Fraser's Dolphin (시라와크돌고래)

 

시라와크돌고래(Lagenodelphis hosei)

영명 : Fraser's Dolphin

 

몸무게 : 2.5m. 몸쪽에 뚜렷한 암회색의 줄이 새겨져 있다. 주둥이가 짧고, 앞머리에 해당하는 부분이 길며, 등지느러미가 뒤로 휘어져 있다. 새끼는 몸길이 약 1m에 20kg 정도 나간다.

 

말레이시아 보르네오섬의 사라와크주에서 채집된 표본에 1956년 명명이 이루어진 진귀한 종류였으나, 남아프리카와 대서양, 태평양 동부, 일본 등지에서도 채집되어 어느 정도 서식하고 있는 것으로 추정된다. 1속 1종이 있다. 남북 위도 40° 선 사이의 해역에 서식하고 있다.

 

◈ 북태평양 (동부 열대역) : 289,300 마리.

 

                   

 

 

 

 

 

 

Description
The body of Fraser's dolphin is stocky, the beak short but distinct and the dorsal fin small, triangular and slightly falcate. The flippers and flukes are also comparatively small. The striking colouration varies with age and sex: a distinctive black stripe extending from the eye to the anus is absent or faint in juveniles, wider and thicker in adult males and variable in adult females. A similar pattern is observed with the facial stripe or "bridle". The back of Lagenodelphis hosei is brownish grey, the lower side cream-coloured and the belly is white or pink.
The one species in this genus was not recognized until 1956, when it was described from a single skull which had been picked up on a beach in Sarawak in 1895. It remained unknown to science as a living animal until 1971, when the species was "rediscovered". Once its external features became known, it turned out that tuna fishermen in the eastern tropical Pacific were already familiar with it. Fraser's dolphin belongs to the subfamily delphinidae. Based on cytochrome b mtDNA it is more closely related to Stenella, Tursiops, Delphinus, and Sousa than to Lagenorhynchus.

 

Distribution
Lagenodelphis hosei is pantropical and ranges north to the Gulf of Mexico, Islas Canarias, West Africa, Sri Lanka, Taiwan, southern Honshu, and Jalisco in Mexico and south to Uruguay and Brasil, Natal, Queensland, and Peru.
The distribution of this species is poorly known. It appears to be most common near the equator in the eastern tropical Pacific and at the southern end of Bohol Strait in the Philippines. It seems to be relatively scarce in the Atlantic Ocean, where it is known from the Lesser Antilles and the Gulf of Mexico and recently from Venezuela. Lagenodelphis hosei may range across the Indian Ocean, though confirmed sightings exist only from the east coast of South Africa, Madagascar, Sri Lanka, and Indonesia. It also occurs away from equator as far north as Taiwan and Japan and, in small numbers, off Australia. It is rarely seen in inshore waters, except around oceanic islands and in areas with a narrow continental shelf. Dolar et al. report sightings between the Philippines and Malaysia, which, however, were so infrequent that they did not allow to estimate population density.

Strandings in temperate areas (Victoria in Australia, Brittany and Uruguay) may represent extralimital forays connected with temporary oceanographic anomalies such as the world-wide el Niño phenomenon, during which a mass stranding occurred in France. Bones et al. report on a stranding on the coast of Scotland.

 

Population size
Estimates of abundance for the eastern tropical Pacific yield 289,500 Fraser's dolphins in that region. Gerrodette and Wade found that their 1989 relative abundance estimates in the eastern tropical Pacific Ocean were substantially higher than the 1988 estimates. In the Eastern Sulu Sea, Dolar estimated a total abundance of 8,700 Fraser dolphins.

