Stenella attenuata (Gray,
English: Pantropical spotted dolphin
Spanish: Delfín manchado
French: Dauphin tacheté
Stenella attenuata © Würtz-Artescienza
Pantropical spotted dolphins can be identified by their long beak
sharply demarcated from the melon, slender body, strongly recurved
fin and spotted body. The ventral spots fuse and fade to a medium
grey, and the dorsal light spots intensify, sometimes to the point
of making the animal appear nearly white above. The tip of the beak
is white. Details of coloration and spot intensity vary regionally.
The newborn calf is unspotted. Adults range from 166 to 257 cm and
weigh up to 119 kg. Males are on average slightly larger than females.
As opposed to S.
frontalis, with which it may easily be confounded, S.
attenuata lacks a light spinal blaze and dark ventral spots
and has a dorsoventral division of the peduncle (Perrin, 2009).
Stenella attenuata is distributed in tropical and
warm-temperate waters around the world, from roughly 40 °N to
40°S (Jefferson et al. 1993). It ranges north to Massachusetts,
the islands of Cape Verde, the northern Red Sea, Persian Gulf, Arabian
Sea, Bay of Bengal, South China Sea, East China Sea, Pacific coast
of northern Honshu, the Hawaiian Islands, and Baja California Sur.
Vagrant to Santa Cruz County in California, and Cold Bay on the
Alaska Peninsula (Rice, 1998). It ranges south to Argentina, Cape
Province in South Africa, Timor Sea, New South Wales, and about
southern Peru (Hammond et al. 2008).
Distribution of Stenella attenuata (Hammond
et al. 2008; © IUCN; enlarge
Tropical and warm-temperate waters of the Atlantic, Pacific and
This species varies geographically in cranial and
postcranial measurements, in body size and coloration, but in most
of its range a division into subspecies has not been attempted because
too few specimens are available (Rice, 1998).
S. a. attenuata is pantropical, found in all oceans between
about 40°N and 40°S, although it is much more abundant in
the lower-latitude portions of its range. The range extends to some
enclosed seas, such as the Red Sea and Persian Gulf, but does not
include the Mediterranean Sea (Perrin 2001, 2002).
S. a. graffmani - The coastal spotted dolphin is found only
in a narrow band (<200 km wide) along the coast of Latin America,
from southern Mexico to Peru (Perrin 2001; Escorza-Treviño
et al. 2005). Recent genetic data suggest there may be several populations
contained within this subspecies (Escorza-Treviño et al.
Silva et al. (2005) describe two aberrant individuals of the genus
Stenella from Fernando de Noronha Archipelago, tropical West
Atlantic. One specimen was presumably an interspecific hybrid between
S. longirostris and S. attenuata, and the second possibly
a hybrid between S. longirostris and S.
3. Population size
S. attenuata is among the most abundant dolphins in the
eastern tropical Pacific (Jefferson et al. 1993). It ranks second
in abundance in the deeper waters of the Gulf of Mexico, the eastern
tropical Pacific and Sulu Sea, and sixth in the tropical Indian
Ocean (Perrin 2009). Perrin and Hohn (1994 and refs. therein) estimated
that in 1979 the population amounted to 1.7 million animals in total,
a value which matches well with current estimates.
The most recent population estimates come from USA EEZ waters:
in the western North Atlantic. Data from two 2004 surveys yielded
a joint estimate of 4,439 (CV=49%) individuals (Waring et al. 2009).
In oceanic waters of the USA Gulf of Mexico the estimate pooled
from 2003 to 2004 was 34,067 (CV=0.18) (Mullin 2007). This is significantly
different from the 1996-2001 estimate of 91,321 (CV=0.16). However,
the 2003-2004 estimate is similar to that for 1991-1994 of 31,320
(CV=0.20) (Waring et al. 2009).
The 1999 estimates of absolute abundance in the eastern Pacific
(Gerrodette, 1999) were 592,000 for the "north-eastern"
stock, 710,000 for the "west/south" stock, and 73,000
for the "coastal" stock (S. a. graffmani). Corresponding
values for 2003 are 737,000 for the "north-eastern" stock
(CV = 14.7), 628,000 (CV = 30.9) for the western/southern offshore
stock and 149,400 (CV = 26.6) for the "coastal" stock
(Gerrodette et al. 2005). In Hawaiian waters, there are an estimated
8,978 (CV=48%) (Barlow 2006).
