Stenella attenuata
Creators: name: Russell A. Mittermeier
name: Don E. Wilson
Publisher: name: Lynx Edicions
Year: 2014
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Mammalia
Cetacea
Delphinidae
Stenella
Stenella attenuata

description: 17.
Pantropical Spotted Dolphin
Stenella attenuata
French: Dauphin bridé / German: Schlankdelfin / Spanish: Delfin manchado tropical
Other common names: Bridled Dolphin, Narrow-snouted Dolphin, Slenderbeaked Dolphin, Spotted Porpoise, Whitespotted Dolphin; Offshore Pantropical Spotted Dolphin (attenuata); Coastal Pantropical Spotted Dolphin, Graffman’s Dolphin (graffmani)
Taxonomy. Steno attenuatus Gray, 1846,
type locality unknown. Later designated by J. E. Gray in 1850 as “Inhab. Cape Horn.”
The genus Stenella is likely polyphyletic, and taxonomy of its member species probably will be revised in the near future.
According to molecular studies, S. frontalis 1s most closely related to S. coeruleoalba and S. clymene, followed by Delphinus and Tursiops aduncus. These species appear to be more closely related to each other than any of them are to 1. truncatus, S. attenuata, or S. longirostris. Similarities in appearance between S. attenuata and S. frontalis are thus likely due to homoplasy or pleisiomorphy. Two subspecies recognized.
Subspecies and Distribution.
S.a.attenuataGray,1846—Pantropicalinalloceansbetweenc.40°Nand¢.40°S,includingRedSea,ArabianSea,PersianGulf,andBayofBengal,butexcludingtheMediterraneanSea.
S. a. graffmani L.onnberg, 1934 — near-shore coastal waters of the Pacific coast of Mexico, Central America, and South America to Peru.
Descriptive notes. Total length 160-260 cm (males) and 160-240 cm (females); weight up to 119 kg. Neonates are 80-85 cm long and weigh c.10-15 kg. The “Coastal Pantropical Spotted Dolphin” (S. a. graffmani) tends to be slightly larger in size than the “Offshore Pantropical Spotted Dolphin” (S. a. attenuata). Overall, the Pantropical Spotted Dolphin has slender body shape, but the Coastal Pantropical Spotted Dolphin tends to be more robust. Beak is long and skinny, but it is slightly thicker in the Coastal Pantropical Spotted Dolphin. Dorsal fin is moderately sized, narrow, and falcate with pointed tip. Flippers are slender and curved, also with pointed tips. Adult males may have slight anal protuberance. The Pantropical Spotted Dolphin has countershaded color pattern underlying numerous white spots. Skin is pale gray except for a dark-gray dorsal cape that sweeps low over flanks. Offshore Pantropical Spotted Dolphins have varying degrees of spotting, and some adults may have very little to none at all. Spotting is generally heaviest in Coastal Pantropical Spotted Dolphin, sometimes obscuring cape entirely. Lips and beak tip are usually white. There are a few dark gray stripes on face and head: “mask” stripe across front of melon above beak between two dark rings around eyes, and stripe extending from anterior base of flipper to corner of mouth and onto lower jaw. Youngest individuals do not have spots, and areas that are pale gray on adults are white on young. As they grow, white areas darken to pale gray, and dark spots develop first on ventral body, followed by white spots on flanks and back. As individuals age, dark ventral spots may fuse and fade, leaving only white flank and dorsal spots. There are 34-48 pairs of slender, conical teeth in each jaw.
Habitat. Offshore and around oceanic islands. The Pantropical Spotted Dolphin is the most abundant delphinid species in the eastern tropical Pacific Ocean. The Offshore Pantropical Spotted Dolphin tends to be more abundant around the Hawaiian Islands, some Caribbean islands, Philippines, and Taiwan. In these regions, they may be observed in shallower waters. Overall, the Pantropical Spotted Dolphin is more abundant in lower latitudes of its distribution. Offshore populations prefer tropical surface water characterized by a shallow thermocline (less than 50 m deep), sea-surface temperatures more than 25°C, and salinity less than 34 ppt. In the south-western Atlantic Ocean, the Offshore Pantropical Spotted Dolphin is found beyond the continental shelf in waters more than 850 m deep, and in the Gulf of Mexico, they are found in continental slope waters more than 1000 m deep.
Food and Feeding. The Pantropical Spotted Dolphin is known to feed on small epipelagic fish, cephalopods, and crustaceans. It is an opportunistic feeder whose diet varies by season and geographical region. The Offshore Pantropical Spotted Dolphin seems to prefer lanternfish (Myctophidae), flying squid (Ommastrephidae), and flying fish (Exocoetidae). They forage nocturnally and exploit the rising deep scattering layer. Around the Hawaiian Islands, nocturnal foraging activity is characterized by deeper dives than those observed during the day. At night, average dive is 57 m, but it is only 12:8 m during the day. Off Taiwan, diet of the Pantropical Spotted Dolphin is known to include some mesopelagic species. The Coastal Pantropical Spotted Dolphin seems to specialize more on larger benthic species. Natural predators may include the Killer Whale (Orcinus orca), the False Killer Whale (Pseudorca crassidens), the Pygmy Killer Whale (Melon-headed Whale), and the Short-finned Pilot Whale (Globicephala macrorhynchus). The Pantropical Spotted Dolphin associates with yellowfin tuna (Thunnus albacares) in the north-eastern tropical Pacific Ocean, which may be related to foraging efficiency.
