the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area , the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region and the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia.

Risso's dolphins are protected in the United States under the Marine Mammal Protection Act of 1992. Currently, Japan, Indonesia, the Solomon Islands, and The Lesser Antilles hunt Risso's dolphins.
Risso's Dolphin off the Calf of Man A large dolphin growing up to 4 metres.  Seen regularly off the Isle of Man but infrequently. Calf of Man,Grampus griseus,Isle of Man,Rissos dolphin

Behavior

Risso's dolphins do not require cutting teeth to process their cephalopod prey, which has allowed the species to evolve teeth as display weapons in mating conflicts.

Risso's dolphins have a stratified social organisation. These dolphins typically travel in groups of between 10 and 51, but can sometimes form "super-pods" reaching up to a few thousand individuals. Smaller, stable subgroups exist within larger groups. These groups tend to be similar in age or sex. Risso's experience fidelity towards their groups. Long-term bonds are seen to correlate with adult males. Younger individuals experience less fidelity and can leave and join groups. Mothers show a high fidelity towards a group of mother and calves. But, it is unclear whether or not these females stay together after their calves leave or remain in their natal pods.

Habitat

They are found worldwide in temperate and tropical waters, in the Indian, Pacific and Atlantic Oceans, also 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.

Their preferred environment is just off the continental shelf on steep banks, with water depths varying from 400–1,000 m and water temperatures at least 10 °C and preferably 15–20 °C.

The population around the continental shelf of the United States is estimated to be 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.They feed almost exclusively on neritic and oceanic squid, mostly nocturnally. Predation does not appear significant. Mass strandings are infrequent. Analysis carried out on the stomach contents of stranded specimens in Scotland showed that the most important species preyed on in Scottish waters is the curled octopus.

A population is found off Santa Catalina Island where they are sympatric with short-finned pilot whales and both species feed on the squid population. Although these species have not been seen to interact with each other, they take advantage of the commercial squid fishing that takes place at night. They have been seen by fishermen to feed around their boats. They also travel with other cetaceans. They surf the bow waves of gray whales, as well as ocean swells.

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 39.6 years.

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

References:

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Status: Least concern
EX EW CR EN VU NT LC
Taxonomy
KingdomAnimalia
DivisionChordata
ClassMammalia
OrderArtiodactyla
FamilyDelphinidae
GenusGrampus
SpeciesG. griseus
Photographed in
Isle of Man