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Grampus griseus (G.
Cuvier, 1812)
English: Risso's dolphin
German: Rundkopfdelphin
Spanish: Delfín de Risso
French: Dauphin de Risso
Family Delphinidae
Grampus griseus © Würtz-Artescienza
(see "links")
1. Description
Risso's dolphin is the fifth largest of the delphinids. Adults
of both sexes reach 4 m in length. Its anterior body is extremely
robust, tapering to a relatively narrow tail stock and its dorsal
fin is one of the tallest in proportion to body length of any cetacean,
exceeded only by that of the adult male killer whale (Orcinus
orca). The bulbous head has a distinct vertical crease or cleft
along the anterior surface of the melon. Colour patterns change
dramatically with age. Infants are dorsally grey to brown, then
darken to nearly black and lighten while maturing (the dorsal fin
remaining dark). In ageing animals, the majority of the dorsal and
lateral surfaces become covered with distinctive linear scars. Older
animals can appear completely white on the dorsal surface (Baird,
2002). Risso's dolphins are often confused with killer whales, due
to the size of their dorsal fin (Baird, 2009). Adult size ranges
up to 3,8 m long and body mass may reach 500 kg (Jefferson et al.
2008). 
2. Distribution
This is a widely distributed species, inhabiting deep oceanic and
continental slope waters 400-1,000 m deep (Baird, 2002) from the
tropics through the temperate regions in both hemispheres (Jefferson
et al. 1993). Sighting records indicate this species occurs roughly
between 60°N and 60°S latitudes, where surface water temperature
are above 10 °C (Kruse et al. 1999). It ranges north to Newfoundland,
the Shetland Islands, the North Sea (Weir et al. 2001), the Mediterranean
Sea, Ostrov Iturup in the Ostrova Kuril'skiye, Koman-dorskiye Ostrova,
56°, 146° in the northern Gulf of Alaska, and Stuart Island
(50°N) in British Columbia; and south down eastern South America
as far as Cabo de Hornos in Chile, to Cape Province in South Africa,
Geographe Bay (33°S) in Western Australia, Sydney in New South
Wales, North Island in New Zealand, and Valparaiso in Chile (Rice,
1998).
World-wide distribution of
Grampus griseus (Taylor et al. 2008; © IUCN): tropical
and
warm temperate waters in both hemispheres (Click
here for large map).
3. Population size
There are examples of long term changes in abundance and distribution,
e.g. in the Southern California Bight (Kruse et al. 1999 and refs.
therein). In the late 1950s, Risso's dolphins were rarely encountered
in this area, and between 1975 and 1978 they were still considered
to be a minor constituent of the cetacean fauna of the Bight, representing
only 3% of the cetaceans observed. Since the El Niño of 1982/83,
however, numbers of Risso's dolphins have increased, especially
around Santa Catalina Island where they are now considered to be
common. Forney and Barlow (1998) observed that the abundance of
Risso's dolphins off California was almost an order of magnitude
higher in winter (n = 32,376) than in summer (n = 3,980). The 2001-2005
geometric mean abundance estimate for California, Oregon and Washington
waters based on the two most recent ship surveys, however, is 11,621
(CV = 0.17) Risso's dolphins (Barlow and Forney 2007, Forney, 2007).
Currently, there is no evidence of a trend in abundance for this
stock. In the EEZ waters around Hawaii, Barlow (2006) estimated
abundance at 2,375 (CV=0,65). An earlier estimate for the entire
eastern Tropical Pacific was 175,000 (Wade and Gerrodette, 1993).
In the eastern Sulu Sea, Dolar (1999) estimated the population
size at 950 individuals. Population estimates off Sri Lanka ranged
from 5,500 to 13,000 animals (Kruse et al. 1999 and refs. therein).
In three areas of concentrated occurrence off Japan, abundance is
estimated at 83,300 (Taylor et al. 2008).
In the western North Atlantic, Waring et al. (2007) estimated the
stock from 2004 data at 20,500 (CV = 0,59). In the northern Gulf
of Mexico the estimate of abundance for Risso's dolphins in oceanic
waters, pooled from 1996 to 2001, is 2,169 (CV=0.32) (Mullin and
Fulling 2004).
