|Scientific Name:||Anguilla japonica|
|Species Authority:||Temminck & Schlegel, 1846|
Anguilla angustidens Kaup, 1856
Anguilla breviceps Chu & Jin, 1984
Anguilla manabei Jordan, 1913
Anguilla remifera Jordan & Evermann, 1902
Anguilla sinensis McClelland, 1844
Muraena pekinensis Basilewsky, 1855
|Taxonomic Source(s):||Eschmeyer, W.N. (ed.). 2014. Catalog of Fishes. Updated 10 March 2014. Available at: http:// research.calacademy.org/research/ichthyology/catalogfishcatmain.asp.|
Typical diagnostic features of A. japonica include a fleshy and smooth elongate body that is cylindrical anteriorly and laterally compressed posteriorly. The mouth corner extends to the posterior margin of the eyes with the lower jaw slightly longer than the upper. The snout is depressed and stout and the distance between dorsal fin origin and vent varies from 9.0 to 13.5 % of total body length and the head length 11.2 to 11.9 % of the body length. The length of pectoral fin, at least during the yellow eel stage, is between 2.2 and 3.7 % of head length. The dorsal and anal fins are confluent with the caudal fin. The distance between origins of dorsal and anal fins is shorter than the head length. The body is covered with rudimentary embedded scales that are arranged in small groups and placed obliquely at right angles to those of neighbouring groups. Overall, A. japonica is plain-coloured and neither marbled or mottled (FAO).
|Red List Category & Criteria:||Endangered A2bc ver 3.1|
|Assessor(s):||Jacoby, D. & Gollock, M.|
|Reviewer(s):||Tzeng, W., Miller, J, Han, Y., Aoyama, J., Harrison, I.J. & Tsukamoto, K.|
|Contributor(s):||Ahn, H., Bennett, L., Casselman, J., Crook, V., DeLucia, M., Kaifu, k., Kurwie, T., Sasal, P., Silfvergrip, A., Uchida, K., Walker, M & Turnock, S.|
Anguilla japonica exhibits facultative catadromy (Tsukamoto and Arai 2001), has multiple life stages, is semelparous (Tsukamoto et al. 2011) and panmictic (Ishikawa et al. 2001, Han et al. 2010, Minegishi et al. 2012) and these life history traits made application of the IUCN Red List criteria more challenging.
Ideally, IUCN Red List criteria would be applied to mature eels at their spawning areas, and in the absence of such data, the criteria would be applied to silver eels starting their spawning migration (in the case of Japanese Eels, leaving ‘continental’ waters), as these represent the maximum estimate of spawning stock biomass. These data sets however, are very rare. As such data from eel fisheries and monitoring data (glass, yellow and silver eel stage) from freshwater and coastal habitats have been used as a proxy, although the relationships between recruitment, yellow eel populations, silver eel escapement and spawner stock biomass are poorly understood. The IUCN Red List criteria have to be applied to an amalgamation of multiple life stages, which may not exactly mirror the mature spawning stock. The Japanese Eel is a panmictic species, i.e. they come from one spawning stock. Assuming equal importance of the continental populations, the difficulty that this potentially poses is that escapement from a specific river/country/region is not equivalent to the subsequent recruitment as this relies on the spawning stock as a whole, irrespective of escapement location. However, there are hypotheses that certain regions may have greater importance for the spawning stock, and as data are only available from certain parts of the species' range it is important that conservation initiatives and management actions are adjusted as new data become available. Due to the location of the spawning area for the Japanese eel, which has been identified as west of the Mariana Islands in the Western Pacific Ocean (Tsukamoto et al. 2011), the waters of Japan are likely to receive a high proportion (20-50%) of the total recruitment of eel larvae/glass eels annually with China, Korea and Taiwan also recruiting a significant proportion of the remainder (Y-S Han; K Tsukamoto 2014 pers. comm.). In addition, it is thought that those individuals that do silver in and around Japanese waters are more likely to succeed in reaching the spawning grounds on their return migration (Otake et al. unpub.) and as such Japan’s – and to a lesser extent, China, Korea and Taiwan’s - proportion of A. japonica are a crucial indicator for the health of the population as a whole.