 

Habitat
This dolphin is typically a high-seas animal ; it has not been observed close to shore in shallow water. However, it may approach very close to shore (100m) of some islands surrounded by deep water, e.g. Lesser Antilles, Indonesia and Philippines. In the eastern tropical Pacific, it forms part of an equatorial cetacean community that also includes Physeter catodon, Globicephala macrorhynchus, Delphinus delphis, Stenella coeruleoalba and Peponocephala electra. This community is more or less complementary in occurrence to another group of species that includes Stenella attenuata, Stenella longirostris and Steno bredanensis. The latter group is found primarily in so-called tropical surface water, where a stable, shallow mixed layer and thermocline ridging are dominant features. The former group occurs more often in Equatorial - southern subtropical surface water and other waters typified by upwelling and generally more variable conditions. Off South Africa, records are associated with the warm Agulhas Current that moves south in the summer.

 

Behaviour
Analysis of prey suggests that Fraser's Dolphin is a deep diver, hunting at depths of at least 250~500m. In some areas, it is considered shy and difficult to approach; in others it is a bit more approachable. It does not bowride in the eastern tropical Pacific, but it does in most other areas. Running herds create a great deal of white water.

 

Reproduction
The life history of Fraser's dolphin was examined by Amano et al. (1996) based on 108 specimens from a school captured by the driving fishery in Japan. The sex ratio was approximately 1:1. The annual ovulation rate was 0.49. The estimated neonatal length (110cm) predicts a gestation period of about 12.5 mo. and calving peaks in spring and probably also in fall. The calving interval was estimated to be about 2 yr. Life history parameters are similar to those of the striped and pantropical spotted dolphins, but reproductive rate of this species may be lower than that of other pelagic delphinids, if the observed shorter longevity is real.

 

Schooling
Herds tend to be large, consisting of hundreds or even thousands of dolphins, often mixed with other species, such as melon-headed whales (Peponocephala electra), short-finned pilot whales (Globicephala macrorhynchus), Risso's dolphin (Grampus griseus), spinner dolphin (Stenella longirostris) pantropical spotted dolphin (S. attenuata), bottlenose dolphins (Tursiops truncatus) and sperm whales.

 

Food
In the eastern Pacific, Fraser's dolphin feeds on mesopelagic fishes, shrimps and squids. It rarely associates there with bird flocks or tuna schools, which correlates well with the absence of surface-dwelling prey from its diet. In other regions, e.g. the southern Indian Ocean and the western Pacific, it may also feed far below the surface. The stomachs of animals stranded in Brittany contained only the remains of fish (4~24cm ; four species) and the cephalopod Sepia sp. indicating benthic or mesopelagic feeding preferences. Based on stomach contents, prey in the eastern tropical Pacific may be taken at between 250 and 500 m water depths. Santos and Haimovici report on the preference for loliginid squids in the diet of L. hosei stranded in southern Brazil. Watkins et al. report on co-operative hunting techniques observed in the Caribbean.

 

Migration
There are no detailed reports on migratory behaviour, although this pelagic species regularly approaches islands where it is captured for human consumption (see below).

 

Threats
◈ Direct catch
Small numbers of Fraser's dolphins are taken in local subsistence harpoon fisheries in the Lesser Antilles, Indonesia, the Philippines and probably elsewhere in the Indopacific. A few are taken in drive fisheries in Taiwan and Japan. Dolar et al. investigated directed fisheries for marine mammals in central and southern Visayas, northern Mindanao and Palawan, Philippines from archived reports and visits to sites where such fisheries are conducted. Some of the hunters take only dolphins, for bait or human consumption and the species taken include Fraser's dolphins. These are taken by hand harpoons or, increasingly, by togglehead harpoon shafts shot from modified, rubber-powered spear guns. Around 800 cetaceans are taken annually by hunters at the seven sites, mostly during the inter- monsoon period of February-May. Dolphin meat is consumed or sold in local markets and some dolphin skulls are cleaned and sold as curios.

 

◈ Incidental catch
Some are killed incidentally in the tuna purse-seine fishery in the eastern tropical Pacific : 26 were estimated taken during the period 1971~75. A few are also taken in gill nets in Sri Lanka, the Philippines, and likely in other tropical gillnet fisheries as well. Some are killed by anti-shark nets. Gerrodette and Wade note that Lagenodelphis hosei is taken incidentally by tuna purse seiners for the yellowfin tuna (Thunnus albacares) fishery in the eastern tropical Pacific.