Stenella attenuata (Gray, 1846)

English: Pantropical spotted dolphin
German: Schlankdelphin
Spanish: Delfín manchado
French: Dauphin tacheté

Family Delphinidae

Stenella attenuata © Würtz-Artescienza (see "links")

1. Description

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).

2. Distribution

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 map):
Tropical and warm-temperate waters of the Atlantic, Pacific and Indian Oceans.

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. 2005).

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. clymene.

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 after dark.

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).

5. Migration

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).

6. Threats

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, 2005).

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, 1999).

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 less important:
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 takes.

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).

7. Remarks

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 Sahara; Yemen.

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 2.

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© Boris Culik (2010) Odontocetes. The toothed whales: "Stenella attenuata". UNEP/CMS Secretariat, Bonn, Germany. http://www.cms.int/reports/small_cetaceans/index.htm
© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.