In the southern part of the Sulu Sea and north-eastern Malaysian
waters, Dolar et al. (1997) estimated abundance at 3,500 individuals.
Dolar et al (2006) estimated about 14,930 (CV=41%) for the eastern
Sulu Sea and 640 (CV=27%) for the Tañon Strait between the
islands of Negros and Cebu.
Other reports are rather qualitative: S attenuata was one
of the most numerous species recorded off Costa Rica in the Caribbean
(MayCollado et al. 2005), off the island of St Helena (McLeod and
Bennett, 2007) and off Angola (Weir, 2007) in the tropical south-eastern
Atlantic, off Mayotte in the Indian Ocean (Kiszka et al. 2007),
and off the Marquesas Islands in French Polynesia (South Pacific)
(Gannier, 2002). .
4. Biology and Behaviour
Habitat: In the Atlantic, S. attenuata
is primarily a dolphin of the high seas and oceanic islands, but
in the eastern Pacific a large-bodied race occurs along the coast
from Mexico to Peru; it may feed on larger prey than does the oceanic
form and may be an ecological counterpart of the large form of the
endemic S. frontalis in Atlantic coastal waters (Perrin and
Hohn, 1994 and refs. therein).
The pantropical spotted dolphin of the eastern Pacific inhabits
the tropical, equatorial and southern subtropical water masses.
The waters in which it occurs with greatest frequency are those
underlain by a sharp thermocline at depths of less than 50 m and
with surface temperatures over 25°C and salinities less than
34 parts per thousand. These conditions prevail year round in the
region north of the Equator called the "Inner Tropical"
waters of the eastern Pacific. Occurrence in this core habitat is
correlated with apparent multi-species foraging and feeding behaviour.
The species also occurs in closely similar waters south of the Equator
that expand and contract greatly with season and year to year (Perrin
and Hohn, 1994 and refs. therein).
Photo © Robert L. Pitman
Schooling. A "school" (all of the animals seen
at one time, or captured in one purse-seine set) may consist of
from just a few dolphins to several thousand. Observations of schools
captured in purse seines show that they are often formed of distinct
subgroups containing cow-calf pairs, adult males, or juveniles (Perrin
and Hohn, 1994 and refs. therein).
Spotted dolphins in the oceanic eastern tropical Pacific aggregate
with yellowfin tuna, Thunnus albacares. Other participants
in the aggregations include spinner dolphins (S. longirostris),
skipjack tuna (Katsuwonus pelamis), oceanic birds of several
families, and less commonly other small cetaceans, sharks and billfish.
The reason for these associations is not known but may have to do
with foraging efficiency, protection from predators, orientation
in the pelagic void, or some other factor or circumstance not yet
understood. Tuna fishermen take advantage of the dolphin-tuna association
in finding and catching tuna (Perrin and Hohn, 1994 and refs. therein).
Similarly, in the western tropical Indian Ocean (WTIO), Balance
and Pitman (1998) recorded 26 mixed-species cetacean schools, 43
schools with which seabirds associated, and 17 schools associated
with tuna. Notable among these were mixed aggregations of S.
attenuata, S.longirostris, yellowfin tuna, and seabirds. In
Hawaiian waters spinner dolphins were 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).
Reproduction: Females reach sexual maturity at 9-11 years
and males at 12-15 years. Gestation lasts approx. 11.5 months (Perrin,
2009). Spotted dolphins begin to take solid food at approximately
6 mo of age, or 115 cm, but continue to suckle until they are nearly
2 years old. Calves tend to feed more frequently on squid as they
get older, which suggests there is a shift in diet during weaning.
The average age and total body length at weaning is estimated to
be 0.8 yr and 122 cm. The oldest suckling calf was almost 2 yr old
(Archer and Robinson, 2004).