In the Pelagos Sancuary in the North-Western Mediterranean Sea,
Risso's dolphin was not very abundant, accounting for only 4.3%
of all cetacean encounters (Moulins et al. 2008). In the central
Spanish Mediterranean, Gomez de Segura et al. (2006) determined
a surface- estimated density of 0.015 dolphins / km² (95% CI
= 0.005-0.046) and a mean abundance of 493 (95% CI = 162-1,498).
4. Biology and Behaviour
Habitat: Risso's dolphins are pelagic, mostly occurring
seaward of the continental slope. They frequent subsurface sea-mounts
and escarpments where they are thought to feed on vertically migrant
and mesopelagic cephalopods. In Monterey Bay, California, Risso's
dolphins are concentrated over areas with steep bottom topography.
Currents and upwelling causing local increases in marine productivity
may enhance feeding opportunities, resulting in the patchy distribution
and local abundance of this species worldwide (Kruse et al. 1999
and refs. therein). Davis et al. (1998), Baumgartner (1997) and
Baumgartner et al. (2001) reported that in the Gulf of Mexico, Risso's
dolphins were mostly found over the steeper sections of the upper
continental slope (200-1000 m) concentrating along the upper continental
slope, which may reflect squid distribution.
In the western Ligurian Sea, Mediterranean, Azzelino et al. (2008)
found that Risso's dolphins were strongly associated with depth
and slope gradient characteristics of the shelf-edge and the upper
and lower slope. Their data showed clear and not overlapping habitat
preferences for Risso's dolphin and Cuvier's beaked whale. A temporal
segregation in the use of the slope area was also observed for sperm
whales and Risso's dolphins. In the Spanish Mediterranean, Risso's
dolphins preferred waters more than 1500 m deep (Gomez de Segura
et al. 2008) and depths of around 1000 m were hot spots in the Pelagos
Sanctuary (north-western Mediterranean Sea; Moulins et al. 2008).
Blanco et al. (2003) assumed that due to distribution records of
prey in the western Mediterranean, Risso's dolphins more frequently
inhabit the outer continental slope and shelf break region. The
preference for this habitat may be explained by the high marine
productivity with enhanced feeding opportunities and this agrees
with results from other countries and sightings in the area.
Behaviour: G. griseus are often seen surfacing slowly,
although they can be energetic, sometimes breaching or porpoising,
and occasionally bowriding (Jefferson et al. 1993).
Reproduction: In the North Atlantic and western Pacific,
there appears to be a summer calving peak (Jefferson et al. 1993)
and a winter calving peak in the eastern Pacific (Baird 2009).
Schooling: Herds tend to be small to moderate in size (1-100
individuals), averaging 30 animals, but groups of up to 4,000 have
been reported, presumably in response to abundant food resources.
Limited data on subgroup composition obtained from mass strandings
and observations of captive animals suggest that cohesive subgroups
may be composed of same-sex and similar-age individuals. Risso's
dolphins commonly associate with other species of cetaceans such
as gray whales, Pacific white-sided dolphins, northern right whale
dolphins, Dall's porpoises, sperm whales, short-finned pilot whales,
bottlenose dolphins, common dolphins, striped dolphins, spotted
dolphins, false killer whales, and pygmy killer whales (Kruse et
al. 1999 and refs. therein).
Hartman et al. (2008) found that individuals at the Azores, central
Atlantic, formed stable, long-term bonds organised in pairs or in
clusters of 3-12 individuals. Social structure showed strong associations
between adult males and between adult females. Males were organised
in stable, long-term associations of varying size that occurred
throughout the complete range of behavioural states observed. For
females, associations could be of similar strength, but the time
scale could vary depending on the presence of nursing calves. As
subadults, associations also occurred (pair formation), but were
less stable than those observed for adults.
Frantzis and Herzing (2002) observed in the Gulf of Corinth, an
almost-enclosed sea in Greece in the eastern Mediterranean, that
Risso's dolphins associated with striped and common dolphins. However,
in all mixed-species sightings, Risso's dolphins and common dolphins
were always, and by far the minority species, present. To date,
no single-species groups of Risso's or short-beaked common dolphins
have been observed in the Gulf of Corinth. Interspecific rake marks
on the Risso's dolphinsand behaviours observed through video analysis
indicated potentially complex and regular interspecific interactions.
