Last modified: Thursday September 24th, 1999
Stenella attenuata (Gray, 1846)

Pantropical spotted dolphin

Taxonomy

The taxonomy of the spotted dolphins has been quite confused and still is the subject of discussions. Currently 2 species of spotted dolphins are recognized: the pantropical spotted dolphin, Stenella attenuata and the Atlantic spotted dolphin, Stenella frontalis (Perrin et al, 1987). A number of specimens have been described under a wide variety of names. S. attenuata has a number of valid synomyms (Delphinus velox, Delphinus pseudodelphis, Delphinus brevimanus and Steno attenuata), but Perrin et al (1987) argue that S. attenuata should be the accepted name for the species. S. frontalis has been described as Delphinus frontalis, but the former is accepted as the valid species name. The spotted dolphins in the coastal Eastern Tropical Pacific (ETP) have also been referred to as S. attenuata graffmani, a separate subspecies (Perrin and Hohn (1994), Minasian et al (1984)).

Appearance

At birth, this dolphin is unspotted, with a strongly defined cape, a flipper stripe ending at the mouth and a peduncle with a dark upper part and a lighter lower part. Spots appear ventrally first and later light dorsal spots appear. The dark ventral spots increase in size and number and merge in some areas. The same happens to the light dorsal spots. There is a lot of regional variation in coloration and spotting. Coastal dolphins are in general more heavily spotted than pelagic dolphins. In some areas, such as Hawaii and St. Helena, spotting is poorly developed and is only visible at close range.

There is also regional variation in size. The coastal ETP dolphins are the largest (average 223 cm for males and 207 cm for females). The oceanic ETP dolphins are the smallest at 200 cm for males and 187 cm for females. The pantropical spotted dolphins in the Atlantic are on average 215 resp. 188 cm long (Perrin et al, 1987). They have 40-45 pairs of small conical teeth per jaw (Minasian et al, 1984).

Distribution

This species can be found worldwide in tropical and some subtropical waters. There are records of stranded specimens in Alaska and New Zealand. In the ETP this dolphin usually ranges from 25°N to 17°S. It occurs around the Pacific islands and is seasonally found around Japan. In the Indian Ocean it ranges south to about 33°S and it has been recorded in the Red Sea as well. There is not much information about its distribution in the Atlantic. It by and large seems to occur in the same areas as the Atlantic spotted dolphin, Stenella frontalis (tropical, subtropical and warm temperate waters of the Atlantic) (Klinowska, 1991).

Population dynamics and life history

Females reach sexual maturity at 9-11 years, males at 12-15 years. The gestation period is between 11 and 11.5 months. The calving interval is about 3 years. Lactation lasts 1-2 years. The pregnancy rate is estimated at 0.3. There are no direct estimates of natural mortality. A 9.8% mortality (ASR: 0.912) was derived indirectly. Other methods yielded estimates of 3, resp. 6.9% mortalities for immature females and males. Maximum longevity may be as high as 45 years (Perrin et al, 1987). Gross reproductive rates are estimated at 11-12% for the ETP and 10% for the Japanese waters (Perrin and Hohn, 1994).

Population status

The population of pantropical spotted dolphins has decreased as a result of a large incidental mortality in the tuna fisheries. The original ETP population was probably about 4.8 million. In 1979, this had been reduced to 1.7 million, a 65% decline. The northern ETP stock in 1992 was estimated to be 738,100 (574,800-989,600) animals, 1,299,300 (910,100-2,121,400) for the western and southern stock and 29,000 (16,200-87,000) for the coastal (S. a. graffmani) stock (Perrin and Hohn, 1994). There are no abundance estimates for populations in other areas.

Exploitation

There is a directed fishery for this species for human consumption in Japan. In 1982 and 1983, 3,799 and 2,945 animals were taken. For 1992, 1993 and 1994 the takes were 637, 565 and 449 animals (IWC, 1996). Small numbers of this species are taken in local subsistence fisheries in several areas.

A large number of spotted dolphins has been killed as incidental catches in the ETP tuna fisheries. The following numbers were taken from the National Research Council (1992). In the period 1959-1972 an estimated 350,000-653,751 dolphins were killed per year.

Dolphin Mortality in Tuna Purse Seine Fisheries
Year Total dolphin mortality Mortality in US fleet
1974 103,000-175,000  
1975 110,000-194,000  
1976 108,000-128,000  
1977 22,000-51,000  
1978 12,000-31,000  
1979 21,426 17,938
1980 31,970 15,305
1981 35,089 18,780
1982 29,104 23,267
1983 13,493 8,513
1984 40,712 17,732
1985 58,847 19,205
1986 133,174 20,692
1987 99,187 13,992
1988 78,927 19,712
1989 96,979 12,643
1990 52,531 5,083

Note that these figures are total dolphin mortalities in the tuna fisheries. It includes incidental catches of pantropical spotted dolphins, spinner dolphins and common dolphins. The spotted dolphins are killed most often in these fisheries. Averages yearly mortalities for 1986-1990 were: 33,900 spotted dolphins, 12,400 spinner dolphins and 4,900 common dolphins. The IWC(1996) reports incidental catches of pantropical spotted dolphins for 1992, 1993 and 1994 as 6,531-6,790, 1,896 and 2,160.

Feeding

In the ETP, this dolphin commonly associates with yellowfin tuna, spinner dolphins (Stenella longirostris) and sea birds. It is not known why these associations occur, although foraging efficiency, protection from predators and orientation in the "pelagic void" have been suggested (Perring and Hohn, 1994). Analysis of stomach contents revealed a large variety of small fish species, squid, worms and crab larvae. Pregnant females appear to have different feeding habits than lactating females.

References

International Whaling Commission (1996)
Report of the sub-committee on small cetaceans. Rep. Int. Whal. Commn. 46:160-179
Klinowska, M. (1991)
Dolphins, Porpoises and Whales of the World. The IUCN Red Data Book. IUCN, Gland, Switzerland.
Minasian, S.M., Balcomb III, K.C. and Foster, L. (1984)
The world's whales. The complete illustrated guide. Smithsonian Books, Washington D.C.
National Research Council (1992)
Dolphins and the Tuna Industry. National Academy Press, Washington D.C.
Perrin, W.F. and Hohn, A.A. (1994)
Pantropical spotted dolphin Stenella attenuata In: S.H. Ridgway and R. Harrison: Handbook of Marine Mammals. Volume 5: The first book of dolphins, pp. 71-98. Academic Press, San Diego
Perrin, W.F., Mitchell, E.D., Mead, J.G., Caldwell, D.K., Caldwell, M.C., van Bree, P.J.H. and Dawbin, W.H. (1987)
Revision of the spotted dolphins, Stenella spp. Marine Mammal Science 3(2):99-170


Note on the pictures

The photographs on this page were take on a whale-watching tour with Dan McSweeney off the Big Island of Hawaii in November 1993.


Back to the Dolphin Page Back to the Main Page