Pantropical Spotted Dolphin (Stenella attenuata)
The Pantropical spotted dolphin (Stenella attenuata) is a species of dolphin found in all the world's temperate and tropical oceans. The species was beginning to come under threat due to the killing of millions of individuals in tuna purse seines. The 1980s saw the rise of "dolphin-friendly" tuna capture methods in order to save millions of the species in the eastern Pacific Ocean and it is now one of the most abundant dolphin species in the world.
The species was first described by John Gray in 1846. Gray's initial analysis included the Atlantic spotted dolphin in this species. They are now regarded as separate. Both the genus and specific names come from Latin words meaning thin or thinning.
There are three subspecies recognised in Rice's 1998 survey of cetacean taxonomy. Two of these have not been formally named
S. a. subspecies A, the off-shore form found in the eastern Pacific
S. a. subspecies B, a form found around the Hawaiian islands.
S. a. graffmani, coastal form found from Mexico to Peru
The Pantropical Spotted Dolphin varies significantly in size and colouration throughout its range. The most significant division is between coastal and pelagic varieties. The coastal form is larger and more spotted. (These two forms have been divided into subspecies only in eastern Pacific populations. The most distinctive feature of the Pantropical spotted dolphin is, as its name suggests, the spots that speckle the body of adults. Newborn calves are unspotted, but by adulthood, a varying amount of light spots cover the upper surface, and dark spots cover the dolphin’s underside. Underneath this spotting, the slender, stream-lined body is grey, with a darker grey cape extending back from the head and sweeping low underneath the dorsal fin. The dorsal fin is narrow and sickle-shaped.. The long, thin beak of the Pantropical spotted dolphin is separated from the melon by a distinct crease. In most adults, the tip of the beak is white . Male Pantropical spotted dolphins are slightly larger than females. A subspecies of the Pantropical spotted dolphin is recognised, Stenella attenuate graffmani, which inhabits more coastal areas and can be distinguished in appearance by its larger, stockier body, thicker beak and more extensive spotting.
Spots are key defining characteristics in adults, though immature individuals are generally uniformly coloured and susceptible to confusion with the bottlenose dolphin. Populations around the Gulf of Mexico may be relatively spot-free even in adulthood. In the Atlantic, confusion is possible with the Atlantic spotted dolphin.
Broadly speaking the dolphin has a long thin beak. The upper and lower jaws are darkly coloured but are separated by thin white "lips". The chin, throat and belly are white to pale grey with a limited amount of spots. The flanks are separated into three distinct bands of colour — the lightest at the bottom, followed by a thin grey strip in the middle of the flank and a dark grey back. The tall concave dorsal fin is similarly coloured. The thick tail stock matches the colour of the middle band.
The Pantropical spotted dolphin is very active and is prone to making large splashy leaps from the sea. It is a common breacher and will often clear the water for a second or more. Bow-riding and other play with boats is common.
In the eastern Pacific, the Dolphin is often found swimming with yellowfin tuna (hence the problem with dolphin deaths caused by tuna fishing — see the human interaction section). However they do not feed on that fish. In fact the two species have a similar diet of small epipelagic fish. In other areas the species may also feed on squid and crustaceans.
Population and distribution
The Pantropical spotted dolphin, as its very name implies, is found across all tropical and sub-tropical waters around the world — roughly speaking all oceans and seas between 40° N and 40° S. The total world population is in excess of three million — the second most abundant cetacean after the bottlenose dolphin — of which two million are found in the eastern Pacific. However, this represents a decrease from at least 7 million since the 1950s.
Centres of highest population density are the shallow warmest waters (water temperature in excess of 25 °C). There is also a tendency for groups to concentrate where there is a high temperature gradient.
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) al. 2005).
