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Anguilla marmorata

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA CHORDATA ACTINOPTERYGII ANGUILLIFORMES ANGUILLIDAE

Scientific Name: Anguilla marmorata
Species Authority: Quoy & Gaimard, 1824
Common Name(s):
English Marbled Eel, Giant Long-finned Eel, Giant Mottled Eel
French Anguille marbrée
Synonym(s):
Anguilla fidjiensis Günther, 1870
Anguilla hildebrandti Peters, 1881
Anguilla labrosa Richardson, 1848
Anguilla mauritiana Bennett, 1831
Anguilla ohannae Günther, 1867
Muraena manillensis Bleeker, 1864
Taxonomic Notes: Anguilla marmorata has a robust body which is compressed at the rear. The mouth reaches to under the rear edge of eye with a projecting lower jaw and a visible patch that conspicuously narrows in its middle on the roof of the mouth. Other characteristics include thick lips and minute teeth in bands separated by a toothless groove on the upper jaw separating the inner and outer teeth. This species has large pectoral fins, a dorsal fin origin well behind the pectoral fin and dorsal and anal fins continuous with the tail fin. The lateral line is complete with very small pores and small embedded scales arranged in basket-weave pattern present in large eels but not in small ones. Skin is a yellowish to olive or brown, mottled with dark greenish brown with lighter colouration below. All fins are dark. Anguilla marmorata is thought to have between 100-110 vertebrae and can reach up to 2 m in length. Seven synonyms exist for the species, described from the Indo-Pacific.

Assessment Information [top]

Red List Category & Criteria: Least Concern ver 3.1
Year Published: 2014
Date Assessed: 2013-06-20
Assessor(s): Jacoby, D. & Gollock, M.
Reviewer(s): Feunteun, E., Larson, H., Schabetsberger, R., Tsukamoto, K., Turnock, S. & Weyl, O.
Contributor(s): Ahn, H., Bennett, L., Bills, R., Cambray, J., Casselman, J., Crook, V., DeLucia, M., Engelbrecht, J., Kaifu, k., Kottelat, M., Kurwie, T., Larson, H., Mailautoka, K., Marshall, B., Sasal, P., Silfvergrip, A., Uchida, K., Vishwanath, W. & Walker, M
Justification:

This species has a wide distribution throughout most of the tropical and subtropical western-central Pacific and Indian Oceans and associated freshwater systems. It is considered Least Concern at present as there is no evidence of population declines, nor have any major threats to the whole population been identified, however exploitation, water management, reclamation of wetlands, dams and pollution does occur across many of the countries in its range so increased monitoring of the population is required. Additionally, there is still relatively little known about the extent to which changing oceanic currents and/or temperature changes impact the recruitment of glass eels to maturing grounds, as is the case for all anguillids. Further research is certainly needed in this area. Regionally, the species is impacted by local threats, and local declines have been shown in some areas such as Réunion Island due to over fishing and Madagascar due to river management (e.g., rice culture in Madagascar which has led to the disappearance of silver eel stocks in the Highlands of Tananarive).

This species is a highly commercial food fish and as such yellow and silver life stages could be vulnerable to over fishing whilst glass eels of A. marmorata are used for seed-stock for farming. In Madagascar, an extensive glass eel fishery (several tens of tons) is in operation. Restrictions on supplies of other species, such as the ban on export of Anguilla anguilla outside of the European Union, to global markets could also stimulate increasing exploitation. Habitat loss and degradation and migration barriers could also become a problem in many parts of its range. Thus recruitment, populations, habitats and fisheries should be carefully monitored, and the species reassessed if further data become available as the current level of perturbation could, if neglected, easily push A. marmorata into a Near Threatened or threatened category.

Debate over the number of isolated subpopulations and the number of spawning grounds that exist continues and will likely prove a significant area of future research. Once resolved the spawning population(s) will require localised management and conservation strategies considering each as a distinct ecological unit (Watanabe et al. 2013).

