|Scientific Name:||Grampus griseus (Mediterranean subpopulation)|
See Grampus griseus
|Red List Category & Criteria:||Data Deficient ver 3.1|
|Assessor(s):||Gaspari, S. & Natoli, A.|
|Reviewer(s):||Reeves, R. & Crespo, E.|
Risso’s Dolphin is widely distributed in temperate and tropical waters worldwide but little is known about the species. Estimates of abundance are available for only a few regions and details of distribution are generally lacking.
This is also true of Risso’s Dolphins in the Mediterranean Sea. No large-scale surveys have been conducted to assess their range, distribution or numbers although their presence has been established from small-scale surveys in particular areas (specified in later sections). The current assessment for the Mediterranean subpopulation is Data Deficient.
Long-term studies in the Ligurian-Corso-Provençal basin indicate that:
1- Risso’s Dolphins in the Mediterranean are genetically differentiated from those in the eastern Atlantic (Gaspari et al. 2007).
2- Genetic characteristics of animals sampled in the Ligurian-Corso-Provençal basin were variable but suggestive of intra-basin structuring (Gaspari et al. 2007).
3- In the Ligurian-Corso-Provençal basin, a core group of individuals is present during the summer and this group shows a degree of site fidelity (Airoldi et al. 2005). However, additional Risso’s Dolphins probably visit the area.
4- Preferred habitat is continental slope waters with steep relief (Azzellino et al. 2008).
5- Risso’s Dolphins are taken as a bycatch in gillnets and on longlines.
The following types of data are needed to support an assessment of the subpopulation:
1- Distribution and abundance, preferably in relation to habitat features and population substructure.
2- Genetic population structure, which requires more extensive sampling to cover a larger proportion of the species’ range within the Mediterranean.
Risso’s Dolphins occur in much of the Mediterranean Sea although most reported sightings have been in the western basin. The greatest concentration is in the Ligurian-Corso-Provençal basin, where the species is present all year-round. In general, Risso’s Dolphins prefer deep offshore waters and continental slope areas. The Ligurian-Corso-Provençal basin is one of the few areas in the Mediterranean Sea where the continental shelf is close to the coast, giving especially good opportunities to observe and study this species. In this Area, Azzellino et al. (2008) suggested a transient use of the habitat. Risso’s Dolphins also occur seasonally in the southern Tyrrhenian Sea off the west coast of Ischia and between the island of Ustica and the Aeolian islands. They are observed regularly in the Balearic Sea and in the eastern half of the Alborán Sea (mainly from Seco de los Olivos to the Gulf of Vera) all year-round.
The apparent scarcity of Risso’s Dolphins in the eastern Mediterranean may be partly due to the paucity of observational effort there. A few strandings have been recorded in the northern Adriatic Sea and few sightings and strandings have been recorded along the coast of Israel and in the western Ionian Sea (Sicily). Risso’s Dolphin has been observed in the eastern Ionian Sea (Greece), around the western side of Crete and in the Aegean Sea (Frantzis et al. 2003). No data are available for the southern Mediterranean Sea.
Native:Algeria; France; Greece; Israel; Italy; Malta; Monaco; Spain
|FAO Marine Fishing Areas:||
Mediterranean and Black Sea
|Range Map:||Click here to open the map viewer and explore range.|
Although Risso’s Dolphins are regularly sighted in the western Mediterranean, no population estimates exist for the species in this region. They are generally considered scarce. Population identity and structure Risso’s Dolphins in the Mediterranean Sea are genetically differentiated from those in the eastern Atlantic. This implies that gene flow between the two areas is limited or negligible and that the Mediterranean animals constitute a distinct subpopulation (Gaspari et al. 2007). There is also some evidence of structuring within the Mediterranean. Most of the samples analysed were from the Ligurian Sea, so it is possible that multiple populations use this area as a foraging ground (Gaspari et al 2007).
Abundance and TrendLine-transect abundance estimates exist only for the western central Mediterranean, where aerial surveys from 2001–2003 resulted in an estimate of 493 Risso’s Dolphins (95% C.I. 162–1,498) in an area of 32,270 km² (Gómez de Segura et al. 2006). In all surveyed areas, encounter rates have been low (i.e., Ligurian-Corso-Provençal basin, 0.098 per km – Tethys Research Institute; southern Tyrrhenian Sea, 0.2 per nmi – B. Mussi, pers. comm.; Alborán Sea, 0.0032 per km - Cañadas et al. 2005).
There is no baseline information on abundance and therefore it is not possible to assess trends for the Mediterranean population.
|Habitat and Ecology:||
Risso’s Dolphins show a preference for deep pelagic waters, in particular over steep shelf slopes and submarine canyons (Azzellino et al. 2008, Gaspari 2004, Cañadas et al. 2002).
Individual Associations and Kinship
Risso’s Dolphins in the Ligurian Sea (1990–2000) were encountered in groups of variable size, with mostly weak inter-individual associations but also some consistent relationships between individuals over periods of months and, in a few cases, years (Gaspari 2004). Limited evidence on genetic similarity among individuals within and among groups (n = 30) in the northwestern Mediterranean suggests that Risso’s Dolphins have a fluid social structure (Gaspari 2004).
In the Mediterranean Sea, Risso’s Dolphins are among the cetacean species frequently found entangled in fishing nets. Bycatches in longlines and gillnets have been reported in Spain (Valeiras et al. 2001) and Italy (Notarbartolo di Siara 1990).
Sound pollution is a threat to deep-diving pelagic cetaceans, including Risso’s Dolphins. Although there are no records of Risso’s Dolphin strandings in the Mediterranean Sea directly attributable to noise, evidence consistent with a syndrome related to exposure to high-intensity sonar has been described in this species in the UK (Jepson et al. 2005).
Like other odontocetes, Risso’s Dolphins in the Mediterranean carry substantial contaminant burdens (Kim et al. 1996, Marsili and Focardi 1997, Shoham-Frider et al. 2002, Fossi and Marsili 2003).
To date, no specific conservation measures have been taken for Risso’s Dolphins in the Mediterranean Sea.
The existence of a Marine Sanctuary for cetaceans in the Corso-Ligurian Basin, declared by the Governments of Italy, France and Monaco, has proved to be of great value for the study of Risso’s Dolphins. Most of the detailed studies of Risso’s Dolphins within the Mediterranean have taken place there. Research on Risso’s Dolphin within the Pelagos Sanctuary should be expanded, and additional areas in the region should be identified where protective measures would benefit the species. The first step toward this has taken place in southern Spanish waters, where habitat preference modelling has been undertaken to define Areas of Special Interest for the Conservation of Cetaceans in the Spanish Mediterranean (Cañadas et al. 2005).
|Citation:||Gaspari, S. & Natoli, A. 2012. Grampus griseus (Mediterranean subpopulation). The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 02 October 2014.|
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