Spotted dolphin - Stenella attenuata and S. frontalis

Spotted dolphins include two tropical species with widely varying habitats, external appearance, and habits.  However, it is likely that there will be further revision of their taxonomy as more becomes known. Currently, the Pantropical spotted dolphin (Stenella attenuata), is found in the Pacific, Atlantic and Indian Oceans, as well as the Persian Gulf and Red Sea; and the Atlantic Spotted dolphin (Stenella frontalis), is limited warm-water regions of the Atlantic.  Pantropical spotted dolphins are further divided into two recognized subspecies, the offshore spotted dolphin (S.a. attenuata), which has a global distribution, and the Coastal spotted dolphin (S.a. graffmani)¹, which is only found along the Pacific coast of Mexico and Central America.  To further complicate matters, these dolphins are closely related to, and sometimes difficult to distinguish from many of the species with which they share their range and often form large mixed species groups.  Pantropical spotted dolphins are best known for their longstanding association with the tuna fishing industry in the Eastern Tropical Pacific, which caused significant population declines in the past

Distribution

Pantropical spotted dolphins (Stenella attenuata) are found in the Pacific, Altantic and Indian Oceans, as well as the Persian Gulf and Red Sea.  They are generally limited to latitudes between 40°S and 40°N.  They are mostly found in deep offshore waters, although they can be observed around oceanic islands or other coastlines where there is deep water near to the coast (such as Central America and Mexico, the Hawaiian Islands, Tawian and the Philippines)^2-4. The Atlantic Spotted dolphin (Stenella frontalis) is limited to warm-water regions of the Atlantic.   Both species are absent in the Mediterranean Sea.^3,5,6. In the Bahamas a population of Atlantic spotted dolphins unusually spends a lot of time over sand flats in water only 6-12 m deep, rendering them more available for detailed study than most other spotted dolphins⁶.

Pantropical spotted dolphins are native to the following countries and territories: American Samoa; Argentina; Australia; Bahamas; Bangladesh; Belize; Bonaire, Sint Eustatius and Saba (Saba, Sint Eustatius); Brazil; Cambodia; Cape Verde; Cayman Islands; China; Colombia; Comoros; Cook Islands; Costa Rica; Côte d'Ivoire; Cuba; Curaçao; 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; Mauritius; Mexico; Mozambique; Myanmar; 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 Martin; Saint Vincent and the Grenadines; Saudi Arabia; Senegal; Seychelles; Singapore; Solomon Islands; Somalia; South Africa; Sri Lanka; Taiwan, Province of China; Tanzania, United Republic of; Thailand; Togo; Tonga; Trinidad and Tobago; Turks and Caicos Islands; Tuvalu; United Arab Emirates; United States; Uruguay; Vanuatu; Venezuela, Bolivarian Republic of; Viet Nam

Atlantic spotted dolphins are native to:  Angola; Antigua and Barbuda; Bahamas; Barbados; Belize; Benin; Bonaire, Sint Eustatius and Saba (Saba, Sint Eustatius); Brazil; Cape Verde; Cayman Islands; Colombia; Costa Rica; Côte d'Ivoire; Cuba; Curaçao; Dominica; Dominican Republic; Equatorial Guinea; French Guiana; Gabon; Gambia; Ghana; Guinea; Haiti; Honduras; Jamaica; Martinique; Mauritania; Mexico; Montserrat; Nicaragua; Panama; Portugal (Azores, Madeira); Puerto Rico; Saint Helena, Ascension and Tristan da Cunha; Saint Lucia; Saint Martin (French part); Saint Vincent and the Grenadines; Senegal; Sierra Leone; Sint Maarten (Dutch part); Spain(Canary Islands); Togo; United States; Venezuela, Bolivarian Republic of; Virgin Islands, British; Virgin Islands, U.S.

Biology and Ecology

Feeding

Pantropical spotted dolphins feed on small schooling fish, and sometimes squid or crustaceans that live in the deep scattering layer – a dense layer of small marine animals of many different species that descends during the day and rises toward the surface at dusk to feed on plankton⁷.  This diet brings pantropical dolphins into frequent association with various species of tuna, as well as spinner dolphins and other oceanic predators that feed on the same prey^3,8.  Atlantic spotted dolphins have similar diets in their open ocean habitats, but may feed on a wider variety of bottom-dwelling fish and invertebrates when closer to shore.⁵  Atlantic spotted dolphins are also known to form large mixed species groups with common dolphins, bottlenose and striped dolphins in response to prey availability⁹.

Social structure, Reproduction and growth

Different poupaltions of spotted dolphins around the world appear to have evolved different social structures and habits.  Nearshore Atlantic bottlenose dolphins in the Bahamas comprise one of the best studied poulations of the species group with long-term studies completed on their social structure and communication ^10,11. Yet, little is known about Atlantic spotted dolphins’ life history: Females are thought to give birth an average of once every 3 years, and nursing is thought to last up to five years⁵.  Much more is known about the life history of pantropical spotted dolphins, due to the high number of dead dolphins that have been examined by scientific observers in the Eastern Tropical Pacific (ETP) tuna fishery¹².  Gestation is 11.2-11.5 months, and females have calves every 2-3 years.  Calves are normally weaned at 9 months but can continue nursing for up to 2 years³.

Threats and conservation

Natural Predators

Predators of both species of spotted dolphin include killer whales and sharks.  Pygmy killer whales, false killer whales, and short-finned pilot whales are also possible predators^3,5.

Human induced threats

Pantropical spotted dolphins have been subject to direct takes in Japanese drive and harpoon fisheries.  They are also the species most severely impacted by entanglement in tuna purse seine fisheries in the Eastern Tropical Pacific (ETP).  Because of the species’ frequent association with tuna schools, purse seine fishers look for the dolphins at the surface of the water, and intentionally set their nets around the school of dolphins, encircling them together with the tuna. Between the 1959 and 1972 over three million pantropical spotted dolphins were killed in the tuna fishery in the ETP¹³.  Since then, various measures have been put in place to reduce the levels of mortality, including changes to the fishing gear and procedures that allow dolphins to escape the nets.  Reported dolphin deaths in the ETP are now thought to be sustainable, with only a few thousand individuals per year, rather than hundreds of thousands, but the ETP spotted dolphin population does not appear to be recovering as quickly as would be expected¹⁴.   There is concern that their interactions with tuna fisheries, even if no longer fatal, still cause stress that reduces their fitness and ability to reproduce¹⁵.  Both species of spotted dolphins also suffer mortality from bycatch in other fisheries throughout their range.³

Conservation status

Despite the heavy rates of mortality suffered by pantropical spotted dolphins in the 1960‘s and 70’s, the species is thought to be one of the most numerous dolphins in open ocean habitats throughout the tropics.  It is designated as Least Concern by the IUCN Red List of Threatened species, and is on Appendix II of the Convention on Migratory Species (CMS).  Atlantic spotted dolphins have not been studied in enough detail to generate population estimates throughout most parts of their range, and are designated as Data Deficient on the IUCN Red List.  They are not listed on either appendices of the CMS.

Spotted dolphins and dolphin watching

Please see the IWC Whalewatching Handbook

References