American Samoa (American Samoa); Argentina; Australia; Bahamas; Bangladesh; Belize; Brazil; Cambodia; Cape Verde; Cayman Islands; China; Colombia; Comoros; Cook Islands; Costa Rica; Côte d'Ivoire; Cuba; Djibouti; Dominica; Dominican Republic; Ecuador; Egypt; El Salvador; Equatorial Guinea; Fiji; French Polynesia; Gabon; Ghana; Grenada; Guadeloupe; Guam; Guatemala; Guinea; Haiti; Honduras; Hong Kong; India; Indonesia; Jamaica; Japan; Kenya; Kiribati; Madagascar; Malaysia; Maldives; Marshall Islands; Martinique; Mauritania; Mauritius; Mexico; Mozambique; Myanmar; Netherlands Antilles; New Caledonia; New Zealand; Nicaragua; Northern Mariana Islands; Oman; Pakistan; Panama; Papua New Guinea; Peru; Philippines; Puerto Rico; Réunion; Saint Helena, Ascension and Tristan da Cunha; Saint Lucia; Saint Vincent and the Grenadines; Saudi Arabia; Senegal; Seychelles; Singapore; Solomon Islands; Somalia; South Africa; Sri Lanka; Taiwan, Province of China; Tanzania, Thailand; Togo; Tonga; Trinidad and Tobago; Turks and Caicos Islands; Tuvalu; United Arab Emirates; United States; Uruguay; Vanuatu; Venezuela, Bolivarian Republic of; Vietnam
FAO Marine Fishing Areas: Native:
Atlantic – eastern central; Atlantic – northwest; Atlantic – southeast; Atlantic – southwest; Atlantic – western central; Indian Ocean – eastern; Indian Ocean – western; Pacific – eastern central; Pacific – northwest; Pacific – southeast; Pacific – southwest; Pacific – western central
In the eastern Pacific, there were an estimated 228,038
coastal spotted dolphins in 2000 (CV=34%; Gerrodette and Forcada 2002a). The
north-eastern offshore spotted dolphin (the form most affected by the ETP tuna
fishery) numbered about 737,000 in 2003 (CV=15%; Gerrodette et al. 2005), a
reduction of 76% from original size in 1959 (Reilly et al. 2005). This
population is not showing clear signs of recovery despite the dramatic decline
in mortality in recent years (Gerrodette and Forcada 2005). The western/southern
offshore stock (which is less affected by the fishery) numbered about 876,075 in
2000 (CV=31%; Gerrodette and Forcada 2002b). In Hawaiian waters, there are an
estimated 8,978 (CV=48%) (Barlow 2006). About 438,000 inhabited Japanese waters
in the early 1990s (Miyashita 1993). There are estimated to be 34,067 (CV=18%)
in the northern Gulf of Mexico (Mullin 2006), and 4,439 (CV=49%) along the east
coast of the United States (Waring et al. 2006). 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.
Population Trend: Unknown
The gregarious Pantropical spotted dolphin forms schools that
can range in size from less than one hundred to thousands of individuals;
although it has been observed that these impressively large herds are less
common in the eastern tropical Pacific than they once were, as exploitation
takes its toll. The Pantropical spotted dolphin is well known for its tendency
to associate with schools of tuna in this region. While this may be due to an
overlap in diet, other reasons for this association have also been suggested,
such as increased protection from predators, as there is safety in numbers. In
the eastern Pacific the Pantropical spotted dolphin is an inhabitant of the
tropical, equatorial and southern subtropical water masses. The waters in which
the animal 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 similar
waters south of the Equator that expand and contract greatly with season and
year to year (Perrin and Hohn 1994). In the Atlantic, S. attenuata is primarily
a dolphin of the high seas and oceanic islands, but in the eastern Pacific a
large-bodied subspecies occurs along the coast from Mexico to Peru. Detailed
analysis of oceanographic correlates of distribution will be necessary in order
to understand fully the habitat requirements of these pelagic dolphins, often
the most conspicuous elements of tropical cetacean communities around the world
(Ballance and Pitman 1998).
Offshore spotted dolphins feed largely on small epi- and mesopelagic fishes, squids, and crustaceans that associate with the deep scattering layer (Robertson and Chivers 1997). In some areas, flying fish are also important prey. The diet of the coastal form is poorly known, but is thought to consist mainly of larger fishes, perhaps mainly bottom-living species (Perrin 2001, 2002).