Food. The prey of the pantropical spotted dolphin is made
up primarily of small epipelagic fish, squid and crustaceans, with
some take of mesopelagic animals (Perrin and Hohn, 1994 and refs.
therein). In the eastern tropical Pacific, significant differences
in prey composition by season and geographic region indicate that
they are flexible in their diet and may be opportunistic feeders.
Identified prey include 56 species of fish and 36 species of cephalopods
(Roberston and Chivers, 1997). The most frequently found fish were
lanternfish (family Myctophidae) at 40%, and the most frequently
found cephalopods were trying squids (family Ommastrephidae) at
65%. The dominance of these primarily mesopelagic prey species and
a significantly higher stomach fullness index for stomachs collected
during the morning hours suggest that pantropical spotted dolphins
feed at night when many mesopelagic species migrate toward the surface:
Near the islands of Maui and Lana'i, Hawai'I, dives at night were
deeper (average 57.0m, maximum depth 213 m) than during the day
(average 12.8m, maximum depth 122m), and swim velocity also increased
Together with the series of deep dives recorded immediately after
sunset this suggests that pantropical spotted dolphins feed primarily
at night on organisms associated with the deep-scattering layer
as it rises up to the surface after dark (Baird et al. 2001).
Off eastern Taiwan, mesopelagic prey species also dominate the stomach
contents. Sixty-four species of fish made up 67.5% and 21 species
of cephalopods made up 32.5% by number. Myctophid lanternfishes
and enoploteuthid squid accounted for 78.3% of all prey consumed.
The enoploteuthid squid Enoploteuthis chunii was the primary
prey and represented 25.8% by number of the total prey, with an
overall occurrence of 66.7% (Wang et al. 2003).
In the pelagic eastern tropical Pacific (ETP), Reilly (1990) studied
large-scale patterns of dolphin distribution and oceanography from
research-vessel surveys. The species was sighted in abundance west
of 120°W along 10°N coincident with seasonal shoaling of
a thermocline ridge. Highest-density areas for the different species
were clearly separated spatially, and the thermocline depths surface
temperatures of sighting localities were statistically different
between spotted/spinner dolphin schools and common dolphin schools.
Tagging experiments in the ETP show that movements of pantropical
spotted dolphins may generally be onshore in fall and winter and
offshore in late spring and summer. The minimum distance travelled
by the tagged animals ranged from 7 to 582 nautical miles (Reyes,
1991, and refs. therein). Offshore spotted dolphins may be found
as close to the coast as 16 nautical miles, where they overlap with
the coastal form (Reyes, 1991, and refs. therein). A recent radio-tracking
study shows that the animals associate with areas of relatively
high biological productivity. The movement data suggested that close
to shore the dolphins moved along the continental slope, while some
dolphins travelled farther offshore along thermocline 'ridges' (Scott
and Chivers, 2009).
Seasonal migrations have been observed for the population in the
coastal waters of Japan. Here, spotted dolphins move north in summer
and probably concentrate at the northern boundary of the Kuroshiro
current. In winter they move south, reaching a migration peak in
late October and early November (Reyes, 1991, and ref. therein).
Direct catches: Only Japan takes large numbers of spotted
dolphins for human consumption in drive and harpoon fisheries. The
catch in 1982 was 3,799, and annual catches between 1995 and 2004
averaged 129 (Kasuya, 2007). The drive fishery for spotted dolphins
began in 1959 and is thought to have caused a slight decline in
the minimum age attainment of sexual maturity in females. Pantropical
spotted dolphins are also taken in hand-harpoon fisheries in the
Philippines, Laccadive Islands and Indonesia and Sri Lankan gillnet
and harpoon fisheries (e.g. Dolar et al. 1994). A drive fishery
at Malaita in the Solomon Islands took several hundred or thousands
of spotted dolphins annually in the 1960s and still operates. Small
numbers are taken in numerous small subsistence fisheries for dolphins
and whales around the world, e.g. at St Vincent in the Lesser Antilles
(Perrin and Hohn, 1994 and refs. therein).
