Scientific name: Grampus griseus
Distribution within the Mediterranean:
Conservation status in the Mediterranean:
Conservation status in the world:
Between 2,6 and 4 meters in adults and between 1,1 and 1,7 meters at birth
Can be confused with:
Individual identification characteristics:
The Risso’s dolphin (Grampus griseus) is a cetacean of the family Delphinidae, whose main characteristic is the bulbous morphology of its forehead, although it is not as bulbous as the melon of the long-finned pilot whale (Globicephala melas).
It is a medium-sized dolphin, with a robust and rather compact body, with a bulbous-shaped head and without a distinct peak. It’s mouth line slopes upward and backward. The melon has a squarish profile and shows a vertical crease on its front. It has a tall, slender and pointed dorsal fin located in the middle of the body. Pectoral fins are rather long, pointed and recurved. The tail stock is shallow and shows a big contrast between the anterior part of the body, which is thicker.
Adults show a general grey coloration, although it may vary, the oldest individuals being almost all-white, due to the scars caused by the interactions with other individuals of the species or due to marks made by their prey. This is the main characteristic of the species, which is probably the most heavily-scarred cetacean species. It also has a white anchor-shaped patch on the chest and another white patch around the genital area. The colour of young individuals varies from pale grey to brownish dark grey. They do usually not show as many scars as adults do.
The Risso’s dolphin has a unique dentition. It has between 2 and 7 pairs of rounded teeth in the front of the lower jaw. It usually doesn’t have teeth on the upper jaw.
It is a medium-sized dolphin. Adults measure between 2,6 to 4 metres and weigh between 300 and 500 kg. Newborn individuals measure between 1,1 and 1,7 metres. There appears to be little sexual dimorphism, females being slightly smaller than males. There is some evidence of geographic variation.
DISTRIBUTION AND MIGRATORY PATTERNS WITHIN THE MEDITERRANEAN SEA
The Risso’s dolphin is a widely-distributed species in tropical and temperate oceans, especially in waters warmer than 10° C. It has been associated with waters close to the continental slopes from Newfoundland, Norway, the Kamchatka Peninsula and the Gulf of Alaska in the north, to the Cape Horn (America), the Cape Agulhas (Africa) the south of Australia and the south of New Zealand (Jefferson et al., 2008; Perrin et al., 2009; Jefferson et al., 2013). Recent studies suggest that the species shows a circumglobal distribution, preferring waters associated to the continental slope (Jefferson et al., 2013).
Its distribution in the Mediterranean Sea is rather uncertain. Although it is an easy species to study (Jefferson et al., 2013), there is very little information about the species (Bearzi et al., 2011). Different authors believe that the species is more abundant in the western part of the Mediterranean basin, although the number of individuals in the population is not high. Due to the lack of knowledge on the species, its presence in the different ares is unpredictable (Gaspari, 2004; Reeves and Notarbartolo di Sciara, 2006 & Bearzi et al., 2011). In general, its presence is higher in the western part of the Mediterranean. The areas where the species shows a higher abundance, perhaps due to a greater research effort, are the Alboran and the Ligurian seas, a stable population exists in the latter. It has also been seen in the Tyrrhenian sea, in the Balearic sea and in the Greek seas (Notarbartolo di Sciara et al., 1993; Cañadas and Sagarminaga, 1994; Frantzis et al., 2003; Gaspari, 2004; Cañadas et al, 2005; Gannier, 2005; Gomez de Segura et al., 2006 & Bearzi et al., 2011).
The Mediterranean population would be genetically distinct from the north Atlantic population and it will have some internal structure (Gaspari, 2004 & Gaspari et al., 2007)
HABITAT AND FEEDING
The Risso’s dolphin inhabits waters between 400 and 1.000 metres deep, which are close to the continental slope and platform or associated to offshore archipelagos (Perrin et al., 2009 & Bearzi et al., 2011). There are different studies suggesting that the species would prefer the waters above the continental slope in the Mediterranean, especially those areas with steep slopes (Cañadas et al., 2002 & Azzellino et al., 2008). Although, the mean depth for this species varies between 700 and 1,280 metres with a peak at 1,000 metres, it has also been spotted in waters from 400 to 1,700 metres deep. (Notarbartolo di Sciara et al., 1993; Cañadas and Sagarminaga, 1994; Cañadas et al., 2002; Frantzis et al., 2003; Gannier, 2005 & Azzelino et al., 2008). The species shows small variations in its habitat depending on the area. (Bearzi et al., 2011).