Available data for glass, yellow and silver eels indicate that this species has declined in abundance across its range over the last 30 years (ca. three generations) and has been assessed as Endangered under A2bc indicating that the spawning population has at least halved in this time period. Across East Asia, A. japonica is an important fishery resource with a long cultural tradition and thus is of high economic and cultural value. As such one of the proposed main drivers of the declines is exploitation for consumption and stocking culture facilities. Another factor that has been proposed as causing declines in the Japanese Eel is short term changes in oceanic conditions (e.g., El Niño, typhoons, mid-scale eddies, bifurcation, global warming) which influence larval survival or transport. These in turn are potentially exacerbated by multiple and more localised threats that include barriers to upstream and downstream migration, loss of habitat, pollution caused by industrial and agricultural runoff and modifications of coastal, riverine and other suitable habitat.
Estimating population dynamics in anguillid eels is particularly challenging and due to a considerable lack of fisheries-independent monitoring data for eels in East Asia, for this assessment we predominantly used fisheries catch data with some standardised catch per unit effort (CPUE) data from countries/regions across the distribution of A. japonica. The quality and quantity of data is disproportionate across the species’ range with the most extensive data sets coming from Japan. It is generally agreed that more fisheries-independent monitoring of eel populations is required throughout the range of the Japanese Eel. However, until such data become available a precautionary approach must be to assess trends in the fisheries time series in order to highlight potential threats and apparent trends as a guide for future research and conservation. Furthermore, as with other anguillid species, the available data relating to A. japonica is gained from fisheries statistics in freshwater. Increased monitoring of the brackish and sea water residents and also of individuals migrating to open ocean for spawning is clearly required to accurately and fully assess the impact of the proposed threats on the population as a whole.
While more monitoring is clearly needed, conservation progress is being made. In Japan in 2009, the Eel River Project was set up by the East Asia Eel Resource Consortium (EASEC) designed to sample and monitor glass eel recruitment year round across a number of prefectures in Japan and Taiwan. To date however, the EASEC remains without official support from the Japanese government and as such their influence to implement change can be limited. More recently in 2012, the capture of silver eel was prohibited or restricted in three of the principal glass eel-fishing prefectures (Miyazaki, Kumamoto and Kagoshima), with the intention of preserving spawning eels in these productive areas. And furthermore, restocking of young eels also occurs in some localities throughout its range and since 2013 Japan, Korea, China and Taiwan just started to plan co-operation to manage fisheries and introduce traceability of eel products at the international level. Reassessment of this species in five years will hopefully gauge the success of some of these current conservation initiatives at a time when there is hoped to be more monitoring data available to better understand the population status of this species.
Assessment of the species was carried out during a workshop held at the Zoological Society of London from July 1st-5th, 2013.
It is known that A. japonica occurs as a native species in Japan, China, Taiwan and Korea. The range of this species extends from the southern Pacific coast of Japan and further south to Hainan Island covering large areas of mainland China, Taiwan and the Republic of Korea. There are some records of recruitment along the coasts of the Sea of Japan, Yellow Sea and rarely in Northern Luzon Island, Philippines. It is a rare vagrant to the Chao Phraya basin in Thailand, Viet Nam and Cambodia. It is thought that these vagrant individuals might not contribute greatly to the population as they are outside the influence of currents that are essential to the successful completion of the life cycle and thus recruitment to countries such as the Philippines can fluctuate substantially.
Preleptocephali for this species have been collected in the North Equatorial Current (NEC) along the western side of the West Mariana Ridge (14–17o N, 142–143o E) to the west of the Mariana Islands and were only captured during new moon periods (Tsukamoto 2006, 2009; Tsukamoto et al. 2011). This implicates this area, west of the Mariana Islands, as the spawning area for this species. The latitude of spawning events, however, changes among years depending upon oceanic conditions (Kimura and Tsukamoto 2006, Miller et al. 2009, Tsukamoto et al. 2011) although the NEC and Kuroshio are the predominant transport mechanisms for this species (Tzeng et al. 2012). Spawning is thought to occur between April and August (Shinoda et al. 2011) and the recruitment of glass eels to estuaries occurs in the following order in East Asia. The first few Japanese glass eels are found around the north-eastern coast of Luzon Island in the Philippines and then in greater abundance in Taiwan in late October, with the main fishing season there being between November and February. After 2-4 weeks, recruitment occurs in the Fujian Province of China and the Pacific coast of southern Japan, where the main fishing season is between December and February. In the Zhejiang and Guangdong Provinces of China, Jeju Island, and Pacific coast of central Japan, the main fishing season occurs between January and March. Later, from February to April, recruitment is concentrated in the southern Jiangsu Province of China (around Changjiang River estuary), the southern coast of Korea, and east coast of Japan. In the northern Jiangsu Province of China and the west coast of Korea, the main fishing seasons are between March and May. The final recruits occur on the northwestern coast of Korea and Yalu River between April and June (Cheng and Tzeng 1996, Han 2011).
