The Risso’s dolphin (Grampus griseus, Cuvier, 1812) is the smallest member of the subfamily Globicephalinae (Delphinidae). Adult size varies up to 3.8 m in length and body mass can reach 500 kg.

The anterior part of the body is extremely robust, tapering to a relatively narrow tail. The dorsal fin is one of the tallest in proportion to the body length of any cetacean, surpassed only by that of the adult male killer whale (Orcinus orca). The head is bulbous with a distinct vertical crease or cleft along the anterior surface of the melon. Color patterns change dramatically with age.

Hatchlings at birth are between 1.1 and 1.5 m in length with gray to brown coloration, then darken to almost black and lighten as they mature (the dorsal fin remains dark). As they age, most lateral and dorsal surfaces become covered with distinctive linear scars. Older animals may appear completely white on the dorsal surface.

Global distribution

This cosmopolitan species inhabits temperate and tropical waters worldwide approximately between 64°N and 46°S. The Risso’s dolphin is defined as a highly mobile pelagic species, with a relatively limited diet based primarily on cephalopods. Although without a complete lineage classification, according to mtDNA analysis, the Mediterranean Risso’s dolphin population is genetically differentiated from that inhabiting eastern North Atlantic waters and other oceanic populations, suggesting that even between these two neighboring areas gene flow is limited or negligible. Within the Mediterranean, there is some evidence of a possible subpopulation structure as studied by some authors, however, more samples from underrepresented areas, particularly from the eastern and southern part of the basin are needed to confirm this.

Levantine-Balearic Marine Demarcation

It is a rare species, although present throughout the year mainly in the spring and summer months. It is distributed from the coastline to 1000m. Although they are present in almost all the Mediterranean Sea, from Gibraltar to the Levantine basin, including the Adriatic Sea and the Aegean Sea, most of the reported sightings have been in the western basin, as shown in the prediction map made during the ADI project. The presence of Risso’s dolphin has been reported in the Ligurian Sea and the marine waters of the Gulf of Lions.

In 2004, in the waters of Murcia and the Valencian Community, Risso’s dolphin population size of 493 individuals (95% CI = 162-1498) was estimated, with a relative density of 0.015 individuals per km 2 in the area (95% CI = 0.005-0.046).

In the Marine Mammal Group Document of the Marine Strategies (2012) a prediction of potential habitats for Risso’s dolphin in the Levantine-Balearic Demarcation was made from data obtained from several authors.

Habitat

This species seems to be associated with coastal waters and oceanic islands where the bottom is steep, in those areas with a narrow shelf and in the European Atlantic. It has been sighted in deep offshore waters (200-1200 m depth), concentrating on the upper continental slope and around steep shelf edge areas. They are often found near the edge of the slope, around pronounced bathymetric formations such as submarine canyons and seamounts and oceanic trenches.

Diet

Data on the feeding behavior of Risso’s dolphin have been obtained from the stomach contents of stranded animals. They are considered to have a teutophagous diet, i.e. they concentrate on cephalopods, including squid, octopus and cuttlefish. They feed on both neritic and oceanic species and appear to feed mainly at night.

Group size

They generally form small groups of between 10 and 100 individuals, the average being about 30 animals, although aggregations of up to 4,000 Risso’s dolphins can form, which have been related to areas where food is concentrated.

Reproduction

The age of sexual maturity of females is estimated to be between 8-10 years, while males mature later, between 10-12 years.

The interval between births has been estimated at 2.4 years (observations of females in the western Mediterranean are 3-4 years) and females may reproduce up to 38 years of age. The annual survival rate for juveniles is 0.87 and 0.95 for adults.

The spatial and temporal patterns of movements observed in the vicinity of Pico Island (Azores Islands) suggest a mating system based on groups of males defending areas to which females periodically come. A gestation period of 13-14 months is estimated.

Currently, the only information available on lactation and calving in Risso’s dolphins is related to calving seasons and weaning age. Possible summer and fall calving peaks have been reported off Japan and fall and winter off California. In the North Atlantic, there appears to be a calving peak between April and September.

Acoustics

The first recordings of Risso’s dolphin vocalizations began in the 1960s and determined the possibility of an acoustic signature, through in an adult male. More recently, data from a captive juvenile Risso’s dolphin housed with several bottlenose dolphins also showed that Risso’s dolphins produce signature whistles; in this case perhaps through vocal learning from the other species.

The acoustic repertoire of the Risso’s dolphin was first described in Australia where they found that they produce a wide range of vocalizations between 30 Hz and 22 kHz. These include broadband clicks, pulse-burst vocalizations (barks and buzzes that are stereotyped vocalizations), low-frequency narrow-band sounds (grunts and chirps), and the simultaneous production of whistles and click-burst sounds.

In 1995, hearing measurements were taken from a captive adult Risso’s dolphin and a stranded calf. The hearing threshold of the juvenile was lower than that of the adult for high frequencies; for example, for a frequency of 110 kHz the threshold for the juvenile was 76dB re 1μPa, and for the 30-year-old animal it was 122.9 dB re 1μPa. However, these measurements were performed under different conditions and using different methods.

The temporal resolution of the long-finned Risso’s dolphin’s auditory system is very high, beyond that of most terrestrial animals, but still similar to that of other odontocetes.
In general, the Risso’s dolphin produces during echolocation broadband clicks of the short duration of 40 µs with peak frequencies of around 50 kHz, centroid frequencies of between 60 and 90 kHz, and levels of 202- 222 dB re 1 µPa _peak-to-peak.