The Risso’s dolphin feeds almost exclusively on cephalopods, mainly mesopelagic squids. It probably feeds at night, when its prey migrates to surface waters (Perrin et al., 2009; Soldevilla et al., 2010 & Bearzi et al., 2011). In fact, the reduction of the number of teeth comparing with other delphinid species is related to its diet, which is manly composed of cephalopods. Its main prey species belong to the families Histioteuthidae and Ommastrephidae, the flying squid (Todarodes sagittaus), Ancistroteuthis lichtensteinii, Histioteuthis reversa or H. bonnellii being the most important (Blanco et al., 2006 & Praca and Grannier, 2008). A study that analyzed 15 stranded Risso’s dolphins suggests that the diet of these individuals was composed by 125 different cephalopod species belonging to 13 different families (Blanco et al., 2006). Amongst them, the octopods were the most important one, representing 49,9% of the intake, above all Argonauta argo, which represented more than 45 % of the intake (Blanco et al., 2006). The same study suggested that squid species such as Todarodes sagittatus or Ommastrephes bartramii would be more important in terms of biomass due to its bigger size (Blanco et al., 2006). Most of the prey species of the Risso’s dolphin inhabit waters between 600 and 800 metres of depth, which overlap with the habitat of the species (Blanco et al., 2006 & Praca and Garnier, 2007).
REPRODUCTION AND LIFE HISTORY
There is not much information about the reproduction and the life history of the Risso’s dolphin. Births vary depending on the region. In Japanese waters there is a peak between summer and autumn, but in Californian waters, the peak takes place between autumn and winter (Jefferson et al., 2008). Gestation lasts 13 to 14 months and the calving interval lasts 2.4 years. Females achieve sexual maturity at the age of 8 to 10 years and males a little bit later, at the age of 10 to 12 (Perrin et al., 2009). Some estimates indicate that Risso’s dolphin could live up to 35 years.
ECOLOGY, BEHAVIOUR AND IDENTIFICATION
Although some groups of more than 4.000 animals have been seen in areas with high prey concentration, Risso’s dolphins generally form small groups of 10 to 100 individuals, the mean being around 30 animals (Jefferson et al., 2008; Perrin et al., 2009 & Bearzi et al., 2011). Groups tend to be rather small in the Mediterranean. They are usually composed of 1 to 130 animals, the mean being between 4 and 37 individuals (Notarbartolo di Sciara et al., 1993; Cañadas and Sagarminaga, 1994; Cañadas et al., 2002; Gannier, 2005; Gomez de Segura et al., 2006 & Azzellino et al., 2008).
Risso’s dolphin associates with other cetacean species such as the pacific white-sided dolphin (Lagenorhynchus obliquidens), the dusky dolphin (L. obscurus), the common bottlenose dolphin (Tursiops truncatus), the common dolphin (Delphinus spp.) different species of the genus Stenella and even the Gray whale (Eschrichtius robustus). There are some interactions with the common bottlenose dolphin and the striped dolphin (Stenella coeruleoalba) documented in the Mediterranean (Bearzi, 2011).
The Risso’s dolphin is easy to distinguish from other cetacean species at sea, thanks to the huge amount of scars that some individuals, above all adults, show on the dorsal parts of their bodies and their tall and prominent dorsal fin. It can be confused with some cetacean species such as the common bottlenose dolphin, but Risso’s dolphin lacks a distinct beak and its head has a bulbous-shaped melon. These characteristics, together with the white scars they accumulate during their lives are helpful in order to distinguish the species (Shirihai and Jarret, 2006).
The Risso’s dolphin is an active species at the surface, where immature individuals may perform breaches. Spyhopping is a common behaviour. They generally swim slowly at the surface and ignore vessels so porpoising and bowriding behaviours are rarely seen. They can be seen side by side in a linear disposition when hunting. The swimming pattern is also characteristic for the species. The Risso’s dolphin surfaces out of the water showing a 45° angle to the water surface. It usually doesn’t show its fluke out of the water before a dive. Dive usually last for 1 to 2 minutes, although they can be up to 30 minutes long (Carwardine, 2004 & Shirihai and Jarret, 2006).
INDIVIDUAL IDENTIFICATION CHARACTERISTICS
There are many cetacean species that show specific marks on their bodies that allow researchers to identify the different individuals. These marks should be stable over time in order to allow recognition of an individual every time a photograph is taken. Risso’s dolphins show different traits that allow for their individual identification (Würsig and Jefferson, 1990 & Hartman et al., 2008).
Dorsal fin: The shape of the dorsal fin and the notches on its posterior margin can be used as photo-identification traits for the species. In addition, the scars on the sides of the dorsal fin can also be used as individual identification marks. For that reason photographs of both sides should be included in a photo-identification catalog.
Coloration patterns: Risso’s dolphins show individual-specific scarification patterns that alter their coloration. At birth individuals are rather dark, but their coloration becomes lighter with age, especially in adult animals, the older ones being almost totally white. This scarification is caused by the teeth of other Risso’s dolphins or by the beaks of cephalopods, the main prey of the species.
Scars: Risso’s dolphins acquire scars during their lives. Their distribution, size and number can be used in order to identify specific individuals, but knowing that their number will increase with the age of the individual and older scars may be covered or altered by new ones.
