Risso’s dolphin in the Mediterranean Sea

As promised, I now hand over to Sabina Airoldi from Tethys Research Institute in Italy, who explains in more detail the work of her and her colleagues on Risso's dolphins in the Mediteranean Sea. Having had the pleasure of joining them in the field during previous seasons, I can attest to the quality and professionalism with which they operate. I can also only dream of similar environmental conditions within which to conduct our own Risso's research ...! Scotland may be beautiful and magical but it's not the tropics - at least not for more than a few days a year!

Since 1990 the Tethys Research Institute has been carrying out a long term project focusing on the ecology of the cetacean species living in the Pelagos Sanctuary, an international Sanctuary for the protection of the Mediterranean marine mammals. In order to provide a scientific basis for management recommendations to the policy makers, the final goal of Tethys' research is to gain a basic understanding of this complex ecosystem, studying the distribution, abundance, habitat use and ecology of the different cetacean species inhabiting the Ligurian Sea, together with their social organization, behaviour and foraging ecology. One of the eight species investigated is the Risso’s dolphin (Grampus griseus).

In the Mediterranean Sea Risso’s dolphins are relatively widespread, but not abundant and their occurrence can be unpredictable. This factor, together with generally low densities, has precluded sustained and focused investigations, therefore, information on this cetacean species remains relatively scant and the distribution, ecology, status and trends of this species in the Mediterranean remain somewhat mysterious. A regional IUCN Red List assessment workshop in March 2006 concluded that the Mediterranean subpopulation is Data Deficient.

Risso's dolphins are usually about 3 m long but can reach 4 m, with no significant sexual size dimorphism. The adults are heavily scarred, mostly from superficial wounds caused by the teeth of conspecifics. Depigmentation can persist for a long time, and this facilitates species identification at sea and also allows single individuals to be photo-identified. During the surveys conducted in the western Ligurian Sea by the Tethys Research Institute over the last 20 years, more than 350 individuals have been photo-identified in an area of approximately 24.000 km2. These data suggest relatively wide movements but do not preclude some degree of fidelity or regular use of specific areas.





Risso’s dolphins can be energetic and active at the sea surface, breaching, porpoising and occasionally riding the bow of a vessel. In the Mediterranean Sea they are often seen ‘head-standing’, i.e. staying almost motionless in a vertical position with the tail stock out of the water, sometimes for 10-20 sec. The reason behind this behaviour is unknown. In the Ligurian Sea, during daytime, Tethys researchers observed Risso’s dolphins moving mostly at about 6-7 km h-1, with surfacings of about 7-15 sec followed by dives lasting 5-7 min, and occasionally longer.







The sound repertoire of Risso’s dolphins includes a variety of pulsed and whistle-like sounds, used for both echolocation and communication. Echolocation clicks in free-ranging animals average 40 ?s with peak frequencies around 50 kHz and source levels of 202–222 dB re 1 ?Pa. Risso’s dolphins may be capable of hearing frequencies above 100 kHz.

Based on mitochondrial DNA analyses, Risso’s dolphins in the Mediterranean Sea are genetically differentiated from those in U.K. waters, with limited gene flow between the two areas and genetic analyses on 33 samples from the Mediterranean region (27 collected by Tethys from the Ligurian Sea) suggested relatively high diversity.

Risso’s dolphins have reduced dentition that is specialised to capture cephalopods, their main prey. They seem to feed predominantly during the night, probably to take advantage of the circadian vertical movements of their prey. Diet consists primarily of cephalopods, with a clear preference for mesopelagic squids.

In the Mediterranean Sea, similarly to other areas around the world, Risso’s dolphins prefer steep slopes and submarine canyons and are observed over mean bottom depths of 1280 m. Most observations come from the northwestern part of the basin. During the summer Risso’s dolphins are found regularly in continental slope waters of the Ligurian-Corso-Provençal basin, in the Alborán Sea and in the Gulf of Vera, where their range includes deep offshore waters. In the eastern Mediterranean, most information comes from the Greek seas.

