Overview

Distribution

Range Description

Anguilla bicolor has diverged between the Indian and Pacific Oceans giving rise to two subpopulations (Ege 1939). The population found in the Indian Ocean (sometimes referred to as A. bicolor bicolor in the literature) is genetically homogeneous in this ocean, but significantly different from the Pacific Ocean clade (referred to as A. bicolor pacifica) (Minegishi et al. 2012). There is also continued debate about whether the Indian Ocean subpopulation might also be considered as two separate management units (K. Tsukamoto, 2014 pers. comm.).

The Pacific population is native to the coasts of China, Taiwan, Vietnam (Hue province, Quang Ngai city, Phu My (Binh Dinh province), Ba river (Gia Lai, Dac Lac, Phu Yen provinces)), Philippines and the islands of Borneo, Sulawesi Island, New Guinea, and the Marianas. In a study by Sugeha and Suharti (2008), conducted around the Indonesian Archipelago, it was found from the western Sulawesi Island to the western Papua Island whilst the Indian Ocean population was found from the western Sumatera Island to the southern Jawa Island (Sugeha and Suharti 2008).

The Indian Ocean subpopulation is distributed from the east coast of Africa, the Arabian Peninsula (Oman and Yemen, including Socotra, in coastal drainages of the Gulf of Oman, Arabian Sea, and Gulf of Aden (EPAA 2002)), it is widespread in the tropical Indian Ocean (Seychelles, Madagascar and Mascarenes), east to India, Sri Lanka, Bangladesh, Myanmar. It is found across to northwestern Australia (known only from streams in the Kimberley regions of northwestern Western Australia and Kakadu National Park in the Northern Territory (Allen et al. 2002, Larson and Williams, in prep.)), and greater Sundaland. Recent studies suggest that it is even more widespread also occupying the western and northern parts of the Malaysian Peninsula (Arai et al. 2012). In Africa, it is widespread but relatively uncommon along the east and southeast African coast from South Africa to Tanzania and Madagascar, including the Lower Shire River System of the Lower Zambezi in Malawi (Tweddle and Willoughby 1979) and Tana River (Copley 1958). In southern Africa it occurs in the eastern part of the region, only present in lower coastal plain sections. In Zimbabwe it is only known from the Save-Runde confluence. It may be present in eastward flowing rivers of east Africa, although it has not been recorded from Kenyan rivers (Seegers et al. 2003). There is a known locality in Somalia, which also suggests the subspecies is present the entire length of the African east coast.

Anguilla bicolor is thought to have potentially three spawning grounds, two of which are in the Indian Ocean (North-East of Madagascar and South-West Sumatra) with speculation of another somewhere in the Pacific (Robinet and Feunteun 2002, Robinet et al. 2003). As a result of the two Indian Ocean spawning sites, it has further been proposed that this subpopulation, may itself be further divided into Eastern and Western units due to oceanographic processes (the South Equatorial Current of this ocean bifurcate into to two gyres, which easily separates the population into two), and while research is required to establish this, it could be potentially be important with regard to conservation and management (K. Tsukamoto, pers. comm.).

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Western Indian Ocean, Africa and India: South African and East African watersheds and islands in Western Indian Ocean (Seychelles, Madagascar and Mascarenes) east to India and Sri Lanka.
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Ecology

Habitat

Habitat and Ecology

Habitat and Ecology

Anguilla bicolor spawn in deep tropical and subtropical oceans. Subpopulations of A. bicolor in Java and Sumatra are thought to spawn off the southwest coast of Sumatra (Mentawai Trench) during a broad, protracted spawning season, with these areas a relatively short distance to where they recruit to their growth habitats (Jespersen 1942, Robinet and Feunteun 2002, Aoyama et al. 2007, Kuroki et al. 2007). The growth rate of A. bicolor lepcephali collected in the eastern Indian Ocean west of Sumatra was slower than other tropical eels such as A. celebesensis or A. borneensis, suggesting that this subpopulation from the eastern Indian Ocean have a longer leptocephalus stage during oceanic migration (Kuroki et al. 2007). But the small leptocephali of this species (less than 10 mm TL) have never been collected in the ocean and there is no information on the spawning area and their early life history.

