Latin Grampus griseus
Manx Lheimmeyder marrey garroo
The Risso’s dolphin is a large, robust dolphin with an adult body length of up to 4 metres and a square, beakless head. The dorsal fin is relatively large and can be sickle shaped or tall and straight. Body colour can range from dark grey to almost white, depending on the level of scarring on an individual.
Heavy scarring is almost always apparent in adults and is caused by two main factors; feeding and fighting. Scarring can be inflicted by the tentacles and beaks of their chosen prey; squid, cuttlefish and octopus (collectively known as cephalopods), but can also be inflicted by the teeth of other Risso’s dolphins. The amount of scarring on an individual can help us estimate age; often the scars build up to such an extent that some very old Risso’s dolphins are almost completely white. Life span is estimated at between 20-40 years. The oldest stranded Risso’s dolphin examined was estimated to be 34.5 years old; analysis was made by looking at layers in the teeth.
The Risso’s dolphin is recognisable at the surface by its tall dorsal fin in the centre of the back. Behaviour varies widely and can include breaching, head slapping, spy hopping and logging. They are generally much slower than our other dolphin species when travelling, but are known to be highly active when feeding.
Group size in Manx waters
Risso’s dolphins are deep divers and prey on bottom dwelling animals such as squid, cuttlefish and octopus. Like other dolphins they locate their prey using echolocation.
Abundance and distribution
Little is known about the Risso’s dolphin and no worldwide population estimates exist. They have been observed in temperate and tropical waters as far north as Greenland and as far south as New Zealand and are found throughout the Mediterranean. They are generally found on continental shelves and favour steep shelf edge habitats between 400-1000 metres deep where they prey on bottom dwelling animals including squid and cuttlefish. There are only a handful of inshore populations in Europe, of which the Isle of Man has one.
Risso’s dolphins are the most commonly seen dolphin species in summer months despite our distinctly shallower waters compared to their usual habitat. They are often observed extremely close inshore in areas such as Port St Mary, Marine Drive and Langness. They have even been seen passing through the Calf sound. These coastal areas have water depths ranging from only 22m-35m. The reason for such abundance in our shallower waters is not yet known but the island clearly has an important and unusual population.
Risso’s dolphins provide our primary source of photo-ID in Manx waters, the catalogue currently containing at least 102 individual dolphins.
This dolphin has such heavy scarring that it has turned almost completely white
A very young Risso’s dolphin breaching