Incidental catches: The tuna fishery in the eastern tropical
Pacific targets the pantropical spotted dolphin to catch yellowfin
and skipjack tuna that often swim below the herds. This ecological
association of tuna and dolphins is not clearly understood (Gerrodette,
2002). Wade (1995) estimated that 4.9 million dolphins were killed
by the purse-seine fishery over the fourteen-year period 1959-72,
an average of 347,082 per year. Nearly all of the fisheries kill
of pantropical spotted dolphins was of the northeastern stock, totalling
3.0 million (211,612 per year). In the early 1990's, the kill declined
to around 15,000 due to improved rescue techniques (Perrin and Hohn,
1994 and refs. therein).
Although tuna and dolphins are still herded and captured together
in the net, the crew attempt to release the dolphins by a procedure
called "backdown," while utilising various dolphin safety
gear. Though a great majority of the dolphins are released unharmed,
some die during the fishing operation. The Tuna-Dolphin Program
of the Inter-American Tropical Tuna Commission (IATTC) is charged
with monitoring this incidental mortality, studying its causes,
and encouraging fishermen to adopt fishing techniques which minimise
it. Analyses of observer data show that many factors cause dolphin
mortality, such as fishing areas; dolphin species and herd sizes;
environmental factors; gear malfunctions; and crew motivation, skill,
and decision-making. A combination of major and minor technological
developments, training in their use, better decision-making skills,
and constant pressure to improve performance led to a significant
progress: since 1986, dolphin mortality has been reduced by 97%
(Bratten and Hall, 1997). Recently, a series of factors such as
the presence of hazardous net conditions (net canopies and net collapses),
the duration of the backdown procedure (the primary method of releasing
dolphins from the net), the size and species composition of the
encircled dolphin herd and the amount of tuna encircled, were all
found to require improvements in order to consistently reduce dolphin
mortality per set (Lennert-Cody et al. 2004).
Gosliner (1999) reported that as the US brought dolphin mortality
by its fishermen under control in the 1980's, the numbers of dolphins
being killed again skyrocketed as a shrinking US fleet was replaced
by those from Mexico, Venezuela, and other nations. Through the
use of trade sanctions, and ultimately international co-operation,
dolphin mortality was reduced to levels believed to be biologically
insignificant by 1997 (0 dolphins in US fishery, ca. 3,000 in non-US
fisheries). Since the Inter-American Tropical Tuna Commission (IATTC)
implemented per-vessel mortality limits on the international fleet,
the combined annual mortality for all spotted dolphins in the ETP
has decreased greatly, e.g. to 373 in 2005 (IATTC 2006).
The estimates for offshore spotted dolphin mortalities in 2007
are 187 animals of the northeastern, 116 of the west-southern and
6 of the coastal stocks a total of 309 animals (IATTC, 2009).
Nevertheless, the use of dolphins to locate and catch tuna will
remain controversial as long as any of these cetaceans are killed
or injured in the process (Gosliner, 1999). Gerrrodette (2002) stated
that by 1999, there was no clear indication of a recovery for northeastern
offshore spotted dolphins, and the same assessment was repeated
in 2005. 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 spotted dolphin population dynamics (Gerrodette and Forcada,
The intense fishing pressure on tuna supports these hypotheses:
Schools of 1,000 or more dolphins are estimated to be set on approximately
once a week each on average, but such schools are estimated to represent
just under one tenth of the animals in the northeastern offshore
stock. Schools set on most often by tuna purse-seiners, containing
from about 250 to 500 dolphins, are estimated to be set on between
two and eight times each per year and are estimated to include approximately
one third of the stock. An estimated one half of the stock occurs
in schools smaller than 250 animals; schools of this size are estimated
to be set on less than twice per year each (Perkins and Edwards,
Aerial photographs taken between 1987 and 2003 show that the proportion
of adults with calves decreased steadily from 1987 to 2003. Annual
number of purse-seine sets on dolphins is a predictor of both proportion
with calves and length at disassociation. Because northeastern pantropical
spotted dolphins are the main species targeted by the fishery, the
link between fishing activity and both measures of reproductive
output indicates that the fishery still has population-level effects
beyond reported direct kill (Cramer et al. 2008; Wade et al. 2007;
Archer et al. 2004).
In other regions and fisheries, related mortality is fortunately
in the Hawaii-based longline fishery annual mortality and serious
injury for during 1994-2005 included one pantropical spotted dolphin
(Forney and Kobayashi, 2007).
