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Steno bredanensis (G.
Cuvier in Lesson, 1828)
English: Rough-toothed dolphin
German: Rauhzahndelphin
Spanish: Delfin de dientes rugosas
French: Sténo
Family Delphinidae
Steno bredanensis^© Wurtz-Artescienza (see
links)
1. Description
The species derives its common name from the vertical ridges in
the teeth, which give them a roughened appearance. S. bredanensis
is the only long-beaked dolphin with a smoothly sloping melon that
gently blends into the upper beak. The body is not very slender
and the anterior portion may be stocky. The large flippers are set
farther back on the body than in most other delphinds. The dorsal
fin is tall and only slightly recurved. Some large males may have
a hump posterior to the anus resembling a keel. Rough-toothed dolphins
are countershaded with white bellies and black to dark grey backs.
The sides are medium grey and separated from the a cape on the back.
Size reaches 255 cm in females and 280 cm in males, and body mass
may reach 155 kg (Jefferson, 2009).
2. Distribution
S. bredanensis is distributed in tropical and warm temperate
waters around the world. It ranges north to the Gulf of Mexico,
Virginia, Brittany on the French coast, Mediterranean Sea, Gulf
of Aden, Arabian Sea, Bay of Bengal, East China Sea, Northern Japan,
Hawaiian Islands, and California (Hammond et al. 2008). Its southern
range extends to Rio Grande do Sul in Brazil, about 32°S in
the eastern Atlantic, Natal, Timor Sea, Coral Sea, New Zealand,
and Botija (24°30'S) in northern Chile (Rice, 1998). Monteiro
et al. (2000) and Ott and Danilewicz (1996) confirm a few sightings
and by-catches of S. bredanensis off Brazil.
Distribution of Steno bredanensis: deep
tropical, subtropical and warm temperate
waters around the word (Hammond et al. 2008; © IUCN; enlarge
map).
Recently, the species has been observed in the US
Exclusive Economic Zone (EEZ) waters of American Samoa (Johnston
et al. 2008), off the Marquesas and Windward Islands Islands in
French Polynesia as well as around Tahiti and Moorea. (Gannier 2002;
Gannier and West, 2005), off the Island of St Helena in the tropical
south-eastern Atlantic (Macleod and Bennet 2007), and off northern
Angola (Weir 2007).
Ocasionally vagrant north to Oregon and Washington in the eastern
Pacific (Ferrero et al. 1994; Rice, 1998), but see abundance below;
visitor to the Mediterranean Sea (Hammond et al. 2008).
The distribution of S. bredanensis remains poorly known,
and distributional maps are based on relatively few sightings spread
over a wide area. The species does not appear to be particularly
numerous anywhere, although researchers have worked intensively
mostly in the eastern tropical Pacific and may simply have missed
areas of high abundance elsewhere (Carwardine 1995).
3. Population size
There are few recent abundance estimates. S. bredanensis
is one of the most common delphinids of the U.S. Exclusive Economic
Zone (EEZ) around the Hawaiian Islands where its abundance was estimated
at 8,709 (CV = 0.45) from a 2002 survey (Barlow, 2006). The best
available abundance estimate for the species in the northern Gulf
of Mexico is the combined estimate for both the outer continental
shelf and oceanic waters obtained in 2003-2004 which is 2,653 (CV=0.42)
(Waring et al. 2008). More to the north, however, abundance is unknown,
as the species was no longer seen during surveys conducted off the
eastern U.S. and Canadian Atlantic coast after 1999 (Waring et al.
2008).
Previous estimates indicate that 146,000 (CV = 0,32) rough-toothed
dolphins inhabit the eastern tropical Pacific (Wade and Gerrodette,
1993). During a number of survey cruises conducted in the region
over a period of approximately 20 years, 176 of 4,006 schools of
small cetaceans seen were of rough-toothed dolphins; the species
was encountered less often than Stenella attenuata, S.longirostris,
S.coeruleoalba, Delphinus delphis, Globicephala macrorhynchus, Grampus
griseus, and Tursiops truncatus but more often than Peponocephala
electra, Orcinus orca, Pseudorca crassidens, Feresa attenuata, Kogia
spp. and beaked whales. However, this ranking could be affected
by relative sightability as well as by abundance (Miyazaki and Perrin,
1994 and refs. therein).
4. Biology and Behaviour
Habitat: Most often S. bredanensis is found in deep
water far offshore, usually beyond the continental shelf (Maigret,
1995). Off the Canary Island of La Gomera, S. bredanensis was found
in waters of 506 m mean depth, but mean distance from shore was
only 4.4 km (Ritter, 2002). In the Pacific, around the main Hawaiian
Islands, sighting rates were highest in waters of >1,500 m and
associated with upwelling (Baird et al. 2008). Near the Windward
Islands of French Polynesia rough-toothed dolphins were usually
sighted 1.8 to 5.5 km from the barrier reef, in water depths between
1,000 and 2,000 m (Gannier and West, 2005). Ritter (2002) reported
that the year-round abundance off La Gomera, Canary Islands, indicates
that this species might endure temperatures well below 25°C.