Assessment of this species was carried out during a workshop held at the Zoological Society of London from July 1st-5th, 2013.
History:
2012 Least Concern

Geographic Range [top]

Range Description:

Anguilla marmorata is the second largest species of anguillid with the widest geographical distribution across two different ocean basins (tropical and subtropical western-central Pacific and Indian Oceans). Its range covers the East Coast of Africa, east across the Indian Ocean (Ege 1939), including India and Sri Lanka, the Indo-Pacific  region (including Indonesia, Philippines, Papua New Guinea) and as far East as the island chains in the central South Pacific (e.g., the Marquesas Islands, French Polynesia). Latitudinally, this species ranges from Southwestern Japan, Taiwan and southeastern China (e.g., Upper Mekong, Yangtze, Xijiang and Zhujiang rivers - Kuang 1991, Wang 1998) south through Viet Nam, Malaysia with its most southern distribution being the Southern Cape in South Africa (Watanabe et al. 2009). In Africa it is restricted to southern Africa, more common south of the Limpopo River (Skelton 2001). Individuals have recently been found in the Pacific in the Palmyra Atoll and as far east as the Galapagos Islands (McCosker et al. 2003), although it is not known if these are the result of natural migration, vagrant individuals, or introduction. The species is at the edge of its range here and inland distribution is limited (Tesch 2003).

According to Minegishi et al. (2008), the proposed North Pacific subpopulation is fully panmictic whereas the South Pacific and Indian Ocean populations have a metapopulation structure. Genetic studies have revealed the non-native occurrence of A. marmorata in Hawaii, reported in the 2006 Records of the Hawaii biological Survey (James and Suzumoto 2006).

Countries:
Native:
American Samoa (American Samoa); Cambodia; China (Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hainan, Hubei, Hunan, Jiangxi, Yunnan, Zhejiang); Comoros; Cook Islands; Fiji; French Polynesia (Society Is.); Hong Kong; India (Andhra Pradesh, Orissa, Tamil Nadu, West Bengal); Indonesia (Bali, Jawa, Kalimantan, Papua, Sumatera); Japan; Kenya; Korea, Republic of; Lao People's Democratic Republic; Madagascar; Malaysia (Peninsular Malaysia, Sabah, Sarawak); Mauritius; Mayotte; Micronesia, Federated States of ; Mozambique; Myanmar (Myanmar (mainland)); New Caledonia; Northern Mariana Islands; Palau; Papua New Guinea (Papua New Guinea (main island group)); Philippines; Réunion; Samoa; Solomon Islands; South Africa; Sri Lanka; Swaziland; Taiwan, Province of China; Tanzania, United Republic of; Thailand; Tonga; United States (Hawaiian Is. - Present - Origin Uncertain); Vanuatu; Viet Nam; Wallis and Futuna; Zimbabwe
Present - origin uncertain:
Ecuador (Galápagos)
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:

This is a widely distributed species, with a range spanning two oceans, and as such there is considerable debate as to how many subpopulations of A. marmorata exist and equally how many different spawning grounds there are. Morphological analyses carried out on specimens from multiple localities throughout its range suggest that there are at least four subpopulations (North Pacific, Micronesia, Indian Ocean and South Pacific) with metapopulation structure evident in the Indian Ocean and South Pacific (Watanabe et al. 2008, 2009). Molecular genetic research varies somewhat: Ishikawa et al. (2004) found differences among geographic samples that revealed the existence of five geographic subpopulations around North Pacific, Madagascar, Sumatra, Fiji, and Tahiti. Minegishi et al. (2008) were more consistent with the molecular analyses proposing four genetically different subpopulations (North Pacific, South Pacific, Indian Ocean, Guam region), offering that the North Pacific population is fully panmictic with some meta-population structure in the South Pacific and Indian Ocean populations. More recently however, Gagnaire et al. (2011) suggest the existence of just three genetically distinct subpopulations, spawning in the North Pacific, South Pacific and Southwest Indian Ocean, showing partial reproductive and hence genetic isolation but with some inter-population gene flow occurring due to long-distance migration mixing. Thus the species is probably panmictic at the regional scale and partial mixing may promote complete panmixia over very long-time scales. It is thought that the different subpopulations are closely associated with the water mass structure of the oceans with new current systems being exploited as the species expanded its range (Ishikawa et al. 2004). Despite the uncertainty surrounding the sub-population structure, the spawning localities and thus the taxonomy of this species, spawning populations should be considered separate from a management and conservation perspective, regardless of the taxonomic category (Watanabe et al. 2013).