The true range of A. japonica within countries such as Japan is difficult to determine because of widespread restocking by fisheries cooperatives. Research is currently under way to determine the relative abundance of wild and stocked eels in a few specific localities. Early indications suggest that there is little recruitment of wild eels to the Sea of Japan coastal localities such as the Hayase River system and that therefore most of the eels present in these areas are stocked eels. To date it has not been proved that stocked fish are able to contribute to the wild spawning stock and often these individuals comprise eels which have be deemed unsuitable for farming. It has been inferred that there has been a recent decline in recruitment, comparable to historical data, along the coastline of the Sea of Japan (Kaifu et al. 2013a).
Native:China; Japan; Korea, Republic of; Philippines; Taiwan, Province of China
|FAO Marine Fishing Areas:|
Pacific – northwest; Pacific – western central
|Range Map:||Click here to open the map viewer and explore range.|
Population genetic studies indicate that A. japonica consists of one genetically homogenous, panmictic population (Ishikawa et al. 2001, Satoshi et al. 2001, Han et al. 2010, Minegishi et al. 2012). Several studies using microsatellite analysis have suggested that Japanese juvenile yellow eels show little genetic variation among years. A genetic study carried out by Tseng et al. (2003) for example suggests that the effective population size (EPS) of the Japanese Eel is declining and the authors speculate that historically this has been due to large scale oceanographic changes. Bonhommeau et al. (2008), however, give a clearer account of the potential impact of changing climate-driven processes on temperate eel recruitment. While EPS does not offer a true measure of the number of mature, reproductive individuals in the population, a reduction in this value suggests an overall loss of adaptive genetic variation. There remains considerable uncertainty, however, surrounding the role of changing oceanographic conditions on all anguillid eel population dynamics and further research is required in order to understand the influence of natural climatic fluctuations (such as the El Niǹo) and anthropogenic effects. Because changing oceanic conditions are beyond our direct control, fisheries management is the most feasible rapid response to these declines, although these actions should not be carried out in isolation from other management measures e.g. habitat restoration and/or mitigation against pollution.
Although numbers of glass eels may fluctuate in 5 to 6 year cycles in relation to their reproductive cycle (Han et al. 2009) or to solar activity in Taiwan (Tzeng et al. 2012), fisheries catches of wild Japanese glass eels have been steadily declining since the 1970s (FAO 2013). Catch effort can be variable in fishing data, and under-reporting and, in some cases, an absence of reporting of landings is a serious problem across the range of the Japanese Eel. Thus landings data cannot be accepted as a precise measure of stock status. However, trends in the reported catch data will, to some extent, reflect true changes in fishing yields. These declines are thought to be caused by multiple factors (see Threats).
According to a study that modelled eel larvae transport and recruitment, of the spawning stock of Anguilla japonica, the Japanese population constitutes between 20-50% of the larvae that hatch due to the proximity of Japan to the spawning area and the likelihood of successful recruitment to growth habitats, which may decline as a function of distance travelled by the leptocephali (Kim et al. 2007). Media reports in Japan suggest that the Japanese Fisheries Agency recently announced that the catch of eel fry in December-April 2013 in Japan was 25% lower than the previous season’s haul. Due to a poor catch for the fourth year in a row (2009-2013), the trading price for young eels, has again exceeded the previous year, reaching record highs for this species. According to MAFF statistics from between 1957 and 2010 there has been a remarkable decrease in the numbers of 'natural seeds', namely glass eels and young yellow eels in inland Japanese waters with an as much as 90% decline over the last 30 years (three generations). The proportion of the population that comprise eels living in estuaries and nearby marine habitats however, is a huge discussion point.
When considering glass eel CPUE data from five different Japanese prefectures prominent for eels, and from glass eel statistics in Taiwan, the declines in catches range between 54 and 74%, with one study in Lake Kasumigaura reporting numbers that are stable but at very low levels. While Japan appears to be the epicentre for glass eel recruitment, mainland China has also seen an 80% decline in glass eel recruitment over the last six years (S. Z. Dou, pers. comm.). More data, however, are required from the proportion of the population that reside in Chinese inland and coastal waters.