Behaviour: S. bredanensis is a fast swimmer, sometimes
porpoising with low, arc-shaped leaps. It may swim rapidly just
under the surface, with dorsal fin and a small part of the back
clearly visible. Sometimes it bow-rides, especially in front of
fast-moving vessels, though not as readily as many other tropical
dolphins. Steno may associate with bottlenose dolphins and
pilot whales and, less frequently, with spinner dolphins and spotted
dolphins. They also may occur with shoals of yellowfin tuna, Thunnus
albacares (Carwardine, 1995; Miyazaki and Perrin, 1994).
Off La Gomera, Canary Islands, behavioural data collected for 26
sightings showed that the reaction of the animals to the observation
vessel varied from no response to interaction. Predominant types
of boat-related behaviours were approaching (46%), bowriding (21%),
and scouting (20%) (Ritter, 2002). This is confirmed by observations
off the coast of Utila, Honduras, where dolphins sometimes expressed
interest in the research vessel and other boats by approaching and
on separate occasions examining a hydrophone and slow moving propeller
visually and by echolocation (Kuczaj and Yeater, 2007).
Schooling. Schools of up to 50 animals have been reported
in the eastern tropical Pacific and central Atlantic (Ritter, 2002),
but smaller groups of 10-20 seem more usual (e.g. Gannier and West,
2005). Five schools in Japanese waters contained from 23 to 53 animals.
However, these small schools may be parts of larger, dispersed aggregations;
one such aggregation of "schools" observed from the air
off Hawaii contained an estimated 300 dolphins, and another seen
in the Mediterranean contained approximately 160 animals in eight
groups of about 20 each (Miyazaki and Perrin, 1994 and references
therein). Near Utila, Honduras, behavioural observations suggest
synchronous behaviours and 'tight' groupings during travelling,
tactile contact as an important aspect of social interactions, and
cooperative behaviour during play (Kuczaj and Yeater, 2007). In
the eastern tropical Pacific, they tend to associate with other
cetaceans (especially pilot whales and Fraser's dolphins) (Miyazaki
and Perrin 1994).
Reproduction: Males reach sexual maturity at about 14 years
and females at about 10 years. Animals may reach 32-36 or more years
of age (Miyazaki and Perrin, 1994).
Food. The diet in the wild includes fish and squid. Cephalopods
reported from stomach contents include Teuthowenia sp. and
Tremoctopus violaceus. The alga Sargassum filipendula
was found in the stomachs of several stranded animals; the significance
of this is unknown. The stomachs of animals stranded in Hawaii contained
the atherinid Pranesus insularum, the scomberesocid Cololabis
adocetus, the belonid Tylosurus crocodilus, all nearshore
species, and squid. Other, larger fish may be taken in deeper water.
Co-operative food gathering has been reported (Miyazaki and Perrin,
1994 and refs. therein). Pitman and Stinchcomb, 2002) reported on
four separate observations of rough-toothed dolphins apparently
preying on adult-sized (>1 m) mahimahi (Coryphaena hippurus)
in the eastern Pacific. Maximum reported dive depth was 70m, but
they may dive deeper. Maximum dive duration was 15 min (Jefferson,
2009).
5. Migration
The species is difficult to observe at sea; schools are extremely
difficult to follow, staying submerged for as long as 15 min (Miyazaki
and Perrin, 1994). Around Hawaii frequent within- and between-year
resightings indicate a small local population with high site fidelity.
Resighting rates were lower off Kaua'i-Ni'ihau, indicating a larger
population size, but with some site fidelity. Two individuals were
documented moving from Kaua'i to Hawai'i, a distance of 480 km,
the largest travelling distance reported for the species (Baird
et al. 2008).
6. Threats
Mass strandings: Miyazaki and Perrin (1994 and references
therein) posited that mass stranding may reduce population size.
A school of 17 stranded in Hawaii in 1976. Further mass strandings
have been summarised by Maigret (1995). The reasons for such mass
strandings are to date poorly understood. A possible cause is disorientation,
caused by parasites affecting the inner ear, by damage due to military
sonar or geological prospection, or by variability in the earth's
magnetic field, coupled with altruistic behaviour (herd members
not abandoning one another).
In the late 1990's, IMMRAC (the Israeli Marine Mammal Research and
Assistance Center) examined 7 strandings of rough-toothed dolphins
along the entire Mediterranean Israeli coastline. The species is
considered rare in the Mediterranean, and this regional clustering
seems rather unusual. It is interesting to notice that all the standings
occurred between the months of February and April: presumably during
a seasonal migration (Aviad Scheinin, pers. comm.).
Directed fisheries: Small numbers were taken in drive fisheries
at Okinawa in the Ryukyus and in the home islands of Japan, the
Solomon Islands and Papua New Guinea. Some may still be taken by
harpoon in Japan, Taiwan, at St Vincent in the Lesser Antilles and
in West Africa. However, only 23 rough-toothed dolphins were captured
in Japan (Okinawa) during the period 1976-81 (Miyazaki and Perrin,
1994 and refs. therein; Hammond et al. 2008).