There is no detailed information on population size, but numbers would be expected to decrease with distance from spawning grounds and with distance from the sea in freshwaters, as in other anguillids. A recent study shows that when estimated using molecular techniques, the effective population size of A marmorata is much smaller than in all other eel species (Delgado 2013). This suggests a higher vulnerability of A. marmorata to bottlenecks and population decline than in any other species. In the Indian Ocean, upstream migration of elvers into freshwater habitats does occur year round but is most pronounced between January and April. In this region particularly, upstream invasion patterns of A. marmorata elvers appear to be synchronized with regular cyclonic floods (Robinet et al. 2003). Subpopulations are impacted by local threats, and local declines have been shown in some areas such as Réunion Island and Madagascar due to river management (rice culture in Madagascar which has lead to the disappearance of silver eel stocks in the Highlands of Tananarive and over fishing in Réunion Island) (E. Feunteun pers. comm. 2012). Population declines have also been reported from China (E. Feunteun pers. comm. 2013) and Japan (K. Tsukamoto pers. comm. 2014).

Population Trend: Unknown

Habitat and Ecology [top]

Habitat and Ecology:

Anguilla marmorata is facultatively catadromous, being found in freshwater, brackish and saline habitats during its continental growth stages (Briones et al. 2007, Lin et al. 2012). The yellow eel growth stage may be as short as two to three years in warm productive habitats, but about six to 20 years or more in more northerly latitudes, e.g. in the Pearl River, China (Williamson and Boëtius 1994). Maturing silver eels migrate to deep oligotrophic sea regions, spawning at depths of 150 to 300 m.

Spawning occurs in the same oceanic areas as other anguillids, such as Anguilla japonica (Kuroki et al. 2009a, Pous et al. 2010, Tsukamoto et al. 2011). The spawning areas of the other proposed subpopulations in this species have not been well identified yet (Réveillac et al. 2008, Robinet et al. 2008, Pous et al. 2010, Kuroki et al. 2008, E. Feunteun pers. comm. 2012), except for the north Pacific population, where adult spawners were found west of the Mariana Islands (Chow et al. 2009) and the early-stage larvae (pre-leptocephali) were found in the same area (Kuroki et al. 2009a). Extensive collections were made of the larvae of A. japonica and A. marmorata of the North Pacific population in an overlapping area of the North Equatorial Current region of the western North Pacific Ocean, but their spawning strategies appear to be different (Kuroki et al. 2009a). The geographic distribution of the growth stages of the species in relation to other anguillids is affected by gyre and coastal currents, spawning times and growth rates and duration of the larval (leptocephalus) stage. The leptocephali of A. marmorata spend about 114-132 days drifting in the plankton before recruiting to river mouths. Ages at metamorphosis from the larval stage and recruitment are thought to be approximately 125 and 155 days respectively (Arai et al. 2002). The recruitment is relatively greater on the east coast of Taiwan compared to the west (Han et al. 2012, Leander et al. 2012). The species may also show preferences for upstream freshwater habitats, but competition with other eel species and environmental factors may influence choices (Shiao et al. 2001, Briones et al. 2007, Robinet et al. 2007).

Leptocephali follow oceanic gyre currents and during this time they metamorphose into glass eels before migrating towards continental habitats (e.g., Robinet et al. 2008, Kuroki et al. 2009b, Pous et al. 2010). They then develop pigment during the elver stage, commence feeding and become yellow eels in either brackish or freshwater habitats. Regarding recruitment to freshwater habitat, as a general rule if the edge of the continental plate is steep (this usually results in short high gradient streams and low-diversity fauna) the elvers ascend the streams (Annamite slope, Philippines, east Borneo, Sulawesi, Indian Ocean slope of Sumatra and Java, Thailand between Phuket and Ranong, etc). If it is an extensive shallow coastal sea (e.g., Sunda shelf), only occasional stray individuals enter freshwater (for example, the Chao Phraya and Mekong estuaries) (M. Kottelat pers. comm. 2011). Most often A marmorata is found in running clear water and seems to avoid still and backwaters. When they are found in sympatry with other anguillid species, they seem to be preferentially located in the mid part of rivers avoiding both high and low altitudes.