This eel spends most of its life in estuaries, brackish waters, freshwater lakes, streams, pools and small rivers, preferring marshy habitats but can also be found over rock bottoms and in deeper pools (Pethiyagoda 1991). Predominantly living in freshwater areas, as maturity approaches, the adult silver eels then migrate seaward to spawn before they die (Seegers et al. 2003). Chino andArai (2010) found that 75% of adults resided in freshwater, such as those found in the upper reaches of the Kurau River in Bukit Merah (Malaysia), 20% in coastal seawater, and 5% in brackish water prior to their spawning migrations. Indeed, in Africa it is noted as being restricted to lowland coastal reaches of river systems (Skelton 1993) and in the Arabian Peninsula to wadis, estuaries and the coastal habitat (EPAA 2002). In the central regions of Vietnam, migrations occur during the storm season, from October to December every year. During their growing stage eels are thought to feed on small fishes, crustaceans and molluscs. The Indian Ocean subpopulation of A. bicolor can grow to 1.2 m which is almost the same as those of temperate eels, and while this species can live for up to 20 years, the average age of maturation is younger than those of temperate eels, suggesting that growth in tropical eels is faster than temperate eels (Arai et al. 2011).

From research conducted on the Pacific subpopulation of A. bicolor in the Dumoga River, North Sulawesi Island, Indonesia, the ages of glass eels at recruitment to the coast ranged from 124 to 202 days, with hatching being estimated as having occurred between November and March. The duration of metamorphosis was estimated as 20 to 40 days (Arai et al. 1999). Two specimens of metamorphosing leptocephali - 42.6 and 46.3 mm - were reported off the Indonesian Archipelago and their ages were estimated as 106 and 112 days, respectively (Kuroki et al. 2006). During a sampling survey from Sumatra in 2003, leptocephali were collected west of Sumatra in the Mentawai Trench. Otolith microstructure of these larvae revealed size and age ranges of 44.1-55.5 mm and 114-158 days respectively (Kuroki et al. 2007). The transport of larvae of this species is likely to be heavily influenced by the South Equatorial Current (SEC), the South Equatorial Counter Current (SECC) and the South Java Current (SJC).


Systems
  • Freshwater
  • Marine
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Depth range based on 23 specimens in 2 taxa.
Water temperature and chemistry ranges based on 21 samples.

Environmental ranges
  Depth range (m): 0.4575 - 8
  Temperature range (°C): 28.885 - 28.954
  Nitrate (umol/L): 0.075 - 0.099
  Salinity (PPS): 32.200 - 34.131
  Oxygen (ml/l): 4.427 - 4.525
  Phosphate (umol/l): 0.113 - 0.339
  Silicate (umol/l): 1.579 - 4.465

Graphical representation

Depth range (m): 0.4575 - 8

Temperature range (°C): 28.885 - 28.954

Nitrate (umol/L): 0.075 - 0.099

Salinity (PPS): 32.200 - 34.131

Oxygen (ml/l): 4.427 - 4.525

Phosphate (umol/l): 0.113 - 0.339

Silicate (umol/l): 1.579 - 4.465
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

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Conservation

Conservation Status

IUCN Red List Assessment


Red List Category
NT
Near Threatened

Red List Criteria

Version
3.1

Year Assessed
2014

Assessor/s
Jacoby, D., Harrison, I.J. & Gollock, M.

Reviewer/s
Tsukamoto, K., Turnock, S., Weyl, O. & Wickström, H.

Contributor/s
Ahn, H., Bennett, L., Casselman, J., Crook, V., DeLucia, M., Kaifu, k., Kurwie, T., Molur, S., Sasal, P., Silfvergrip, A., Uchida, K. & Walker, M

Justification

This shortfin eel species has a widespread distribution. Due to declines in the abundance and availability of both A. japonica and A. anguilla (species which have traditionally been used for farming and consumption), it is believed that A. bicolor is the next preference for plain, bi-coloured eels driving up demand for this species in East Asia. It is believed that the subpopulation found in the Indo-Pacific—sometimes listed by researchers as a subspecies A. bicolor pacifica—is at considerably greater threat than its Indian Ocean counterpart—sometimes listed as a subspecies A. bicolor bicolor—based on the ready supply of glass eels from that region from online trading platforms.

