Pantropical spotted dolphin (Stenella attenuata)

Also known as: bridled dolphin, narrow-snouted dolphin
  
French: Dauphin Tacheté Pantropical
Spanish: Delfín Manchado
KingdomAnimalia
PhylumChordata
ClassMammalia
OrderCetartiodactyla
FamilyDelphinidae
GenusStenella (1)
SizeLength: 166 – 257 cm (2)
Weightup to 119 kg (2)

The pantropical spotted dolphin is classified as Least Concern (LC) on the IUCN Red List (1) and listed on Appendix II of CITES (3).

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 (2). 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 (2) (4). The dorsal fin is narrow and sickle-shaped (4). The long, thin beak of the pantropical spotted dolphin is separated from the melon by a distinct crease (2) (4). In most adults, the tip of the beak is white (2). Male pantropical spotted dolphins are slightly larger than females (2). 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 (4).

The pantropical spotted dolphin is a widely distributed species, occurring in all oceans between 40°N and 40°S (4) (5). Subspecies S. a. graffmani occurs in the eastern tropical Pacific from Mexico to northern Peru (2).

As its name suggests, this dolphin inhabits tropical and warm-temperate seas, where it can be found in both near-shore and oceanic habitats (2).

The gregarious pantropical spotted dolphin forms schools that can range in size from less than one hundred to thousands of individuals (4); 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 (4). 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 (2), as there is safety in numbers.

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 (2) (4). Juveniles in particular are known to make astoundingly high vertical leaps out of the water (2). 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 (2).

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 (2). 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 (2).

The tendency of the pantropical spotted dolphin to associate with tuna schools has been this species’ downfall in the eastern tropical Pacific. Fishermen take advantage of this association to help them locate and catch tuna more efficiently (4), and will intentionally capture both tuna and dolphins together, then release the dolphins from the net (6). Either the dolphin is killed in the process, or this can lead to a single dolphin being chased, captured and released many times during its lifetime, causing a great deal of stress (6).

In the eastern tropical Pacific, tuna fisheries have killed millions of dolphins since the 1960s (2) (5), reducing some stocks to a fraction of their former size (2). Today, mortality rates have been greatly reduced, yet the populations are not recovering from this devastating exploitation as well as could be expected; the stresses of being repeatedly chased and captured, as well as separation of mothers from their young, are possible reasons cited for the slow growth of the populations (5).

Pantropical spotted dolphins are also hunted intentionally in some areas, such as in Japan, Solomon Islands and the Philippines, where they are caught for human consumption and fishing bait (2) (5). They are also taken as bycatch in many fisheries in developing countries around the globe, and in some countries, such as Peru, Ghana and the Philippines, the bycatch is kept and used for human consumption. This has lead to the evolution of directed catches as the markets for the meat develop, resulting in a growing conservation problem (2).

Once it had been noticed what enormous numbers of dolphins were being killed in tuna fisheries in the eastern Pacific, actions were implemented to try and reduce these unnecessary deaths. In the 1970s, the United States employed laws and measures aiming to reduce dolphin bycatch to levels approaching zero through improved fishing methods (6). In 1979, the Inter-American Tropical Tuna Commission began a dolphin conservation program, and soon, the idea of dolphin-safe tuna became popular, with the United States only allowing the sale of dolphin-safe tuna by 1994. In 1999, the International Dolphin Conservation Program Agreement came into force, which meant the major tuna fishing countries in the eastern Pacific were bound to certain measures such as having observers on boats and strict dolphin-mortality limits. As a result of all of these actions, dolphin mortality has fallen drastically (6), which will hopefully give the pantropical spotted dolphins, and other dolphin species, a much needed opportunity to recover.

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Authenticated (17/11/08) by William F. Perrin, Senior Scientist for Marine Mammals, National Marine Fisheries Service, Southwest Fisheries Science Centre.
http://swfsc.noaa.gov

  1. IUCN Red List (November, 2008)
    http://www.iucnredlist.org
  2. Perrin, W. (2008) Pantropical spotted dolphin. In: Perrin, W.F., Würsig, B. and Thewissen, J.G.M. (Eds) Encyclopedia of Marine Mammals. Second Edition. Academic Press, London.
  3. CITES (April, 2008)
    http://www.cites.org
  4. Jefferson, T.A., Leatherwood, S. and Webber, M.A. (1993) FAO Species Identification Guide. Marine Mammals of the World. FAO, Rome.
  5. Reeves, R.R., Smith, B.D., Crespo, E.A. and Notarbartolo di Sciara, G. (2003) Dolphins, Whales and Porpoises: 2002–2010 Conservation Action Plan for the World’s Cetaceans. IUCN/SSC Cetacean Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK.
  6. Gerrodette, T. (2008) Tuna-dolphin issue. In: Perrin, W.F., Würsig, B. and Thewissen, J.G.M. (Eds) Encyclopedia of Marine Mammals Second Edition. Academic Press, London.