Food:Kruse et al. (1999) reported that Risso's dolphins
prey on a mix of neritic, oceanic, and occasionally bottom dwelling
cephalopods. From daily activity patterns observed off Santa Catalina
Island, California, Risso's dolphins are presumably mainly nocturnal
feeders. Santos et al. (2001) found Octopus vulgaris in the
stomachs of animals stranded in NW Spain.
Blanco et al. (2003) analysed stomach contents of 13 Risso's dolphins
stranded on the western Mediterranean coast between 1987 and 2002
and found only cephalopod remains: 25 species belonging to 13 families
were found in the samples, mostly Argonautidae, Ommmastrephidae,
Histioteuthidae and Onychoteuthidae. Despite the numerical importance
and high frequency of small pelagic octopods, mainly Argonauta
argo, Blanco et al. (2003) assumed that greater nutritional
content came from of ommastrephids, mainly O. bartrami and
T .sagittatus because of the larger size of some specimens.
The prey were mainly oceanic and pelagic species with a muscular
mantle.
In the eastern Mediterranean Sea off the Turkish coast, Risso's
dolphins also only feed on cephalopods; Histioteuthis reversa was
the most common species (60.9%of all beaks found), and all the other
species comprised less than 10%. Most of the prey species are oceanic
cephalopods, with wide vertical distribution and diurnal movement.
Many of the cephalopods identified in the diet of these dolphins
are bioluminescent, suggesting that these dolphins use bioluminescence
as a target when feeding on cephalopods (Oeztuerk et al. 2007).
Philips et al. (2003) monitored a trained Risso's dolphin and established
that the species echolocates, and that, aside from slightly lower
amplitudes and frequencies, the clicks emitted are similar to those
emitted by other echolocating odontocetes.
5. Migration
Although Grampus is present year-round in most of its range,
there may be seasonal onshore-offshore movements in some areas (Carwardine,
1995). In more constant environments, e.g. the Azores, Hartman et
al. (2008) found strong site fidelity for at least part of the population.
In seasonally more variable areas, G. griseus seems to show
annual changes in abundance, being e.g. more abundant around northern
Scotland in the summer and in the Mediterranean in the winter (e.g.
Gannier, 1998; Evans, 1998).
Similar seasonal shifts in abundance have been reported from the
Northwest Atlantic, British coastal waters, and the south-east coast
of South Africa. Summer "reproductive migrations" (characterised
by schools of 20-30 animals with empty stomachs and females carrying
large foetuses), and winter "feeding migrations" (characterised
by schools of nearly 200 animals with full stomachs and females
carrying smaller foetuses) have been observed off Japan. Because
some authors maintain that the species is equally abundant in some
areas throughout the year, systematic studies of the distribution
and abundance of Risso's dolphins in localised areas are required
to resolve this conflict (Kruse et al. 1999 and refs. therein).
Water temperature appears to be a factor that affects the distribution
of Risso's dolphins, the acceptable temperature range for the species
being 7.5°C-35°C (Kruse et al. 1999 and refs. therein).
In California, increasing numbers of Risso's dolphin and a shoreward
shift in their distribution have been observed during periods of
warm water, suggesting that seasonal patterns of distribution and
abundance are associated with changing sea surface temperatures
(Kruse et al. 1999).
However, Forney and Barlow (1998) found no significant seasonal
difference in distribution of Risso's dolphins in Californian waters.
In both summer and winter, they were seen most frequently in the
Southern California Bight and were also observed off central California.
Seasonal movement of Risso's dolphins from California into Oregon
and Washington waters in spring and summer has been suggested, and
there is an indication that Risso's dolphins were also common in
offshore waters of northern California. The degree of movement into
Mexican waters is unknown (Forney and Barlow, 1998).
