Stenella longirostris (Gray,
English: Spinner dolphin
German: Ostpazifischer Delphin
Spanish: Estenela giradora
French: Dauphin à long bec
Stenella longirostris © Würtz-Artescienza
Spinner dolphins can be detected from large distances as they spin
high in the air and then land with a loud splash. The body is slender
and the beak is extremely long and thin. Colouration consists of
a dark grey cape, light grey lateral field and white ventral field.
A dark band runs from the eye to the flipper, bordered above by
a thin light line. The rostrum is tipped with black or grey. The
dorsal fin is basically triangular, slightly falcate to erect or
canted forward. The flippers are thin and recurved. Adults range
from 129-235 cm and reach a body mass of 23-80 kg (Perrin, 1998;
Spinner dolphins are pantropical, occurring in all tropical and
subtropical waters around the world between roughly 40°N and
40°S (Jefferson et al. 2008). The geographical variation in
body configuration and colour pattern is more pronounced in spinner
dolphins than in any other species of cetacean.
Distribution of Stenella longirostris.
Four different subspecies occur in tropical and
subtropical waters in the Atantic, the Indian and Pacific Oceans
(Hammond et al. 2008; © IUCN; enlarge
Perrin (1990) and Perrin et al. (1999) recognized
this variation by naming four subspecies:
S. l. longirostris: The nominate subspecies (all spinner
dolphins aside from the other described subspecies) occurs mainly
around oceanic islands in the tropical Atlantic, Indian, western
and central Pacific east to about 145°W. It ranges north to
New Jersey, Senegal, the Red Sea, Gulf of Oman, Arabian Sea, Sri
Lanka, the Andaman Sea, Gulf of Thailand, southern Honshu, and the
Hawaiian Islands (Rice, 1998). Smith et al. (1997a and 1997b) sighted
individuals off Myanmar and Vietnam. It ranges south to Paraná
in Brazil, Saint Helena, Cape Province, Timor Sea, Queensland, and
Tonga Islands and is vagrant to New Zealand (Rice, 1998).
It is found in relatively small and discrete communities
around many islands throughout the Pacific. In island communities
of the Society Archipelago, French Polynesia, gene flow among neighbouring
communities is restricted, although some individual movement was
documented (Oremus et al. 2007). Similarly around Hawaii, dolphins
at almost every island were found to be significantly genetically
differentiated from dolphins at every other island for one or more
tests of population subdivision (Andrews et al., 2006). And finally,
genetic data shows differences between American Samoa and the Hawaiian
Islands (Johnston et al., 2008).
The southernmost record is from New Zealand, more
than 2000 km south of what is thought to be the normal range but
still well north of subantarctic waters. The distribution of S.
l. longirostris in the Atlantic is very poorly known, especially
in South American and African waters (Perrin and Gilpatrick, 1994
and refs. therein). Van Waerebeek et al. (2000) note a lack of recent
sightings, strandings or by-catches off West Africa, whereas Ali
and Jiddawi (1999) report sightings on the coast of Zanzibar in
the Western Indian Ocean. Interspecific hybrids between S. longirostris
attenuata and between S. longirostris and S.
clymene are reported from the Fernando de Noronha Archipelago,
tropical West Atlantic (Silva et al. 2005).
Perrin (1990) proposed the name "Gray's spinner dolphin"
for this race; the "Hawaiian spinner porpoise" is included
here. The "whitebelly spinner porpoise" and the "southern
spinner dolphin" are intergrades or hybrids between this race
and S. l. orientalis (Rice, 1998 and refs. therein).
S. l. orientalis Perrin, 1990: Ranges in pelagic waters
of the tropical Pacific east of about 145°W, from 24°N off
Baja California south to 10°S off Peru, but exclusive of the
range of the following race. This is the "eastern spinner dolphin"
of Perrin (1990).
S. l. centroamericana (Perrin, 1990): Found in coastal waters
over the continental shelf of the tropical Pacific from the Gulf
of Tehuantepec in southern Mexico southeast to Costa Rica. This
is the "Central American spinner dolphin" of Perrin (1990).
