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Anguilla bicolor

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA CHORDATA ACTINOPTERYGII ANGUILLIFORMES ANGUILLIDAE

Scientific Name: Anguilla bicolor
Species Authority: McClelland, 1844
Common Name(s):
English Shortfin Eel
Synonym(s):
Anguilla amblodon Günther, 1867
Anguilla bleekeri Kaup, 1856
Anguilla cantori Kaup, 1856
Anguilla dussumieri Kaup, 1856
Anguilla foochowensis Chu & Yin, 1984
Anguilla malabarica Kaup, 1856
Anguilla moa Bleeker, 1849
Anguilla mowa Bleeker, 1853
Anguilla sidat Bleeker, 1853
Anguilla spengeli Weber, 1912
Muraena halmaherensis Bleeker, 1856
Muraena virescens Peters, 1852
Taxonomic Notes:

This species of shortfin eel is split into two subpopulations: sometimes listed as the subspecies Anguilla bicolor bicolor (McLelland 1844), one subpopulation is found in the Indian Ocean from the east coast of Africa to northwestern Australia and greater Sundaland; while the other subpopulation, sometimes listed as the subspecies Anguilla bicolor pacifica, is found in the Indo-West Pacific from southern China, Philippines and the Indonesian islands of Borneo, Sulawesi and New Guinea (Schmidt 1923).

From a study conducted across the Indonesian archipelago, these two subpopulations were found to share many morphological similarities but significant differences in distribution even at a localised, national scale (Sugeha and Suharti 2008).

Anguilla bicolor is olive to dark bluish-brown dorsally and lighter ventrally from the jaw to the anus. Its dorsal fin soft rays number 240-250 and its anal fin soft rays, 200-220. This species typically has between 106 to 115 vertebrae (Ege 1939). It is most easily confused with Anguilla obscura (and its distribution overlaps with this species in New Guinea). It can only be reliably separated from A. obscura by vertebral count (102-108 for A. obscura) (Ege 1939) and/or genetic analyses (Watanabe et al. 2004).

Assessment Information [top]

Red List Category & Criteria: Near Threatened ver 3.1
Year Published: 2014
Date Assessed: 2013-07-27
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:
2014 Near Threatened
2010 Least Concern

Geographic Range [top]

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.).

Countries:
Native:
Australia (Western Australia); Bangladesh; India; Indonesia (Jawa, Lesser Sunda Is., Sulawesi); Kenya; Madagascar; Maldives; Micronesia, Federated States of ; Mozambique; Myanmar; Northern Mariana Islands; Oman; Papua New Guinea; Philippines; Somalia; South Africa; Sri Lanka; Tanzania, United Republic of; Viet Nam; Yemen (Socotra)
FAO Marine Fishing Areas:
Native:
Indian Ocean – eastern; Indian Ocean – western; Pacific – western central
Range Map: Click here to open the map viewer and explore range.

Population [top]

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

Habitat and Ecology [top]

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

Use and Trade [top]

Use and Trade:

The various life stages, ranging from juvenile to adult, of all Anguilla species are harvested and traded on a global scale for consumption, with current demand predominantly driven by East Asian markets, in particular Japan and mainland China. A concerning pattern of exploitation is already apparent. When one Anguilla species or population becomes over-exploited, industry moves to the next in order to fulfil demand (Crook and Nakamura 2013).  

Anguilla spp. are traded internationally as live eels for farming and consumption, as fresh, frozen and smoked/prepared eels for consumption and as skins and leather products for fashion accessories. Global trade data collated by the FAO for live, fresh, frozen and smoked/prepared Anguilla species (non-species specific) is available for the period 1976-2009. According to FAO data, global annual Anguilla exports averaged around 20,000 tonnes in the late 1970s (valued annually at 55-95 million US Dollars), after which annual exports showed a steady increase to a maximum of over 130,000 tonnes in 2000 (valued at over 1000 million US Dollars). Since then annual exports have been declining, to just over 80,000 tonnes in 2008 and 2009 (valued at over 800 million US Dollars). By weight, China and Taiwan are responsible for nearly 75% of these exports and Japan for over 75% of all imports (FAO, 2013).

Given that this species has such a broad distribution it is likely to be caught alongside multiple other anguillid species. Catch statistics, export/import histories and online trade platforms currently offer the best estimation of use and trade in Anguilla species, but are rarely species specific. Export data for A. bicolor range States are likely to include many of the other Anguilla species, with overlapping ranges (e.g. A. marmorata). According to FAO data for example, annual catches of “River eels nei” (Anguilla spp.) in Indonesia and the Philippines is 2000 tonnes and while a significant proportion of this likely consists of A. bicolor as it is considered common in these countries, A. marmorata is also found and caught in this region. 

Internet searches reveal that A. bicolor (live and frozen) can be easily purchased in bulk online, sometimes referred to as just 'black eels'. At the time of writing (2013), A. bicolor pacifica were the most common freshwater eel found on a popular global trading website, likely due to the fact that they are a favoured replacement species for A. japonica. In October 2013, this species was being offered for sale by over 40 different suppliers on four B2B trading platforms (Alibaba, Weiku, Food and Beverage and EC21) based predominantly in Indonesia (over 50% of all suppliers), Malaysia and the Philippines. This trend appears to have been on the increase in recent years. Eel farming, which is responsible for over 90% of all Anguilla production worldwide (averaging at 280,000 tonnes per year since 2007, (FAO 2013)), is reliant on wild-caught juvenile eels or glass eels, as raising eel larvae to the glass eel stage in captivity has only had limited success to date. A. bicolor glass eels are increasingly being used to stock farms in China. The Philippines is supplying increasingly large quantities of glass eels to East Asia (despite an export ban in place) – nearly 90 tonnes imported alone over the last 10 years (2003-2012), 35 tonnes of that in 2012 alone. According to proportions of different species found in Cagayan river, Philippines (one of the main rivers for glass eels) calculated in 2011 and 2012 (Yoshinaga et al. unpublished data), on average at least 25% is A. bicolor every year (total proportion varying across seasons) therefore ~23 tonnes of this species could have been exported in last 10 years from the Philippines alone. In some parts of its range, such as Sri Lanka, A. bicolor is also reportedly used in the aquarium trade (Pethiyagoda 1991).

Note: double-counting, under-reporting and mis-reporting must be taken into consideration when interpreting all available catch and trade data. See Crook (2010) for explanations of data issues.

Threats [top]

Major Threat(s):

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.


Conservation Actions [top]

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. 


Citation: Jacoby, D., Harrison, I.J. & Gollock, M. 2014. Anguilla bicolor. The IUCN Red List of Threatened Species. Version 2014.3. <www.iucnredlist.org>. Downloaded on 20 December 2014.
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