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 1,000 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).

Anguilla marmorata is found in subsistence and commercial fisheries and is farmed in some regions (e.g., southern Africa, Madagascar and Reunion Island). It is popular and highly sought after in China, where there is high demand for mottled eels and where it is imported from the Mekong in Cambodia, amongst other regions. This species is also used in East Asian medicine.

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. marmorata are likely to include many of the other Anguilla species, with overlapping ranges. According to FAO data for example, annual catches of “River eels nei” (Anguilla spp.) in Indonesia and the Philippines is 2,000 tonnes and while a significant proportion of this likely consists of A. marmorata as it is considered common in these countries, A. bicolor is also found and caught in this region. Anguilla marmorata are currently relatively easy to purchase in bulk via online traders. Sold as glass eels, live adult eels and frozen eels, this species is increasingly common on B2B trade platforms. In October 2013, this species was being offered for sale by over 30 different suppliers on four B2B platforms (Alibaba, Weiku, Food and Beverage and EC21) based predominantly in Indonesia, Malaysia, the Philippines, Viet Nam and Sri Lanka. 

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. Anguilla marmorata 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) with nearly 90 tonnes imported into Hong Kong alone over the last 10 years (2003-2012) - 35 tonnes of this was in 2012. According to proportions of different species found in Philippine waters calculated in 2011 and 2012 (Yoshinaga et al. unpublished data), on average at least 40% is A. marmorata every year (total proportion varying across seasons) and therefore ~35 tonnes of this species have been exported to Hong Kong in the last 10 years from the Philippines alone. Irregular imports of live eel fry into East Asia from Madagascar have been reported from 2005 onwards, reaching a high in 2012 of four tonnes. However studies on the distribution of eel species in Madagascar (Robinet et al, 2008), press releases and online B2B trade platform adverts concerning eel trade from Madagascar (focusing on A. mossambica) suggest that only a small proportion of these are likely to be A. marmorata.

On many islands throughout Oceania freshwater eels are considered mythical creatures and this has led to some local fishing restrictions and taboos. However, with decreasing supplies of temperate species the demand may be growing. 

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): River management, water impoundment, overfishing and pollution could impact this species. In southern Africa, in the upper catchments in Mozambique, all Anguilla species are susceptible to hook and line fishing and poisoning by subsistence fisherman. Within all countries, major impoundments and weirs are barriers to upstream eel movement (Wasserman et al. 2011) and mortalities can occur on downstream passage of silver eels through turbines and pumps. In developing countries, hydroelectric capacity is being rapidly developed and fish passage requirements are scarcely taken into account (e.g., Roberts 2004). Loss and/or deterioration of habitats due to land drainage and reclamation and pollution may become problems in parts of the species range. Development of farming initiatives could locally threaten this species as this will require the collection of glass eels and elvers in estuaries as seed stock. In central Vietnam, this species has been shown to accumulate, in their muscle tissue, high levels of trace metals (Cr, Mn, Co, Cu, Zn, Sr, Cd and Pb) suggesting pollution poses a threat to some subpopulations and consumers (Le et al. 2009).

Conservation Actions [top]

Conservation Actions:

The species has been assessed as 'II Rare and Valuable’ in the Wildlife Conservation Act Republic of China (Taiwan) Article 4 (Wang 1998). In some areas, this species is designated as a “Natural monument” (protected animals) in Japan, clearly indicating this species is quite rare in the country. Further research is required to confirm the species spawning areas, continental distributions, key habitats and vulnerabilities, especially of different populations.

Coordination between countries to define and implement regional management plans are strongly requested and do not exist yet. This species was previously assessed as Least Concern in 2012 (Vishwanath and Mailautoka 2012).

Citation: Jacoby, D. & Gollock, M. 2014. Anguilla marmorata. The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 01 August 2014.
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