However, Perryman and Westlake (1998) examined lengths of spinner
dolphins taken from vertical aerial photographs in the eastern tropical
Pacific and found three unique morphotypes. Two of these forms correspond,
at least in average length and distribution, to the existing eastern
and Central American subspecies. The third form is intermediate
in length between the two recognised subspecies and is found along
the edge of the continental shelf north of Cabo Corrientes, Mexico.
They provisionally called this form the "Tres Marias spinner
S. l. roseiventris (Wagner, 1846): is distributed in shallow
innner waters of Southeast Asia, including the Gulf of Thailand,
Timor and Arafura Seas, and similar waters off Indonesia, Malaysia
and Northern Australia. It is replaced in deeper and outer waters
by the larger pelagic subspecies S. l. longirostris (Perrin
et al. 1999).
Based on morphological data, van Waerebeek et al. (1999) concluded
that Oman spinner dolphins should be treated as a discrete population,
morphologically distinct from all known spinner dolphin subspecies.
Confirmed coastal range states off the Arabian Peninsula include
the United Arab Emirates, the Sultanate of Oman, Yemen, Somalia,
Djibouti, Saudi Arabia, Sudan and Egypt. It is likely that additional
regional subspecies will be split off from the nominate subspecies
in the future.
3. Population size
There are several abundance estimates for this circumglobal species,
most of which are quantitative.
The largest population is reported from the eastern tropical Pacific
(ETP). The 2003 estimate of the eastern spinner dolphin (S. l.
orientalis) was 613,000 (CV=21.9) and for the whitebelly spinner
dolphin (an intergrade between S. l. longirostris and S.
l. orientalis) 442,000 (CV=44.6) (Gerrodette el al., 2005).
This is higher than the 2000 estimates of about 428,000 eastern
spinner dolphins (CV = 0.218). For the whole period from 1979 to
2000, annual estimates of abundance ranged from 271,000 to 734,000
(Gerrodette and Forcada, 2005).
Balance and Pitman (1998) conducted a cetacean survey in the pelagic
western tropical Indian Ocean (WTIO) and reported that the cetacean
community there was similar to that of the ETP and the Gulf of Mexico
(GM). In the central and western Pacific, spinner dolphins are often
the most abundant species. A 2002 shipboard line-transect survey
of the entire Hawaiian Islands EEZ resulted in an estimate of 3,351
(CV=0.74) animals (Barlow 2006). In the nearshore waters of Manu'a
Islands, Rose Atoll and Swains Island, American Samoa, spinner dolphins
(n=34 groups, 46 animals), were the most abundant cetacean species
in summer 2006 (Johnston et al., 2008). In Philippine waters it
is also the most abundant species, with a population estimate of
31,512 (CV=26.63%) in the eastern Sulu Sea and 3,489 (CV=26.47%)
in the Tañon Strait (Dolar et al., 2006). In the waters around
the Marquesas Islands in French Polynesia, spinner dolphins were
the second most abundant species (Gannier, 2002).
Off the Mergui (Myeik) Archipelago of southern Myanmar, spinner
dolphins were the third most frequent species (Smith and Tun, 2008).
In the northern Mozambique Channel around the island of Mayotte,
spinner dolphins were also very numerous;118 animals were observed
(Kiszka et al. 2007).
In the northern Gulf of Mexico (US EEZ), the current estimate of
abundance for spinner dolphins in oceanic waters averaged over 2003
to 2004, is 1,989 (CV=0.48) (Mullin 2007). This estimate is significantly
different (P<0.05) from that for 1996-2001 of 11,971 (CV=0.71),
while the 1991-1994 estimate of 6,316 (CV=0.43) is intermediate.
These temporal abundance estimates are difficult to interpret without
a Gulf-of-Mexico-wide understanding of spinner dolphin abundance
(Waring et al., 2009).