Breeding. The Pantropical Spotted Dolphin may breed throughout the year, but breeding peaks in spring and autumn. Gestation lasts c¢.11-5 months, and young are weaned at 1-2 years of age. Breeding females will typically give birth once every 2-3 years. Females reach sexual maturity at 9-11 years of age, and males at 12-15 years.
Activity patterns. The Pantropical Spotted Dolphin is a fast swimmer (sprint swims may exceed 22 km/h), and itis known to be aerially active. Breaching and side slapping are common, particularly among juveniles. Pantropical Spotted Dolphins will also readily approach boats and bow-ride, but they have learned to avoid boats in regions frequented by tuna fisheries. Dives can last up to 3-4 minutes.
Movements, Home range and Social organization. The Pantropical Spotted Dolphin is gregarious, and group sizes range from a few to several thousand individuals. Group sizes of the Coastal Pantropical Spotted Dolphin tend to be smaller (less than 100) than those of the Offshore Pantropical Spotted Dolphin. Average group sizes from different regions range from 90 individuals to 170 individuals. Sex and age classes may segregate into smaller subgroups within a large group. Mother—offspring pairs, adult males, and juveniles tend to remain within their own subgroups. In the eastern tropical Pacific Ocean, the Pantropical Spotted Dolphin may associate with yellowfin tuna, the Spinner Dolphin (S. longirostris), and skipjack tuna (Katsuwonus pelamas). Tuna fishermen sometimes take advantage of these tuna—dolphin associations. Immature Pantropical Spotted Dolphins and younger adults tend not to associate with tuna as often as full adults do. Tuna association also occurs in the western Indian Ocean. Off Hawaii, interactions between Pantropical Spotted Dolphins and Spinner Dolphins include traveling, aggression, and sexual activity. Little is known about migration and movement patterns of either species. Off coastal Japan, Pantropical Spotted Dolphins are known to migrate seasonally between the northern boundary of the Kuroshiro Current in summer and southern Japan in October-November.
Status and Conservation. CITES Appendix II. Classified as Least Concern on The [UCN Red List. The Coastal Pantropical Spotted Dolphin has not yet been assessed independently. Abundance estimates for the Pantropical Spotted Dolphin total more than 2-5 million, and there are still populations that have not yet been assessed. The Offshore Pantropical Spotted Dolphin in the north-eastern tropical Pacific Oceanhas declined 76% overthe last three generations (c.70 years), but this decline has since stopped and did not contribute to significant overall global decline. Nevertheless, this population does not appear to be recovering. The most recent estimate for this population was 737,000 individuals. Lack of recovery could be due to fishery-related stress and an underreporting of current catch rates, offspring separation or orphaning, or ecosystem change. Abundance of Coastal Pantropical Spotted Dolphins is estimated at 228,038 individuals. The population of Pantropical Spotted Dolphins in the south-western Pacific Ocean consists of 876,075 individuals. There are 8978 individuals off Hawaii, 34,067 individuals in the Gulf of Mexico, and ¢.15,000 individuals off the Philippines. Separate assessments of populations and subspecies have been recommended, given that some, such as the population in the north-eastern tropical Pacific Ocean, will likely have a threatened status at this scale. Japan has taken large numbers of small cetaceans in drive fisheries, including the Pantropical Spotted Dolphin, for human consumption since at least 1959. In 1995-2004, annual catch rate was c.129 individuals, and this is thought to have influenced a decrease in the age of female maturity in the local population of the Pantropical Spotted Dolphin. The Pantropical Spotted Dolphin is also taken by harpoon and gillnets in the Philippines, Laccadive Islands, Indonesia, India, and Sri Lanka. Taiwanese pelagic driftnet fisheries also likely have high incidental catch rates, but these fisheries are not currently monitored. A drive fishery in the Solomon Islands has taken several hundred to several thousand Pantropical Spotted Dolphins yearly since the 1960s and continues today. The now depleted population of Pantropical Spotted Dolphin in the north-eastern tropical Pacific Ocean was once heavily caught in tuna purse-seine fisheries due to tuna—dolphin associations. About 4-9 million Pantropical Spotted Dolphins were estimated killed in this fishery in 1959-1972. This kill was reduced to ¢.15,000 ind/year during the 1990s due to improved dolphin rescue procedures. Implementation of mortality limits by the Inter-American Tropical Tuna Commission also greatly reduced incidental catch rates. In 2005, only 373 Pantropical Spotted Dolphins were incidentally caught in these fisheries. Off Iki Island, Japan, fishermen are given bounties for killing dolphins to keep them from interfering with long-line fisheries. In 1976-1982, 538 Pantropical Spotted Dolphins were killed in these culls. Incidental catch does not appear to be a major threat in other areas. Tourism may be a growing problem in some regions, such as Costa Rica, where presence of dolphin-watching vessels is known to reduce time Pantropical Spotted Dolphins spend feeding and resting.
Bibliography. Archer & Robertson (2004), Archer et al. (2004), Baird et al. (2001), Cramer et al. (2008), Escorza-Trevino et al. (2005), Gerrodette & Forcada (2005), Gray (1850), Hammond et al. (2008;j), LeDuc et al. (1999), Montero-Cordero & Lobo (2010), Noren & Edwards (2007), Perrin (2009a), Perrin & Hohn (1994), Psarakos et al. (2003), Wade et al. (2007), Wang & Yang Shihchu (2007).
description: Published as part of Russell A. Mittermeier & Don E. Wilson, 2014, Delphinidae, pp. 410-526 in Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona :Lynx Edicions on pages 410-526, DOI: 10.5281/zenodo.6610922