Other authors have described whistles, barks, grunts and chirps produced by this species. The former are narrow-band, frequency-modulated sounds that can be of ascending or descending frequency from 4-22 kHz and a duration of 1.6-4.9 s without many harmonics. Whistles sometimes combine them with barks, which have a frequency of 4.3-17.4 kHz and an average duration of 12s. Finally, two types of low-frequency sounds have been described, grunts and chirps, with frequencies of 0.2-0.8 kHz and 2-4 kHz respectively.

In the Canary Islands, where the acoustic behavior of the Risso’s dolphin was studied, a particular behavior has been suggested, which could use a series of burst-pulses to facilitate meetings after long dives.

In conclusion, Risso’s dolphin combine a variety of unusual features in their acoustic faculty. First, they produce highly stereotyped burst sounds (buzzes) and combine their burst sounds with whistles in a highly synchronized manner. This aspect places them closer to killer whales than to any other dolphin species. The suggestion of the presence of characteristic whistles in the Risso’s dolphin places them close to bottlenose dolphins. Thus, the Risso’s dolphin appears to combine characteristics of very different delphinid species. Similarly, its social system appears to lie somewhere between the highly stable matrilineal society of killer whales and the more adept one of bottlenose dolphins. In addition, the Risso’s dolphin also has a unique feature in the anatomy of its vocal apparatus, the vertical slit in the melon that probably impacts sound production.

Conservation status

International level:
– Global category IUCN (2018): Least Concern – LC (Kiszka and Braulik, 2018).
– European category IUCN (2007): Data Deficient – DD.
– Mediterranean Category IUCN (2010): Data Deficient – DD (Gaspari and Natoli, 2012).
– The EU Habitats Directive(92/43/EEC), includes in its Annex IV, Animal and plant species of Community interest that require strict protection, where all species of cetaceans and sea turtles present in our waters are included.

National Level:
– RD 139/2011 for the development of the List of Wildlife Species under Special Protection Regime and the Spanish Catalogue of Threatened Species, includes the Risso’s dolphin among the species catalogued as being of special interest.

Autonomous Community level:
– Law 4/2010, of the Canary Islands Catalog of Protected Species considers the Risso’s dolphin in Annex VI as a species under Special Protection Regime.
– Law 8/2003 of the Flora and Wildlife of Andalusia where the Risso’s dolphin is included in the Andalusian List of Wild Species under Special Protection Regime (LAESPE).
In 2017, 26 areas within the Mediterranean region received the status of Important Marine Mammal Areas (IMMA), defined as “discrete portions of habitat, important for marine mammal species, that have the potential to be delimited and managed for conservation” (IUCN Marine Mammal Protected Areas Working Group 2017). Among the Mediterranean IMMAs, two areas have been designated for their recognized importance for Risso’s dolphin: the Alboran Deep and the Northwestern Mediterranean Sea, Slopes and Canyons System.

Threats

A local estimate of Risso’s dolphin population size in the western Ligurian Sea using the mark-recapture method from 1990 to 2014 was published in 2016. In the last period of the time series, the population size of the Risso’s dolphin decreased by 50%. The results of the study also highlighted a drastic change in habitat use by species in the study area.

Species presence appeared to be declining significantly in the coastal and continental slope areas, while it appeared to be stable in the more pelagic area.
According to the IUCN Red List, the main recognized threat to the Risso’s dolphin in the Mediterranean is bycatch. This is particularly well documented in the Spanish surface longline fishery in the western Mediterranean Sea, although this fishery has declined dramatically in recent years. Stranded animals showed evidence of bycatch in fishing gear.

In the Mediterranean, most of the Risso’s dolphin bycatch is taken with pelagic gillnets (also called driftnets). In the Ligurian Sea, 44% of the Risso’s dolphin stranded between 1986 and 2014 were reported as bycatch or with signs of gillnet gillnetting. In Galicia, 36% of the stranded animals, and that were examined freshly dead (n= 28), were captured or showed signs of capture.

Noise can negatively affect pilot whales in several ways. In the most severe cases (extremely high noise levels, e.g., seismic vessels) this can result in permanent damage or even death.
Chronic noise on various time scales can also affect pilot whales, for example, by inducing temporary displacement and changing short-term and possibly long-term behavior, excluding them from preferred habitat for short or even long periods of time, with the potential to prevent successful feeding and breeding.

Noise from human activities, including seismic surveys, marine construction, and the use of military or other sonar, is of concern to Risso’s dolphin and other cetaceans.
For example, a preliminary study off Pico (Azores) suggested that the resting behavior of Risso’s dolphin was disturbed by whale-watching vessels.
Other potential threats include noise disturbance and the ingestion of plastic debris.

The SCARS Project

The Project “SCARS: expanding the knowledge of the Risso’s dolphin (Grampus griseus) in waters of the Levantine-Balearic demarcation” is being carried out this 2021 and is supported by the Biodiversity Foundation, the Ministry for Ecological Transition and the Demographic Challenge and aims to study the distribution, abundance, and movements of the Risso’s dolphin population in the Levantine-Balearic demarcation and specifically in the submarine canyons of the coast of Catalonia.