4. Biology and Behaviour
Habitat: In most tropical waters, nearly all records of
spinner dolphins are associated with inshore waters, islands or
banks. Around Hawaii spinner dolphins depend on the availability
of sheltered shallow bays for use as resting areas during the day.
In coastal waters of the Society Islands (French Polynesia), they
are observed year-round during daytime in sheltered bays or within
lagoons. Dolphins stay within the bay from early morning until the
early afternoon, when they move slowly offshore. On average, they
stay 400 m from shore, although they approach as close as 100 to
150 m. Dolphin presence and residence time seems to be negatively
affected by surface water turbidity (river flow) and lagoon current
strength. Seasonally, there are slight differences in presence with
81% of dolphin days in from May to November and only 67% between
December and April (Cannier and Petiau, 2006). The dwarf form of
the spinner dolphin in Thai waters apparently inhabits a shallow
coral reef habitat (Perrin and Gilpatrick, 1994 and refs. therein).
In the eastern tropical Pacific, however, spinner dolphins, like
pantropical spotted dolphins, occur in very large numbers on the
high seas many hundreds of km from the nearest land. The spotted
dolphin school may serve as a surrogate "protected bay"
for the spinner dolphins to shelter them from predators during their
daily quiescent period, thus allowing them to exist and make a living
far from land. The habitat there, called by oceanographers "tropical
surface water", is typified by unusual conditions of shallow
mixed layer, shoal and sharp thermocline, and relatively small annual
variation in surface temperature (Reyes, 1991, Perrin and Gilpat-rick,
1994 and refs. therein).
Davis et al. (1998) characterised the physical habitat of cetaceans
found along the continental slope in the north-central and western
Gulf of Mexico. S. longirostris was found over intermediate
bottom depths, its distribution overlapping with that of purely
pelagic and purely coastal species.
Photo: Robert L. Pitman
Schooling: The spinner dolphin society is composed partly
of familial units and more broadly of learned associations beyond
the family group. Mother-calf bonds are persistent, as in other
dolphins. Around Hawaii, social groupings are very fluid, with individuals
moving freely among several sets of companions over periods of minutes,
hours, days or weeks. Large schools form, break down and re-form
with different permutations of subgroups in the course of diurnal
inshore-offshore and longshore movements related to nocturnal feeding.
It is not known whether or not these broader associations are with
members of dispersed kin groups.
There is some segregation by age and sex among schools of spinner
dolphins in the far-offshore eastern Pacific. It has been suggested
that such segregation may be temporary and more pronounced during
migration in dolphins. There appears to be no consistent "leader"
in a spinner dolphin school. Directional movement appears to be
a group process, with direction imparted often from behind, to the
sides or below in the school. In a time of stress, the school becomes
what has been termed a "sensory integration system" (SIS)
and direction may come from anywhere in the school. In the eastern
tropical Pacific spinner dolphins are often found in close association
with pantropical spotted dolphins, yellowfin tuna and birds of several
species; the association varies in percentage occurrence with time
of day (Perrin and Gilpatrick, 1994, and refs. therein). In mixed-species
associations off Hawaii, spinner dolphins are typically present
in greater numbers than spotted dolphins with ratios as high as
75:1. Interspecific behaviours observed include aggression, copulation,
and travelling (Psarakos et al. 2003).
Around the Society Islands (French Polynesia), school sizes ranging
from 15 - 30 to 100 - 140 individuals (Cannier and Petiau, 2006).
At Midway Island, spinner dolphins live in stable bisexually bonded
societies of long-term associates, with strong geographic fidelity,
no obvious fission-fusion, and limited contacts with other populations.
