Stenella attenuata

Pantropical Spotted Dolphin

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Pantropical Spotted Dolphin Pantropical Spotted Dolphin

 

Description

As its common name suggests, the pantropical spotted dolphin is a spotted dolphin that occurs in tropical waters around the world. It is one of the species that fisherman tend to follow as a means of finding yellowfin tuna, which swim with them. Consequently, millions of these dolphins have been killed after becoming entangled in fishing nets. An international effort has reduced the danger in recent years by introducing dolphin-rescue techniques, limiting the accidental kill to a few thousand each year. Why the dolphins and tuna associate is unknown.
 

Geography

Stenella attenuata lives in the Atlantic, Indian, and Pacific oceans. It migrates seasonally to the Japanese coast and is the most common cetacean in the Gulf of Mexico.

Ecosystem

Habitat and Ecology

In the eastern Pacific the pantropical spotted dolphin is an inhabitant of the tropical, equatorial and southern subtropical water masses. The waters in which the animal 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 similar waters south of the Equator that expand and contract greatly with season and year to year (Perrin and Hohn 1994). In the Atlantic, S. attenuata is primarily a dolphin of the high seas and oceanic islands, but in the eastern Pacific a large-bodied subspecies occurs along the coast from Mexico to Peru. Detailed analysis of oceanographic correlates of distribution will be necessary in order to understand fully the habitat requirements of these pelagic dolphins, often the most conspicuous elements of tropical cetacean communities around the world (Ballance and Pitman 1998).

Offshore spotted dolphins feed largely on small epi- and mesopelagic fishes, squids, and crustaceans that associate with the deep scattering layer (Robertson and Chivers 1997). In some areas, flying fish are also important prey. The diet of the coastal form is poorly known, but is thought to consist mainly of larger fishes, perhaps mainly bottom-living species.

Conservation

The abundance estimates available total more than 2.5 million, and additional likely large populations in the Atlantic, Indian and Pacific Oceans have not been assessed. The northeastern population in the ETP declined 76% within the last three generations (69 years), but that decline has ceased and was not large enough to constitute a global decline of 30%. Large impacts of direct catch and bycatch in other regions have not been identified, and it is unlikely that the global population has been reduced by as much as 30%. Therefore, the species is assesed as Least Concern.

 

Threats

Major Threats

Offshore spotted dolphins bore the brunt of the massive dolphin kill by tuna seiners from the late 1950s to the 1980s in the eastern Pacific (although the coastal subspecies was also impacted). For example, in the period 1959 to 1972, nearly five million dolphins were killed, and of this number, about three million were from the north-eastern offshore population (Wade 1995). 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 only 373 in 2005 (IATTC 2006). Although current mortality is greatly reduced, the north-eastern form appears to be recovering very slowly, if at all, and potential factors such as fishery-related stress, unobserved mortality due to calf separation and orphaning during fishing operations (Archeret al. 2001), possible mortality by small vessels that do not carry observers, under-reporting of mortality, and ecosystem change, have been suggested as possible reasons for the species’ slow recovery (Gerrodette and Forcada 2005). 

Spotted dolphins are also taken incidentally in local fisheries along the Central American coast (Palacios and Gerrodette 1996).

Yang et al. (1999) also reported incidental mortality in Chinese fisheries, and Dolar 1994 found incidental spotted dolphin takes in the Philippines. An unknown but suspected large number of pantropical spotted dolphins are taken by the large-mesh pelagic driftnet fishery off eastern Taiwan (J. Wang pers. comm.).

Japan takes large numbers of spotted dolphins for human consumption. The catch in 1982 was 3,799, and annual catches between 1994 and 1997 ranged from 23 to 449 (Perrin 2002). Between 1995 and 2004, the average annual catch was 129 animals (Kasuya 2007). The drive fishery for spotted dolphins began in 1959 and is thought to have caused a slight decline in the minimum age at attainment of sexual maturity in females (Kasuya 1985). 

Pantropical spotted dolphins are also taken in hand-harpoon fisheries in the Philippines (Dolar et al. 1994); in Taiwan, where it is the locally preferred species of cetacean for human consumption (J. Wang pers. comm.); and regularly or opportunistically by gillnet and harpoon in India and Sri Lanka (Perrin and Hohn 1994). Drive hunts at Malaita in the Solomon Islands took several hundred or thousands of spotted dolphins annually in the 1960s; the hunts continue at present (Ross et al. 2003, Kahn 2006). 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 Lamalera in Indonesia (Kahn 2004). Most of these kills have not been adequately monitored and the effects on the subpopulations are usually not known. 

Dolphins and small whales of several species, including S. attenuata, putatively interfere in hook-and-line fisheries for squid and yellowtail in the Iki Island region of Japan (Kishiro and Kasuya 1993). 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 regional population is not known.
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