By-catches: A few rough-toothed dolphins are killed incidentally
in tuna purse seines in the eastern tropical Pacific: 21 were estimated
killed during the period 1971-75, and 36 died in a single net haul
in 1982. Small numbers are also taken as by-catch in gillnet and
driftnet fisheries in Sri Lanka, Brazil, the Central North Pacific,
Taiwan and probably elsewhere around the world in tropical and warm-temperate
waters (Miyazaki and Perrin, 1994 and references therein; Hammond
et al. 2008).
Monteiro et al. (2000) reported on fishery-related mortality along
the coast of Ceara state, Northeast Brazil, commenting on the possible
conservation implications for the local populations. From January
1992 to December 1998, a total of 13 strandings occurred along the
coast, mostly during the austral spring (October-December). Most
animals were recovered in areas where finfish fisheries and stranding
survey efforts were highest.
There has been no reported fishing-related mortality or serious
injury of rough-toothed dolphins during 1992-2006 in the Northern
Gulf of Mexico (Waring et al. 2008).
Pollution: Polychlorinated biphenyls(PCBs) are persistent,
long distant movable and highly bioaccumulative contaminants in
the marine environment. Levels of PCBs and Dichlor-diphenyl-dichlor-ethen
(DDE) in the blubber of two specimens collected in the western Pacific
were lower by two orders of magnitude than those recorded in S.
coeruleoalba and other delphinids (Miyazaki and Perrin,
1994 and refs. therein). However, a subsequent study showed that
the levels of PCBs in marine mammals would reach peak levels between
2000 and 2030. Compared with toxicity equivalents in other dolphinids
from around the world, the toxicity equivalent of PCBs in rough-toothed
dolphin from Dapeng Bay, Gguangdong, China, was at relatively high
levels (Huang et al. 2007). Similarly, Marsili and Focardi (1997)
reported on chlorinated hydrocarbon concentrations in specimens
from the Mediterranean Sea. Concentration of toxaphene and polybrominated
diphenyl ethers from rough-toothed dolphins stranded on the coast
of Massachussetts were 1.4 µg/g and 0.5 µg/g wet mass,
approximately one order of magnitude lower than in L.
acutus (Tuerk et al. 2005).
7. Remarks
Range states (Hammond et al. 2008):
Algeria; American Samoa; Anguilla; Antigua and Barbuda; Aruba; Australia;
Bahamas; Bangladesh; Barbados; Belize; Benin; Bermuda; Brazil; British
Indian Ocean Territory; Brunei Darussalam; Cambodia; Cameroon; 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; Cuba; Djibouti; Dominica; Dominican
Republic; Ecuador; El Salvador; Equatorial Guinea; Ethiopia; Fiji;
France (Corse); French Guiana; French Polynesia; Gabon; Gambia;
Ghana; Gibraltar; Greece (Kriti); Grenada; Guadeloupe; Guam; Guatemala;
Guinea; Guinea-Bissau; Guyana; Haiti; Honduras; Hong Kong; India;
Indonesia; Iran, Islamic Republic of; Israel; Italy (Sardegna, Sicilia);
Jamaica; Japan; Kenya; Kiribati; Kuwait; Liberia; Libyan Arab Jamahiriya;
Madagascar; Malaysia; Maldives; Marshall Islands; Martinique; Mauritania;
Mexico; Micronesia, Federated States of; Morocco; Mozambique; Myanmar;
Namibia; Nauru; Netherlands Antilles; New Caledonia; Nicaragua;
Nigeria; Niue; Northern Mariana Islands; Oman; Pakistan; Palau;
Panama; Papua New Guinea; Peru; Philippines; Pitcairn; Portugal;
Puerto Rico; Qatar; Saint Helena; Saint Kitts and Nevis; Saint Lucia;
Saint Pierre and Miquelon; Saint Vincent and the Grenadines; Samoa;
Sao Tomé and Principe; Senegal; Sierra Leone; Singapore;
Solomon Islands; Somalia; South Africa; Spain (Baleares, Canary
Is.); Sri Lanka; Suriname; Taiwan, Province of China; Tanzania,
United Republic of; Thailand; Timor-Leste; Togo; Tonga; Trinidad
and Tobago; Turks and Caicos Islands; United Arab Emirates; USA;
Venezuela; Viet Nam; Virgin Islands, British; Virgin Islands, U.S.;
Wallis and Futuna; Western Sahara; Yemen.
IUCN Status: "Least Concern" (Hammond et al. 2008). Not
listed by CMS but listed in Appendix II of CITES.
See also recommendations on South American stocks in Hucke-Gaete
(2000) in Appendix
1 and recommendations on Southeast Asian stocks in Perrin et
al. (1996) in Appendix
2.
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© Boris Culik (2010) Odontocetes.
The toothed whales: "Steno bredanensis". UNEP/CMS
Secretariat, Bonn, Germany. http://www.cms.int/reports/small_cetaceans/index.htm
© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.
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