According to FAO landing statistics published by ICES (2002), 2,164 tonnes of A. japonica were caught in 1974 and catch declined year on year thereafter (2,125 t in 1984; 1,042 t in 1994; 765 t in 2000). Recent FAO statistics (2013) suggest that catch is as low as 306 t although there is no indication of effort from these data. More recently the Japanese Eel was described as 'Endangered' on the Japanese Red List (published by the Ministry of Environment, based on the catch data of inland adult eels compiled by the Ministry of Agriculture, Forestry and Fisheries (MAFF)). This report indicated a 70-90% decline in the species over the last three generations. Total annual yellow and silver eel catch across the 47 prefectures significantly decreased from 1,920 t to 229 t during 1981-2011, a reduction rate of between 79.7-88.1% over three generations (Anon. 1958-2011). Catches of yellow and silver stage eels in Okayama for example, have significantly decreased during the period of 1989-2009 and also between 2003-2009, suggesting that the abundance of A. japonica in this water system has not been restored following the closure of the Kojima Bay-Asahi River fisheries, which remains closed today (K. Kaifu unpub. data). Although these data suggest widespread declines across the 47 prefectures of Japan (Anon. 1958-2011), the MAFF data only consider eels in inland waters (i.e., rivers/lakes) and it is estimated that 40-50% of eels in Japan remain in estuarine /saline waters throughout their lives (Kotake et al. 2005, Mochioka et al. 2012). Another study suggested that the proportion of the population which comprise eels living primarily in the marine/estuarine environment was more than 85% of all silver eels (ca. 600 fish) collected in the coastal water around Japan while the eels that grew in the freshwater habitats were only less than 15% (Tsukamoto et al. 2009), suggesting that the eels in this region were was maintained by these eels existing in primarily brackish and marine habitats. The stock status of eels in saline waters, however, is not known and, like many anguillid species, subpopulations are currently only assessed on aspects of their freshwater life histories.
Studies of this species in Taiwan have reported substantial declines in numbers during the last 15-20 years. Han and Tzeng (2006) report an 81-88% female bias in sex ratio of yellow and silver eels caught at one locality, the Kaoping River in south-western Taiwan. Furthermore in the lower reaches of this river population, density was revealed to be as low as 0.01 eels/m² in 2001 and 2002 and the abundance ranged from 7,137 to 31,360 individuals in 2001, and from 3,324 to 22,040 individuals in 2002 (Han and Tzeng 2006). The eels in the Kaoping River were found to be heavily female biased when density was low but males dominated at higher densities.
On consideration of all the available data on its multiple life-history stages, A. japonica is estimated to have suffered at least a 50% decline in mature spawner abundance over a period of three generations (30 years). As such, this species qualifies as Endangered under the IUCN Red List criterion A2bc.
|Current Population Trend:||Decreasing|
|Habitat and Ecology:|
The species is catadromous, spending its lifetime in freshwater, estuaries and coastal environments, including rivers, streams and wetlands, but migrates thousands of kilometres to spawn. While anguillids are often referred to as ‘freshwater eels’, it is known that they can exhibit inter-habitat migration and movements and the proportion of yellow eels that remain in saline waters, rarely, if ever, entering fresh water, is particularly poorly understood which makes management problematic.
Little is known about the marine component of anguillid life histories in general, but A. japonica stands alone in having a well-studied spawning ecology (Kimura and Tsukamoto 2006, Tsukamoto 2009, Tsukamoto et al. 2011). The Japanese Eel’s spawning area has been identified as the region along the western side of the seamount chain of the Mariana Ridge (Tsukamoto 1992, 2006, 2009; Tsukamoto et al. 2003). The eggs and the spawning adults were discovered in the west of Mariana Ridge (Chow et al. 2009, Kurogi et al. 2011, Tsukamoto et al. 2011). The depths where adults and newly hatched larvae were captured indicate that spawning occurs in shallower layers of 150–200 m and not at great depths (Tsukamoto et al. 2011).
Its larvae develop as leaf-shaped leptocephali in the pelagic ocean environment and over the course of five to six months they drift towards their estuarine recruitment areas (Kuroki et al. 2009, Shinoda et al. 2011) on oceanic currents and then metamorphose into transparent glass eels before becoming pigmented elvers in estuaries. Pigmentation stages of glass eels of A. japonica are described by Fukuda et al. (2013). Age upon arrival to estuaries is estimated between 154 and 182 days while age at metamorphosis as between 115-137 days. Therefore, time from metamorphosis to estuarine arrival is thought to be between 32 and 45 days. As such the total time between spawning and completion of metamorphosis is likely to be a principle factor in the long distance dispersal of this species (Cheng and Tzeng 1996). A full description of the timing of glass eel recruitment to different shores is given in the Geographic Range section. The subsequent timing of upstream migration of glass eels depends on the water temperature, tidal current and moon phase (Tzeng 1985). In Korea, glass eels are caught in the Geum River when temperatures are between 5 and 15°C and the annual catch in this river is highly representative of catch throughout the rest of Korean freshwater habitat (T.W. Lee unpub. data).