CONSERVATION STATUS AND MAIN THREATS
There are no estimates of the world’s population of the Risso’s dolphin, but it’s abundance has been estimated in some regions. The biggest populations are found in the Pacific ocean, with around 83.300 in waters off Japan (Miyashita, 1993) and around 175.000 in waters of the Tropical East Pacific (Wade and Gerrodette, 1993). Estimates off the western coast of the United states vary from 4.000 to 16.000 individuals depending on the season, while the population of the eastern coast would be composed of 20.000 individuals (IUCN) and in the northern part of the Gulf of Mexico of 2.700 animals (Jefferson et al., 2008). There would be around 2.372 individuals living in waters surrounding Hawaii (Barlow et al., 2006). The estimates of the population of Sri Lanka vary between 5.500 and 13.000 animals (Jefferson et al., 2008). Although the population trend is unknown, the IUCN (International Union for Conservation of Nature) listed the species under the status of least concern. In the Mediterranean Sea, there are only abundance estimates of the population living off the east coast of the Iberian Peninsula, which consists of 493 individuals (Gomez de Segura et al., 2006). Due to the lack of information about the abundance of the species in the Mediterranean Sea, the IUCN listed the species under the status of data deficient. There are different threats affecting the Mediterranean population:
- Noise pollution: Man-made underwater noise may have negative effects on cetaceans such as injuries or disorientation that may cause the stranding or even the death of the animals (Fernández et al., 2005 & Cox et al., 2006). There are different sources of noise pollution in the Mediterranean sea. The most important are coastal construction works, seismic surveys and intense maritime traffic, which has increased greatly in the last decades (Bearzi et al., 2011 & Notarbartolo di Sciara et al., 2012). Although the exact effects of noise pollution on the species are not known yet, scientists believe that underwater noise could alter the behavior of the Risso’s dolphin and even cause in vivo gas bubble formation (Bearzi et al., 2011).
- Bycatch: Driftnets are by far the most dangerous fishing method for cetaceans, causing the death of many individuals every year. It is a non-selective fishing method consisting of a very long floating net which entraps all kinds of animal that try to cross it. Cetaceans trapped in these nets cannot reach the surface to breath and die there (Bearzi, 2002; E. J. F., 2007 & Cornax and Pardo, 2009). Risso’s dolphins can get caught in these traps (Di Natale and Notarbartolo di Sciara, 1994; Bearzi, 2002; Cornax and Pardo, 2009 & Bearzi et al., 2011). In fact, Risso’s dolphin mortality caused by driftnets has been significant in the past and it has been considered unsustainable in certain regions (Bearzi et al., 2011).
Another fishing method causing a significant impact to Risso’s dolphin populations are the longline fisheries. There are two species of cetaceans which suffer the effects of this fishing method: the pilot whale (Globicephala sp.) and the Risso’s dolphin (Garrison, 2007 & Macías López et al., 2012). Of 57 cetaceans captured by the longline fisheries off the Mediterranean coast of the Iberian Peninsula, 33 (58%) were Risso’s dolphin. The most dangerous longline type for the Risso’s dolphin in the Spanish Mediterranean is the Japanese longline, which consists of a monofilament longline with hooks at 4 different depths and which remains floating at sea for 24 hours. Most of the cetaceans caught by this fishing methods will survive as long as they can reach the surface to breath. However, injuries caused by the lines or the hooks may be a serious issue of concern for the conservation of the species (Macías López et al., 2012). There are some factors, intrinsic to Japanese longlines, that could explain the great impact of this gear on Risso’s dolphin in the Spanish Mediterranean: (1) The length of the line: longer mainlines could represent a more attractive food source for the cetaceans; (2) The time that the line remains in the water: the longer it remains in the water the higher is the probability of a cetacean being caught; (3) bait: squid, which are the main prey of Risso’s dolphin and pilot whales, are mainly used as bait; (4) working depths of hooks (Macías López et al., 2012).
- Water pollution: The Mediterranean sea is one of the most populated regions in the world where very important industrial and agricultural activities have been developed. In addition, the Mediterranean is an almost totally enclosed sea with a very low recycling rate of its water (the Strait of Gibraltar is the only main source of water replenishment). For all these reasons, the Mediterranean Sea has become one of the most polluted seas in the world (Aguilar, 2000 & Fossi and Marsili 2003). Pollution mainly affects the top predators, which tend to accumulate pollutants in their body, due to a process known as bioaccumulation. The cetaceans of the Mediterranean show the highest concentrations of pollutants (Aguilar, 2000). The effects of these pollutants on cetaceans may include immunosuppression, cancer, skin lesions, secondary infections, reduced reproductive success, etc. (Reijnders, 1998 & Fossi and Marsili 2003). Risso’s dolphin shows a great concentrations of organochlorine compounds and traces of heavy metals in its body (Bearzi et al., 2011).
There are many international conventions protecting the Risso’s dolphin, such as the appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), the appendixes I and II of the Convention on Migratory Species (CMS) or the Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and contiguous Atlantic Area (ACCOBAMS). The Pelagos Sanctuary has been an important tool for the conservation of the species considering that it is located in one of the regions where the species is more abundant. In addition, the creation of new Community Interest Areas in the Alboran Sea, another important area for the distribution of the species, could represent a significant step in the conservation of the species.
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