Stenella longirostris

Spinner Dolphin

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Spinner Dolphin

 

Description

Schools of spinner dolphins leap out of the water, twirl, and reenter with a large, noisy splashes that can be heard for long distances underwater. Usually they make several such spinning leaps in a row. The reason is a mystery. It may be to remove parasites from the skin, or to communicate with other animals in the school - or is it just for fun? Spinners are found in tropical waters worldwide. In Hawaii, they rest in shallow bays during the day, and swim out at dusk to forage in deeper waters. Groups of spinners disperse over a wide area when they are feeding, and different individuals may return to a bay in the morning. This suggests that the spinners around an island are a super-school, within which a number of sub-groups - perhaps family groups - move freely. Spinners are among the dolphins caught and killed in tuna fisheries.

Geography

S. l. longirostris occurs mainly around oceanic islands in the tropical Atlantic, Indian, and western and central Pacific east to about 145°W (Rice 1998). However, the distribution in the Atlantic is not well known, especially in South American and African waters; the known range can be expected to expand considerably in those areas with increased attention to the cetacean faunas there. The southernmost record is from New Zealand, more than 2,000 km south of what is thought to be the normal range but still well north of subantarctic waters (Perrin and Gilpatrick 1994).

Ecosystem

Habitat and Ecology

In most tropical waters, nearly all records of spinner dolphins are associated with inshore waters, islands or banks. Around Hawaii spinner dolphins depend on the availability of sheltered shallow bays for use as resting areas during the day. In the ETP, however, spinner dolphins occur in very large numbers on the high seas many hundreds of kilometers from the nearest land. Spinner dolphins favor a specific habitat in the ETP, called by oceanographers "tropical surface water;" it is typified by unusual conditions of shallow mixed layer, shoal and sharp thermocline, and relatively small annual variation in surface temperature (Reyes 1991; Perrin and Gilpatrick 1994). There they are often found in close association with pantropical spotted dolphins, yellowfin tuna (Thunnus albacares) and birds of several species. The dwarf form of the spinner dolphin in Southeast Asian waters apparently inhabits a shallow coral reef habitat (Perrin and Gilpatrick 1994). In the north-central and western Gulf of Mexico, Stenella longirostris is found over intermediate bottom depths, its distribution overlapping with that of purely pelagic and purely coastal species (Davis et al. (1998).

Most spinner dolphins feed predominantly at night, on small (<20 cm) midwater fish of many different families (including myctophids), squids, and sergestid shrimps (Perrin et al. 1973; Dolar et al. 2003). Dwarf spinner dolphins are exceptional, however; they feed (presumably during daylight hours) on small, reef-associated organisms (benthic reef fishes and invertebrates) (Perrin et al. 1999).

Conservation

The eastern spinner population has ceased to decline but shows no clears signs of recovery. While there are few estimates of abundance and takes available in regions other than the eastern tropical Pacific, they are taken throughout their range by a diverse number of direct and indirect fisheries; some of these indirect takes may evolve into directed takes. Annual takes on the order of hundreds or thousands have been reported from countries in the Indian Ocean. These kills may comprise a large proportion of the global population. More information is needed before the possibility of a global decline of 30% or more can be eliminated.

 

Threats

Major Threats

The association of spinner dolphins with spotted dolphins and yellowfin tuna results in their entanglement in tuna purse seines in the ETP. This is the second-most important species of dolphin involved in the tuna fishery (after the pantropical spotted dolphin) (Gerrodette 2002). The population of the eastern spinner dolphin subspecies S. l. orientalis is estimated to have been reduced by 65% by the tuna fishery kills (Reilly et al. 2005). Since the Inter-American Tropical Tuna Commission (IATTC) implemented per-vessel mortality limits on the international fleet, the mortality for the eastern and whitebelly forms combined decreased, to 389 in 2005 (IATTC 2006). Although current mortality is greatly reduced, the eastern form appears to be recovering slowly. Potential factors such as fishery-related stress, unobserved mortality due to calf separation and orphaning during fishing operations (Archer et al. 2001), possible mortality by small vessels that do not carry observers, and under-reporting of mortality have been suggested as possible reasons the eastern spinner’s slow recovery (Gerrodette and Forcada 2005). 

Throughout their range, spinner dolphins are taken as bycatch in purse-seine, gillnet, and trawl fisheries (Perrin et al. 1994; Donahue and Edwards 1996), often in high numbers. Spinner dolphins are the most abundant dolphin in the Indian Ocean (Balance and Pitman 1998) and are taken throughout the region. In the Indian Ocean, annual takes of hundreds of spinner dolphins have been reported bycaught in the few fisheries that have been examined in India (Lal Mohan 1994), and annual takes in the thousands have been reported in Sri Lanka (Leatherwood and Reeves 1991; Lal Mohan 1994). Takes in other areas are unknown, but may be substantial. Unknown numbers have been taken in the tuna purse-seine fishery in the eastern Atlantic (Donahue and Edwards 1996) and in small-scale gillnet fisheries in the western Atlantic (Siciliano 1994). Dolphins taken incidentally in the Philippines and Venezuela are utilized for shark bait and human consumption (Dolar et al. 1994; Perrin and Gilpatrick 1994). Dwarf spinners are caught incidentally in shrimp trawls in the Gulf of Thailand (Perrin et al. 1999). There are likely to be undocumented fisheries interactions off West Africa (Jefferson et al. 1993; Perrin and Gilpatrick 1994). Zerbini and Kotas (1998) report on by-catches in Brazilian drift-net fisheries and Cockcroft (1990) on animals entangled in shark nets off KwaZulu-Natal, South Africa. 

In some cases, human use of by-caught spinner dolphins has led to direct fisheries. Direct kills occur in several areas, including the Caribbean, Sri Lanka, the Philippines (Dolar 1994), Taiwan, and Indonesia (Perrin and Gilpatrick 1994; J. Wang pers. comm., Kahn 2004). Spinners may also be so taken in West Africa (Van Waerebeeket al. 1999). 

Tourist development may affect the habitat and viability of spinner dolphins in some regions, for example at Fernando de Noronha Island, Brazil (Reyes 1991), in Hawaii (Lammers 2004) and in Bali, Indonesia (T. Jefferson pers. comm.). The habit of resting in shallow coastal waters during the day leads to problems of harassment by dolphin-watching boats.
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