Species: Grampus griseus (Cuvier, 1812)
Adults of both sexes reach about 4 meters in length and weigh about 500 kg. The body is proportioned, the rostrum is almost absent and the front is rounded, but not globe-shaped as in the pilot whales. During the life the livery is discolored and in the old individuals the head can become completely white. Adults have many scars on the skin that are clearly visible, especially on the front part of the body, which contribute to its white coloration. Newborns are dark, blackish-brown in coloration.
The Risso’s dolphins form small groups of 7-15 individuals that often spread out while searching for food. These dolphins feed mainly on cephalopods but also occasionally on small fish. From their stomach contents, it has been observed that they feed on neritic species belonging to the Teuthidae family.
The species is abundant in tropical and temperate waters of the world’s oceans and generally lives on the continental shelf. In the Mediterranean, the the Risso’s dolphin is described as a predominantly pelagic species living between 500 and 2000 meters with a preference for the upper part of the escarpment between 500 and 1000 meters.
No global abundance estimate is available for Risso’s dolphins in the Mediterranean, and the species is currently listed by the IUCN as Data Deficient.
Among the most important threats, in the Mediterranean Sea, is bycatch: in Spain and Italy, the gears involved are longline systems and gillnets. Although in the Mediterranean there is no evidence of strandings of the species directly attributable to noise pollution, the Risso’s dolphin, being a deep-diving pelagic species, is considered at risk. Like other odontocetes in the Mediterranean, stingrays are also impacted by chemical pollution.
La popolazione locale
Risso’s dolphin is a rare species in the study area and is not present (or not sighted) every year. However, over time it has been possible, thanks to photo-identification, to catalog a total of 45 individuals.
Some of them show strong fidelity to the site; in particular, 10 individuals have been frequenting the study area for 16 years. The photo-identification analysis on scars and scratches of grampus, also allowed to verify the stability over time of these marks, which are therefore useful for long-term photo-identification.
In the waters of Ischia, Risso’s dolphins have been the subject of a serious episode of anthropogenic disturbance, due to interactions with numerous pleasure boats. The boats, unintentionally, have “pushed” the animals towards land, bottling them up and putting them at risk of stranding.
The Risso’s dolphins frequent both the slope environment, more offshore with depths of 500/600m, and the coastal environment of the island of Ischia.
The acoustic repertoire of the Risso’s dolphin is still poorly studied. The species, like other delphinids, emits both clicks, whistles, and pulsed sounds.
Whistles appear to be rarer, and emitted at frequencies between 5 and 20 kHz; the Risso’s dolphin also emits low-frequency vocalizations with a strong harmonic component.
Our study attempts to characterize the acoustic behavior of the species in the study area to define a vocal catalog of the repertoire of this population. Study objectives include: describing combinations of whistles and pulse sounds, determining if acoustic behavior is characteristic of various behavioral states, and determining how group size and composition affect vocalization rates.