Although there are little data available for estimating population changes in this species, it is clear that the number of glass eels being exported is rapidly rising in order to meet the demand. Indeed A. bicolor can now be bought online from over 40 different suppliers across four B2B trading platforms. Anguilla bicolor is assessed as Near Threatened based on a suspected reduction of close to 30% over the next 24 years (3 generations). Should it continue unmonitored, increasing exploitation of the species is likely to pose a significant threat to the population at a global scale and the species could quickly qualify for a threatened category. In addition to widespread exploitation, changing oceanic currents are also proposed as having influence on the migration and recruitment of these freshwater eels, however, our understanding of the mechanisms and impacts of this remain extremely limited. While increased exploitation is of particular concern it is the cumulative and synergistic effects of multiple threats to multiple life-history stages (including climate change and pollution) across the range of A. bicolor that poses the most significant concern and thus improved monitoring and management of populations is essential. Further research into the taxonomy and population genetic structure of this species is also required to help resolve taxonomic confusion of many tropical anguillid species in the Indo-Pacific region in particular.

Assessment of this species was carried out during a workshop held at the Zoological Society of London from July 1st-5th, 2013.


History
  • 2010
    Least Concern
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Population

Population

There is little quantitative information available on the population status of Anguilla bicolor. The species is widespread throughout the Indian Ocean but is by no means common in any of its localities. In studies conducted in southern Sri Lanka, the population was stated to be fairly stable as reflected in the catch per unit effort (Wickström and Enderlein 1988, Wickström 2006). Population densities of this species are very low in the Arabian Peninsula (EPAA 2002) and considerably more research is required to determine population estimates. In the Philippines, multiple species of glass eels recruit to the coast and between October and December these are dominated by A. bicolor with very few recruiting outside of this period (T. Yoshinaga. unpub data). Kuroki et al. (2006) reported that the leptocephali of this species occurred to some extent in the western Pacific.


Population Trend
Unknown
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Threats

Major Threats

To date, few studies or surveys have considered the threats facing this species. Across some of its range, however, A. bicolor is likely to be impacted by threats proposed to be common to many anguillid species (e.g. changing ocean currents, barriers to migration, mortality at hydropower turbines, pollution, exploitation and habitat reduction). The adult individuals, migrating back to sea for spawning, are the most threatened phase of its life-history as they are vulnerable to fishing pressure. Moreover, many migratory routes in small streams and rivers no longer exist due to multiple impoundments such as turbines. However, in the Arabian Peninsula there is no documentation of significant threats to this species (EPAA 2002). As with the many anguillid eels, the cumulative and synergistic effects of multiple threats can be a major concern and thus monitoring and management of populations is highly recommended.

The major concern for Anguilla bicolor, is believed to be the growing exploitation of this species across much of its range. A. bicolor has been fulfilling demand for plain, bi-coloured eels for consumption in East Asia driven by declines in other species. Increasing exploitation is expected to continue due to reductions of availability of A. japonica and A. anguilla particularly.


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Management

Conservation Actions

Conservation Actions

There is no evidence of any conservation in place for this tropical eel species in part due to a lack of knowledge or data surrounding the threats facing it. The next steps towards conserving this species must surely involve obtaining reliable information about its distribution, conduct population estimates and establish long-term monitoring throughout its range. A. bicolor should be reassessed in five years but sooner should more data become available.

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Wikipedia

Anguilla bicolor

Anguilla bicolor is a species of eel in the genus Anguilla of the family Anguillidae consisting of two subspecies.

Subspecies[edit]

References[edit]

  1. ^ Jacoby, D., Harrison, I.J. & Gollock, M. (2014). "Anguilla bicolor". IUCN Red List of Threatened Species. Version 2014.1. International Union for Conservation of Nature. Retrieved 30 June 2014. 
  2. ^ Froese, Rainer, and Daniel Pauly, eds. (2006). Anguilla bicolor bicolor in FishBase. 05 2006 version.
  3. ^ Froese, Rainer, and Daniel Pauly, eds. (2006). Anguilla bicolor pacifica in FishBase. 05 2006 version.
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