In the western Ligurian Sea, where the continental shelf is particularly narrow, Risso’s dolphins have been described to be distributed along steep sections of the upper continental slope, a few km from the coast, and the Tethys researches observed this species at depths ranging between 370 and 1100 m. Their occurrence in this area during the summer months has not changed across years or months, but encounter rates were higher during certain shorter intervals, suggesting some sort of ‘transient’ use of the study area. This may indicate a strategy to exploit the temporary availability of food resources induced by zooplankton accumulation. Tethys researchers also found a correlation between Risso’s dolphin presence and stormy sea conditions in the preceding days, and speculated that moving along the slope area would be an efficient strategy to exploit prey attracted by the wind-driven accumulation of zooplankton. The frequency of occurrence of Risso’s dolphins in and near the Pelagos Sanctuary was similarly higher near the 1000 m isobaths, but sightings were also made far offshore and in deeper pelagic waters.

Risso’s dolphin groups tend to be small to moderate in size (up to around 100 individuals), averaging perhaps 30 animals. Risso's dolphins in the Ligurian Sea were characterised by low levels of relatedness, and kin-based affiliations were found only within groups of females.



A variety of human activities threaten cetaceans and many of those activities are expected to increase in the foreseeable future. In the Mediterranean, most bycatch of Risso’s dolphins is caused by pelagic gillnets (also called driftnets). These are large, floating nets that target primarily swordfish and tunas. Driftnets can be up to 50 km long and hang vertically 20-30 m from the surface. Because these nets indiscriminately kill protected species such as whales, dolphins, sharks, turtles, rays and sea birds, they have been banned in the European Union (EU) since 2002. The ongoing illegal use of driftnets in several parts of the Mediterranean, including areas where Risso’s dolphins occur, means that incidental mortality almost certainly continues at some scale. Risso’s dolphin mortality in driftnets deployed by the Italian fleet was significant and probably unsustainable in the early 1990s and it likely remains high, although there have been no focused investigations in recent years.

In the Mediterranean Sea there has been a great expansion of recreational boat traffic and shipping in recent decades and there is growing evidence that prolonged direct disturbance and noise caused by boat traffic can affect the behaviour and habitat use of cetaceans. However, the implications for disrupting behaviour and excluding Risso’s and other dolphins from important habitat have never been investigated. Noise from human activities including seismic surveys, marine construction and the use of military or other sonars is a cause for concern for Risso’s dolphins and other cetacean species. However, so far no deaths of Risso’s dolphins have been linked definitively to noise.



As a predominantly teuthophagous species that feeds in continental slope waters, the Risso’s dolphin may be less vulnerable to the prey depletion caused by overfishing in the Mediterranean Sea, since most fishing occurs in continental shelf waters and targets bony fishes. Few of the main cephalopod prey species of Risso’s dolphins are commercially important. Two factors, however, are of concern: 1) the possible expansion of Mediterranean deep-water fisheries and 2) the tendency of fisheries to target species lower and lower in the food web, as populations of higher trophic level species are depleted. These trends could lead to reductions in key prey populations or otherwise disrupt food webs in continental slope waters, where Risso’s dolphins mainly forage.



Toxic contamination can affect reproduction and health at the individual and population level. There is evidence to suggest a causal link between organochlorine exposure and reproductive, endocrine and immunological disorders in cetaceans. Levels of organochlorine compounds in Risso’s dolphins from the Mediterranean Sea have been described as ‘high’, and so have levels of trace metals. It is uncertain, however, whether recent and current levels of exposure to any of these contaminants are affecting Risso’s dolphins in the region. Cephalophods in the stomach of one Risso’s dolphin stranded in Corsica had mercury concentrations about 25 times higher than those of fish found in the stomachs of other delphinid species in the same sample, and 50 times higher than concentrations recommended for human consumption.

Some of the effects of global warming have become dramatically apparent in recent years and they have the potential to affect a range of biological processes and cause significant shifts in marine and other biota. Increased temperature has been observed in Mediterranean deep and surface waters and there is increasing evidence of biological responses to such warming. The effects of climate change on cetaceans, often mediated via changes in prey abundance and distribution, may include shifts in distribution and grouping behaviour but it is uncertain how they might apply to Risso’s dolphins.

Data on abundance, distribution, movements, population dynamics and trends, as well as better information on threats, are still needed to inform conservation efforts and ensure that Risso’s dolphins remain a functioning part of marine ecosystems in the Mediterranean region.

Text extracted from: Bearzi, G., Reevs, R., R., Remonato, E., Pierantonio, N., Airoldi, S. Risso’s dolphin Grampus griseus in the Mediterranean Sea. Mammal. Biol. (2010), doi:10.1016/j.mambio.2010.06.003