6. Threats
Direct catch: In Sri Lanka, Risso's dolphins were apparently
the second most commonly taken cetacean in fisheries, providing
fish and meat for human consumption and fish bait; stocks there
may be adversely affected (Jefferson et al. 1993). An estimated
1,300 Risso's dolphins may have been landed annually as a result
of this fishery and population estimates in these waters range only
from 5,500 to 13,000 animals (Kruse et al. 1999). In Japan, Risso's
dolphins are taken periodically for food and fertiliser in set nets
and as a limited catch in the small-type whaling industry (Kruse
et al. 1999 and refs. therein). However, Endo et al. (2005) surveyed
the total mercury (T-Hg) and methyl mercury (M-Hg) levels in red
meat products from small cetacean species, including Risso's dolphins,
sold for human consumption in markets throughout Japan. Due to high
levels in all species, the consumption of red meat from small cetaceans
could pose a health problem for not only pregnant women but also
for the general population.
Incidental catch: Although they have never been the basis
of a large-scale fishery, Risso's dolphins have been taken periodically
as by-catches in other fisheries throughout the world. There are
reports from the North Atlantic, the southern Caribbean, the Azores,
Peru, and the Solomon Islands. They are also a rare by-catch in
the US tuna purse seine industry, and are taken occasionally in
coastal gill net and squid seining industries off the US coast,
or shot by aggravated fishermen (Kruse et al. 1999 and refs. therein).
Baker et al. (2006) reported on identifying Risso's dolphins via
molecular monitoring of 'whalemeat' markets in the Republic of (South)
Korea based on nine systematic surveys from February 2003 to February
2005. As Korea has no programme of commercial or scientific whaling
and there is a closure on the hunting of dolphins and porpoises,
the only legal source of these products was assumed to be 'bycatch'.
The U.S, East Coast pelagic longline fishery has a history of interactions
with Risso's dolphin, one of the primary species that interact with
longline gear. Waring et al. (2007) found that the annual average
combined mortality and serious injury for 2000-2004 was 46 Risso's
dolphins (CV =0.37). They concluded that the total U.S. fishery
mortality and serious injury for this stock was not less than 10%
of the calculated PBR and, therefore, could not be considered to
be insignificant and approaching a zero mortality and serious injury
rate. Garrison (2007) found that incidental bycatch of marine mammals
is likely associated with depredation of the commercial catch and
is increased by the overlap between marine mammal and target species
habitats. Altering gear characteristics and fishery practices may
mitigate incidental bycatch and reduce economic losses due to depredation
Culling: Off Japan, Risso's dolphins were killed in the
drive fishery (oikomi) in response to competition with commercial
fisheries (Kruse et al. 1999 and refs. therein).
Pollution: Accumulation of butyltin compounds, organochloride
and DDT levels have been analysed in tissue samples from various
specimens (Kruse et al. 1999 and refs. therein). Mercury levels
have been reported by Frodello et al. (2000). Increasing levels
of plastics and other refuse at sea may pose a threat to wild populations:
Necropsies of specimens from Japan revealed that they had eaten
foreign materials such as plastic bags, soda cans, and pieces of
rope, which may have been fatal (Kruse et al.1999 and refs. therein).
Chou and Li (2004) analysed blubber samples of cetaceans from Taiwan
coastal waters for polychlorinated biphenyls (PCB). Total concentrations
of 19 PCB congeners (SIGMAPCBs) were 0.23 µg/g lipid weight
of Risso's dolphin with pentachlorobiphenyls, hexachlorobiphenyls
and heptachlorobiphenyls the predominant PCB congener species. Stranded
cetaceans had significantly higher PCB levels than by-caught cetaceans
because of their higher lipid consumption during starvation or illness.
However, by comparison cetaceans from Taiwan waters had relatively
lower PCB concentrations than those from high-latitude areas.
Chen et al. (2002) analysed total mercury (Hg), organic-Hg and selenium
bioaccumulations in small cetaceans distributed in Taiwanese waters
of the Taiwan Strait and the southwestern Pacific. Volcanic activities
are possibly the major source of mercury to the environments. Muscle
samples of Risso's dolphins had the highest mean concentrations
of Se (1.77 mg/kg +-1.29), while mercury concentrations were low
compared to the other cetaceans.
In a specimen stranded on the Mediterranean coast of Israel, high
concentrations of trace metals (Hg, Cd, Zn, Fe and Se) were found
in the various tissues analysed, while Cu and Mn concentrations
were naturally low. Plastic bags found in its stomach contributed
to the dolphin's poor physical condition (Shoham-Frider et a. 2002).