Their large cohesive groups change little over time and are behaviorally/socially
discrete from other spinner dolphin groups. With deepwater food
resources in close proximity and other atolls relatively far away
for day-to-day access, it may be energetically more beneficial in
remote atolls to remain "at home" than to travel to other
atolls, explaining the observed school stability (Karczmarski et
Behaviour: The spinner dolphin performs spectacular leaps
from the water while rotating around its longitudinal axis up to
seven times. Although twisting of the body while airborne has been
proposed as the mechanism to effect the spin, angular momentum to
induce the spin is generated underwater, prior to the leap. The
high rotation rates and orientation of the dolphin's body during
re-entry into the water could produce enough force to dislodge unwanted
remoras (Fish et al. 2006), one hypothesis of the function of the
Off Oahu, Hawaii, spinner dolphins at night actively aggregate their
prey through cooperative foraging using their preys' avoidance behaviour
to create distinct, high-density patches in the prey. Dolphins swim
around the edge of a 28-40 m diameter circle at least 5 times, concentrating
prey within this area before pairs of dolphins on opposite sides
of the circle swap positions, swimming through the high density
prey `donut' they have formed (Benoit-Bird and Au, 2003).
Food: Spinner dolphins feed primarily on small (generally
less than 20 cm) mesopelagic fish, squids and sergestid shrimps,
diving to at least 200-300m (Dolar et al. 2003). In Hawaii, many
prey organisms become available to spinner dolphins when the deep
scattering layer moves toward the surface at night. Spinner dolphins
in the Gulf of Thailand may have an entirely different trophic ecology,
feeding on benthic and coral reef organisms (Perrin and Gilpatrick,
1994, and refs. therein). At Fernando de Noronha Archipelago in
the southwestern Atlantic, twelve fish species in seven families
are known to feed on dolphin offal. The black durgon (Melichthys
niger) is the most ubiquitous waste-eater, recognizing the postures
a dolphin adopts prior to defecating or vomiting and converging
on an individual shortly before it actually voids. Offal is then
quickly fed upon (Sazima et al. 2003).
Reproduction: Gestation lasts about 10 months, and nursing
duration is 1-2 years. Females reach sexual maturity at 4-7 years
and may calve every 3 years. Males are sexually mature at 7-10 years
(Perrin, 2009). The oldest eastern spinner dolphin by-caught in
the ETP tuna fishery was estimated to be 24.5 years old and the
oldest whitebelly spinner dolphin was 26 (Larese and Chivers, 2008).
Smaller testis size in the eastern spinner than in the whitebelly
spinner suggests that the breeding system in the former may tend
more toward polygyny (Perrin and Mesnick, 2003).
Norris et al. (1994) concluded that spinner dolphin distribution
and abundance is related to certain local oceanographic phenomena.
For example, divergence zones at current margins and current ridges
both concentrate food organisms and are heavily frequented by dolphins
of various species, including spinners. Whereas one scientific view
suggests that populations remain geographically stable over rough
bottom topography, another view suggests that at least some populations
may move widely without reference to the bottom. Where a warm current
swings away from the tropics along an ocean margin - for example
where the Kuroshiro current moves northward along the eastern shore
of Japan - oceanic dolphin populations, including the spinner dolphin,
migrate in such water masses and move considerable distances.
Perrin and Gilpatrick (1994, and refs. therein) noted that in different
regions such as Hawaii and Fernando de Noronha Island (northern
Brazil) spinner dolphins usually spend the daytime hours resting
in shallow bays near deep water. They move offshore at dusk to feed.
During feeding, they may move some distance along the shore, so
the same animals may not be present in the same bay on two successive
days. Not all animals go into the rest coves every day; some move
slowly along the shore between successive nights. Maximum net movement
observed was 113 km over 1,220 days. In general , site-fidelity
in Hawaiian animals is strong. At least one and up to three animals
were re-sighted northwest of Oahu 20 years after the first reported
sighting (Marten and Psarakos, 1999). Capture-recapture analyses
at Moorea (Society Archipelago, French Polynesia) based on long-term
observations of marked individuals and molecular data also indicated
a local and relatively closed community (of about 150 dolphins).
This is also confirmed by resightings of individuals across 15 yr
(Oremus et al. 2007).
