Risso's data deficient dolphin
This blog was written by our project partners at the Ischia Whale and Dolphin Project.
SPECIES: Grampus griseus (Cuvier, 1812)
Grampo, Risso’s dolphin, calderòn gris, dauphin de Risso, Rundkopfdelphin
The adult reaches 3.5 metres in length and weighs about 400/600 kg. The body is proportioned, the rostrum is absent and the forehead bulging but not globular as in pilot whales. Peculiar feature of this species is its livery that looses colour with time. New born dolphins are grey or light brown and become darker grey while collecting white scars (mainly teeth marks caused by interactions) as they age. The adults tend to become whiter and whiter, and the senior ones may appear completely white.
Risso’s dolphins form small groups of 5-20 individuals who often get separated during the search for food. They can be very lively and active on the surface, exhibiting numerous jumps and bow-riding. A common behaviour is spy-hopping, showing only their head above the water to observe. In the Mediterranean Sea, often they are seen “head standing”, which is being head down in an almost still, vertical position with the fluke outside the water. The reason for this behaviour is still unknown.
The Risso’s dolphins are abundant in tropical and temperate waters of the oceans around the world, and, as in the other seas, also in the Mediterranean they generally live on the slope of the continental shelf, and underwater canyons, 200-metre-deep, where they feed mainly on cephalopods but also occasionally on small fish.
In the Mediterranean, associations of Risso’s dolphins with other cetacean species include mainly mixed groups of striped dolphin (Stenella coeruleoalba) and common dolphin (Delphinus delphis), documented in the Gulf of Corinth, Greece; only in the waters off Ischia, has it been recorded and interaction with groups of feeding sperm whales (Physeter macrocephalus).
An estimate of population abundance throughout the whole Mediterranean Sea is yet to be available. Only in the Western – Central parts of the basin, air surveys were performed during 2001-2003 estimating 493 Risso’s dolphins in an area of 32.270 km². In all other areas where the species is investigated (Ligurian-Corse-Provence basin, Central Tyrrhenian Sea, Alborán Sea), encounter rates are very low.
Even in those few areas where Risso’s dolphins are being monitored, information is limited to few sightings or strandings as evidence of their presence. Distribution, ecology and conservation status of this species in the Mediterranean are still a mystery.
As a consequence, the species in the Mediterranean is classified as ‘data deficient’ by the International Union for the Conservation of Nature (IUCN) as information on the species are still scarce. However, presently, no specific conservation measures have been adopted for Risso’s dolphins in the Mediterranean.
The main threat for this species in the Mediterranean is the by-catch in fishing gear. Risso’s dolphins often interact (during foraging) mainly with bait systems, attracted by fresh baits (squids and fish) and end up entangled. In the past, also the drift nets for swordfish had a strong impact on these animals. However, after the European ban in 2002, few illegal fleets still represent a threat to them.
As for all cetaceans, also underwater noise pollution represents a threat for the survival of this species. The waters of the Mediterranean, more and more crowded by commercial and touristic fleets, drilling and military operations etc. certainly do not represent the ideal environment.
Among the documented threats, Oceanomare Delphis Onlus has reported about a severe case of harassment of a group of Risso’s dolphins in the waters off Ischia, in August 2000, where too many pleasure boats dangerously neared 20 Risso’s dolphins sighted near the coast. As the boats increased in number, the Coast Guards and the researchers of Oceanomare Delphis intervened to protect the animals. By the time authority arrived, the dolphins where surrounded by 100 motorboats and where cornered in a bay with about 400 anchored boats.
When the researchers and the Coast Guards reached the animals, they were almost forced to strand on the beach, in waters no deeper than 3 metres. Tens of boats moved towards them at high speed every time the animals surfaced, abruptly changing speed and direction so to try and near the animals to take a picture. Meanwhile, the dolphins showed clear signs of distress, swimming at high speed, with sudden and frequent change of direction, sometimes even bumping into and hitting each other. At least one of the calves was observed rolling vertically on itself. Only after 2 hours, the authority and the researchers managed to send away the boats and create a free channel for the animals to swim away towards the open sea. This dramatic event is symbolic of how pleasure boats approaching the animals can be a significant threat to them.
Another important threat for this species is chemical pollution. As in other cetaceans, in Risso’s dolphins there have been found concentration of organochlorine compounds and heavy metals. The remains of cephalopods found in the stomach of a Risso’s dolphin stranded in Corsica had concentrations of mercury 50 times higher than what is recommended for human consumption.
In the study area of Ischia Dolphin Project this species has been observed almost every year, since 1991, whose latest sighting dates August 2017.
The project’s catalogue includes 68 individuals, among which 21 females, several juveniles, and also calves. Many of them are regularly encountered over the years, thus showing a high level of site fidelity.
It was indeed possible to retrace a matriarchy of a Risso’s dolphin called ‘Gaudi’, which resulted being the son of ‘Vasari’ and the grandson of ‘Modigliani’. Mother and grandmother were photo-identified for the first time in 2005 when Vasari was still young, and have been sighted regularly over the years, until now with their new family member. The presence of mother-calf couples is very significant from a scientific point of view as it allows to characterise the area not only as a feeding ground, but also as a site for reproduction and calving. This underlines the need for a greater conservation effort.