Noise pollution: In early 2004 and in 2005, several unusual
stranding events including Risso's dolphins occurred in Taiwan during
a period when large-scale naval exercises were conducted in and
on nearby waters. The findings of the gross post mortem examination
of the only specimens that were available for study suggested that
nearby naval exercises may have contributed to or caused the death
of at least one cetacean in this region and that species other than
beaked whales may also be susceptible to such activities. With an
increasing number of military exercises in this region, more attention
to the impacts of such activities on cetaceans is needed (Wang and
Yang, 2006).
7. Remarks
Range states (Taylor et al., 2008):
Algeria; American Samoa; Anguilla; Antigua and Barbuda; Argentina;
Aruba; Australia; Bahamas; Bangladesh; Barbados; Belgium; Belize;
Benin; Bermuda; Brazil; British Indian Ocean Territory; Brunei Darussalam;
Cambodia; Cameroon; Canada; Cape Verde; Cayman Islands; Chile; China;
Cocos (Keeling) Islands; Colombia; Comoros; Congo; Congo, The Democratic
Republic of the; Cook Islands; Costa Rica; Côte d'Ivoire;
Croatia; Cuba; Denmark; Djibouti; Dominica; Dominican Republic;
Ecuador; Egypt; El Salvador; Equatorial Guinea; Estonia; Fiji; France;
French Guiana; French Polynesia; Gabon; Gambia; Germany; Ghana;
Greece; Greenland; Grenada; Guadeloupe; Guam; Guatemala; Guernsey;
Guinea; Guinea-Bissau; Guyana; Haiti; Honduras; Hong Kong; India;
Indonesia; Iran, Islamic Republic of; Iraq; Ireland; Isle of Man;
Israel; Italy; Jamaica; Japan; Jersey; Jordan; Kenya; Kiribati;
Kuwait; Lebanon; Liberia; Madagascar; Malaysia; Maldives; Malta;
Marshall Islands; Martinique; Mauritania; Mayotte; Mexico; Monaco;
Morocco; Mozambique; Myanmar; Namibia; Netherlands; Netherlands
Antilles; New Caledonia; New Zealand; Nicaragua; Nigeria; Niue;
Northern Mariana Islands; Norway; Oman; Pakistan; Palau; Panama;
Papua New Guinea; Peru; Philippines; Pitcairn; Portugal; Puerto
Rico; Qatar; Russian Federation; Saint Kitts and Nevis; Saint Lucia;
Saint Pierre and Miquelon; Saint Vincent and the Grenadines; Samoa;
Sao Tomé and Principe; Saudi Arabia; Senegal; Serbia; Sierra
Leone; Singapore; Slovenia; Solomon Islands; Somalia; South Africa;
Spain; Sri Lanka; Sudan; Suriname; Sweden; Syrian Arab Republic;
Tanzania, United Republic of; Thailand; Timor-Leste; Togo; Tonga;
Trinidad and Tobago; Turkey; Turks and Caicos Islands; United Arab
Emirates; United Kingdom; United States of America; United States
Minor Outlying Islands; Uruguay; Vanuatu; Venezuela; Viet Nam; Virgin
Islands, British; Virgin Islands, U.S.; Wallis and Futuna; Western
Sahara; Yemen.
This is a circumglobal species which migrates between summering
and wintering grounds. Off California, where these movements are
best known, they may involve US and Mexican waters. In other areas,
the species is insufficiently known with respect to basic biological
parameters. Abundance, by-catch and behavioural data at sea are
needed in order to enable protection of the natural habitat of the
species. For South American stocks, see further recommendations
in the Hucke-Gaete (2000) report (see Appendix
1). General recommendations on Southeast Asian stocks can be
found in Perrin et al. (1996; see Appendix
2).
The IUCN lists G. griseus as "Least Concern" (Taylor
et al. 2008). The Mediterranean, North and Baltic Sea populations
are included in Appendix II of CMS. However, as described above,
populations off the East and West coasts of North America (Range
states US, Mexico, Canada) also seem to migrate along the coast,
and this is also the case for animals off SE South Africa. It is
therefore suggested not to restrict the inclusion into CMS App.
II to the populations mentioned, but to include G. griseus
as a species.
The species is listed in Appendix II of CITES.
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© Maps by IUCN.
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