In the eastern tropical Pacific, however, tagged spinner dolphins
moved minimum distances of 12 to 275 nautical miles (within 16h
and 365 days, respectively). The number of tag returns (seven of
340) was insufficient to allow detection of a migratory pattern
if one exists. Minimum distances moved were less than for pantropical
spotted dolphins at liberty for similar periods of time; the spinner
dolphin may be less migratory (Perrin and Gilpatrick, 1994, and
Directed fisheries: Small numbers of spinner dolphins are
taken in localised harpoon fisheries in many places around the world,
e.g. the Lesser Antilles, the Philippines, and Indonesia. They were
formerly taken in small numbers in drive fisheries in Japan. 117
by-caught spinner dolphins were landed in India in 1986-87, presumably
for human consumption. Dolphins taken incidentally in Venezuela
are utilised for shark bait and human consumption (Dolar et al.,
1994; Perrin and Gilpatrick, 1994 and refs. therein).
Ilangakoon (1997) reported on the interaction between small cetaceans
and the fisheries industry in Sri Lanka. He found S. longirostris
to be the most abundantly caught species at all investigated sites.
The post-monsoonal period from the end of August to November was
the season when peak catches were recorded. Deliberate harpooning
was found to account for a sizeable proportion of the small cetacean
catch, and the practice seems to be spreading to new areas.
By-catches: There has been no reported fishing-related mortality
of spinner dolphins during 1998-2006 in the Northern Gulf of Mexico
US EEZ (Waring et al., 2009). In the Hawaii-based longline fishery
during 1994-2005, annual fishing effort was roughly constant at
about 12,000 sets through 2001 and then increased to over 18,000
sets through 2005. During 24,542 observed sets, 67 cetaceans were
observed hooked or entangled, of which only 2 were spinner dolphins
(Forney and Kobayashi, 2007).
Since the Inter-American Tropical Tuna Commission (IATTC) implemented
per-vessel mortality limits on the international fleet, the mortality
for the eastern and whitebelly forms combined decreased, from 30,500
in 1986 to 288 in 2007 (IATTC, 2009). Although current mortality
is greatly reduced, the eastern form appears to be recovering very
slowly. Possible reasons include underreporting of dolphin bycatch,
effects of chase and encirclement on dolphin survival and reproduction,
long-term changes in the ecosystem, and effects of other species
on population dynamics (Gerrodette and Forcada, 2005).
Reproductive data from the eastern tropical Pacific shows that
proportion with calves is related to number of dolphins in the school
and/or proportion of the school made up of the focal species. Annual
number of purse-seine sets on dolphins was a predictor of both proportion
with calves and length at disassociation. Because the spinner dolphin
is one of the two main species targeted by the fishery, the link
between fishing activity and both measures of reproductive output
indicates that the fishery has population-level effects beyond reported
direct kill and may be responsible for the failure of dolphin populations
to recover at rates expected after reduction of high bycatch levels
(Cramer et al., 2008).
Dolphin mortality seems to increase with the number of dolphins
encircled, because of increased risk of entanglement and longer
duration of the backdown procedure, including the risk of entrapment
in net-canopies. Therefore, large herds are particularly threatened
by the tuna fishery (Lennert-Cody et al. 2004). Gerrrodette (2002)
also mentioned cryptic effects of repeated chase and encirclement
on survival an/or reproduction (internal injuries, stress, hyperthermia
and separation of nursing calves from their mothers during the fishing
A total of 96 dolphins were reported to have been incidentally
caught in gillnet fisheries off Zanzibar (Unguja Island) between
1995 and 1999, including 29 spinner dolphins (Amir et al., 2003).
Reports on incidental catches monitored at 12 landing sites between
2000 and 2003 numbered 44 (31% of all by-caught cetaceans) spinner
dolphins. Most of the bycatches (71%) were in nets set off the north
coast of Unguja Island (Amir et al., 2005). Significant catches
of spinner dolphins also occur in the Caribbean, Australia, Japan,
the Philippines, and Sri Lanka; in the last area up to 15,000 are
killed each year in gillnets and by hand-harpooning. There are likely
to be fisheries interactions off West Africa (Jefferson et al. 1993;
Perrin and Gilpatrick, 1994; Carwardine, 1995). A trawl shrimp fishery
in the Gulf of Thailand takes a yet unknown number of S. l. roseiventris
(Reyes, 1991). Zerbini and Kotas (1998) reported on by-catches in
Brazilian drift-net fisheries and Cockroft (1990) on animals entangled
in shark nets off Natal.