This ocean mammal is a fast swimmer that often engages in a range of aerial acrobatics and will frequently ride the bow waves of boats, except for in tuna fishing grounds where it has learnt to avoid vessels. Juveniles in particular are known to make astoundingly high vertical leaps out of the water. The Pantropical spotted dolphin feeds mainly at night on small fish, squid and crustaceans that rise to near the surface at dusk, with flying fish forming a major part of the diet in some regions. In turn, this dolphin becomes prey for the killer whale (Orcinus orca) and a number of sharks.
While the breeding system of this species is not known, it is possible that it may be promiscuous, like that of the closely related spinner dolphin (Stenella longirostris) (2). Every two to three years, mature female Pantropical spinner dolphins give birth to a calf, after a gestation period of around 11 months. The calf is nursed for between one and two years. Females reach sexual maturity at 9 to 11 years, while males become sexually mature between the ages of 12 and 15 years. Birth length is 80-90 cm. Adults are about 2.5 m long and weigh 120 kg. Sexual maturity is reached at 10 years in females and 12 years in males. Lifespan is approximately 40 years.
Offshore spotted dolphins bore the brunt of the massive
dolphin kill by tuna seiners from the late 1950s to the 1980s in the eastern
Pacific (although the coastal subspecies was also impacted). For example, in the
period 1959 to 1972, nearly five million dolphins were killed, and of this
number, about three million were from the north-eastern offshore population
(Wade 1995). 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 only 373 in 2005 (IATTC 2006). Although current mortality is greatly reduced,
the north-eastern form appears to be recovering very slowly, if at all, and
potential factors such as fishery-related stress, unobserved mortality due to
calf separation and orphaning during fishing operations (Archer et al. 2001),
possible mortality by small vessels that do not carry observers, under-reporting
of mortality, and ecosystem change, have been suggested as possible reasons for
the species’ slow recovery (Gerrodette and Forcada 2005).
Spotted dolphins are also taken incidentally in local fisheries along the Central American coast (Palacios and Gerrodette 1996). Yang et al. (1999) also reported incidental mortality in Chinese fisheries, and Dolar 1994 found incidental spotted dolphin takes in the Philippines. An unknown but suspected large number of Pantropical spotted dolphins are taken by the large-mesh pelagic driftnet fishery off eastern Taiwan (J. Wang pers. comm.).
Japan takes large numbers of spotted dolphins for human consumption. The catch in 1982 was 3,799, and annual catches between 1994 and 1997 ranged from 23 to 449 (Perrin 2002). Between 1995 and 2004, the average annual catch was 129 animals (Kasuya 2007). The drive fishery for spotted dolphins began in 1959 and is thought to have caused a slight decline in the minimum age at attainment of sexual maturity in females (Kasuya 1985).
Pantropical spotted dolphins are also taken in hand-harpoon fisheries in the Philippines (Dolar et al. 1994); in Taiwan, where it is the locally preferred species of cetacean for human consumption (J. Wang pers. comm.); and regularly or opportunistically by gillnet and harpoon in India and Sri Lanka (Perrin and Hohn 1994). Drive hunts at Malaita in the Solomon Islands took several hundred or thousands of spotted dolphins annually in the 1960s; the hunts continue at present (Ross et al. 2003, Kahn 2006). 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 Lamalera in Indonesia (Kahn 2004). Most of these kills have not been adequately monitored and the effects on the subpopulations are usually not known.
Dolphins and small whales of several species, including S. attenuata, putatively interfere in hook-and-line fisheries for squid and yellowtail in the Iki Island region of Japan (Kishiro and Kasuya 1993). 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 regional population is not known (Perrin and Hohn 1994).
The species is listed in Appendix II of CITES.
Spotted dolphins, as with other species impacted by the ETP tuna purse-seine fishery, are managed both nationally by the coastal countries and internationally by the IATTC. The IATTC has imposed annual stock mortality limits on each purse seine and promulgated regulations regarding the safe release of dolphins (Bayliff 2001).
As the species comprises several subspecies and regional populations, the conservation status of each of these should be assessed separately since the available estimates of abundance and removals suggest that some of them may fall into a Threatened category.