In gillnet fisheries off Zanzibar (Unguja Island), from January
2000 to August 2003, 6 (4%) pantropical spotted dolphins were recorded
in 12 landing sites (Amir et al. 2005).
There has been no reported fishing-related mortality during 1998-2006
in the US Gulf of Mexico EEZ. Total annual estimated average fishery-related
mortality or serious injury to the western North Atlantic US EEZ
stock during 2001-2005 was 6 (CV=1) undifferentiated spotted dolphins.
(Waring et al. 2009).
Yang et al. (1999) reported incidental mortality from Chinese fisheries,
and Dolar et al. (1997) found that 4 of the 7 fishing villages surveyed
in the Philippines reported directed and/or incidental spotted dolphin
Culling: Dolphins and small whales of several species, including
S. attenuata, interfere in hook-and-line fisheries for squid and
yellowtail in the Iki Island region of Japan. Bounties have been
paid to fishermen for dolphins killed since 1957. During the period
1976-1982 a total of 538 spotted dolphins were killed. The effect
of these takes on the population is not known (Perrin and Hohn,
1994 and refs. therein).
Pollution: André (1988, in Perrin and Hohn, 1994)
and André et al. (1990a, 1990b) reported levels, somatic
distribution, and age-related changes in levels of Hg, Cd, Cr, Cu,
Mn, Ni, Se, Zn, sDDT and PCBs in pantropical spotted dolphins from
the eastern Pacific. Calmet et at. (1992, in Perrin and Hohn, 1994)
reported levels of radioactive isotopes of Pb, Cs and K in the same
specimens. Cockcroft et at. (1991, in Perrin and Hohn, 1994) reported
levels of seven organochlorine chemicals in four specimens from
Natal. In the waters of the Coiba archipelago, Panama, blubber levels
of HCB (hexachlorobenzene), tPCB (polychlorinated biphenyls) and
tDDT (dichlorodiphenyltrichloroethane) were 0.064, 2.30 and 6.4
mg / kg respectively. These levels are low and are not considered
to represent a threat to the S. attenuata population (Borrell et
a. 2004). In Taiwanese waters of the Taiwan Strait, muscle samples
from specimens collected in 1994-95 showed the highest mean concentration
(mg/kg wet wt.) of both total mercury (3.64 mg/kg wet wt) and organic-Hg
(2.79 mg/kg) (Chen et al., 2002).
Range states (Hammond et al., 2008) :
American Samoa; Anguilla; Antigua and Barbuda; Aruba; Australia;
Bahamas; Bangladesh; Barbados; Belize; Benin; Bermuda; Brazil; Brunei
Darussalam; Cambodia; Cameroon; Cape Verde; Cayman Islands; China;
Cocos (Keeling) Islands; Colombia; Congo; Congo, The Democratic
Republic of the; Cook Islands; Costa Rica; Côte d'Ivoire;
Cuba; Djibouti; Dominica; Dominican Republic; Ecuador; 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; Indonesia; Iran, Islamic
Republic of; Jamaica; Japan; Kenya; Kiribati; Liberia; Madagascar;
Malaysia; Maldives; Marshall Islands; Martinique; Mexico; Micronesia,
Federated States of; Morocco; Mozambique; Myanmar; Namibia; Nauru;
Netherlands Antilles; New Caledonia; New Zealand; 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; Senegal; Sierra Leone; Singapore;
Solomon Islands; Somalia; South Africa; Sri Lanka; Sudan; Suriname;
Taiwan, Province of China; Tanzania, United Republic of; Thailand;
Timor-Leste; Togo; Tonga; Trinidad and Tobago; USA; Viet Nam; Virgin
Islands, British; Virgin Islands, U.S.; Wallis and Futuna; Western
The eastern tropical Pacific and south-eastern Asian populations
of S. attenuata are listed in Appendix II of CMS. The species
is listed as "Least Concern" by the IUCN Hammond et al.
2008). The species is listed in Appendix II of CITES.
The species also occurs in southern South America, so please see
Hucke-Gaete (2000) for further recommendations in Appendix
1. General recommendations on Southeast Asian stocks can be
found in Perrin et al. (1996) in Appendix
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