Pollution: Relatively high levels of mercury and contamination
with DDT, Dieldrin and PCBs have been reported for the species (e.g.
Tanabe et al. 1993). The high level of Hg has been attributed to
natural sources, but in the case of DDT and PCBs the agricultural
and industrial development in Central America may be the cause (Velayutham
et al. 1994; Velayutham and Venkataramanujam 1995; Perrin and Gilpatrick,
1994 and refs. therein; Reyes, 1991). Blubber samples of animals
from the Bay of Bengal (southeast coast of India) contained considerable
levels of organochlorines with DDT in the range of 3330-23 330 ng/g;
HCHs in the range of 95-765 ng/g; and PCBs in the range of 210-1220
ng/g (wet weight basis) (Karuppiah et al. 2005).
Specimens stranded along the coasts of the lower Gulf of California,
Mexico contained mercury (Hg) and methylmercury (MeHg) in their
tissues, albeit at low levels (Ruelas et al. 2003). Lately, polybrominated
diphenyl ethers (PBDEs), one of the flame retardants widely used
in plastics, textiles, electronic appliances, and electrical household
appliances were detected in the blubber of cetaceans found stranded
along the coasts of Japan, Hong Kong, the Philippines and India
during the period from 1990 to 2001. However, concentrations of
PBDEs in spinner dolphins were low, with 6.0 ng/g lipid wt (Kajiwara
et al. 2006).
Tourism: Tourist development may affect the near-shore habitat
of some spinner dolphin populations, for example, at Fernando de
Noronha Island, Brazil (Reyes, 1991). Although increasing levels
of human activity has a limited but measurable effect on the movement
patterns of Hawaiian spinner dolphin groups at Kealakekua Bay (Delfour,
2007; Timmel et al. 2008), Ali and Jiddawi (1999) reported that
in Zanzibar tourism was beneficial for the species: their touristic
value far exceeds that of using them as bait for sharks. As many
as 2,000 tourists visit the dolphin site at Kizimkazi per month.
Range states (Hammond et al. 2008) :
American Samoa; Anguilla; Antigua and Barbuda; Aruba; Australia
(Queensland); Bahamas; Bangladesh; Barbados; Belize; Benin; Bermuda;
Brazil (Paraná); British Indian Ocean Territory; Brunei Darussalam;
Cambodia; Cameroon; Cape Verde; Cayman Islands; 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 (Galápagos); Egypt;
El Salvador; Equatorial Guinea; Fiji; French Guiana; French Polynesia;
Gabon; Gambia; Ghana; Grenada; Guadeloupe; Guam; Guatemala; Guinea;
Guinea-Bissau; Guyana; Haiti; Honduras; Hong Kong; India (Andaman
Is., Nicobar Is.); Indonesia; Iran, Islamic Republic of; Jamaica;
Japan (Honshu); Kenya; Kiribati; Liberia; 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; Puerto Rico; 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
(KwaZulu-Natal); Sri Lanka; Sudan; Suriname; Taiwan, Province of
China; Tanzania, United Republic of; Thailand; Timor-Leste; Togo;
Tonga; Trinidad and Tobago; United Arab Emirates; USA (Hawaiian
Is., New Jersey); United States Minor Outlying Islands; Uruguay;
Venezuela; Viet Nam; Virgin Islands, British; Virgin Islands, U.S.;
Wallis and Futuna; Western Sahara; Yemen (Socotra).
The species is listed in Appendix II of CITES. The IUCN lists the
species as "Data Deficient" (Hammond et al. 2008). The
eastern tropical Pacific populations and south-eastern Asian populations
of S. longirostris are listed in Appendix II of CMS.
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© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.