Following this metamorphosis, a proportion of the population then moves upstream to feed and grow. However, some A. japonica never enter freshwater, but remain in estuaries or nearby marine habitats and have been referred to as marine residents or ‘sea eels’ (Tsukamoto et al. 1998). Analysis of the strontium;calcium ratios within the otoliths of maturing eels reveals a signature of the transition between these environments and it has been estimated that between 40-50% of the eels in some areas might be estuarine/marine residents (Kotake et al. 2005). This suggests that there is some flexibility in the continental migration of some A. japonica, which shows an ability to adapt to various habitats and salinities and implies that movement into freshwater is clearly not obligatory as evidenced by these different ecophenotypes. As such A. japonica are deemed facultatively catadromous (Tsukamoto and Arai 2001; Tzeng et al. 2002, 2003). During their growth phase, A. japonica feed mostly on invertebrates such as benthic crustaceans and insect larvae, and also on small fishes (Kaifu et al. 2013b). The maximum size this species attains is approximately 100 cm TL and 1.5 kg in weight (Kotake et al. 2007).
As with many eel species, the generation times for A. japonica are highly variable depending on sex, individual variation and locality/latitude. For example, Kotake et al. (2007) studied A. japonica from three different latitudes and found that the age of males at maturity ranged from 4 to 10 years and females from 3 to 17 years, but younger, small males were likely not adequately sampled. Other studies on the average age of silver-phase Japanese Eels, provide estimates of 6.9 and 7.8 years (Kotake et al. 2005) and 8.3 and 9.9 years (Yokouchi et al. 2009) for male and female eels, respectively, suggesting that the average generation length of A. japonica lies somewhere between 7 and 10 years.
As with many eel species environmental conditions may strongly influence recruitment and indeterminate growth rates and individual variability restrict our ability to predict how the Japanese Eel is likely to deal with environmental change.
|Continuing decline in area, extent and/or quality of habitat:||Yes|
|Generation Length (years):||10|
|Movement patterns:||Full Migrant|
|Congregatory:||Congregatory (and dispersive)|
|Use and Trade:||
The various life stages, ranging from glass eels to silver eels, of most anguillid species are harvested and traded on a global scale for farming and consumption, with current demand predominantly driven by East Asian markets, in particular Japan, Korea and mainland China. A concerning pattern of exploitation is already apparent – when one Anguilla species or population becomes over-exploited or fisheries restrictions are imposed, the 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 anguillid 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). Due to these poor catches for the fourth year in a row (2009-2013), the trading price for young eels, has risen year on year during this period, reaching record highs for this species. As with glass eels, the price of live adult eels has also risen. By weight, China and Taiwan are responsible for nearly 75% of these exports and Japan for over 75% of all imports (FAO 2013).
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, because raising eel larvae to the glass eel stage in captivity has presently only had limited success. Together with A. anguilla (the European Eel), A. japonica has one of the longest histories of fishing, stocking and farming for consumption purposes of all the Anguilla species. Japanese Eels are utilized/traded fresh, smoked, canned and frozen and eaten steamed, broiled and baked and also to some extent in East Asian medicines. While consumption drives the main demand for this species, in both Japan and Korea, Anguilla spp. eel skin is also used in the manufacture of fashion accessories such as bags and wallets.
Anguilla japonica was historically caught in waters throughout its range, predominantly in Japan, mainland China, Taiwan and the Republic of Korea, with large eels caught destined directly for consumption or small eels destined for farming, from the early 1900s onwards (Ringuet et al. 2002). Globally, these are also the main eel farming, trading and consuming countries/ territories, with Japan in particular having a long tradition of consumption of A. japonica eels. Declines in the availability of A. japonica in the 1990s, however, resulted in many farms in the region stocking A. anguilla and more recently some other Anguilla species (Crook 2010). Japanese Eel, however, is still the preferred species for consumption in Japan, and high demand for this declining resource has resulted in dramatic increases in price for this species in particular.
Global FAO catch data for Japanese eels is available for the period of 1950-2011. From the 1950s to 1986, annual catches fluctuated between 2,000 and 3,600 tonnes (in 1969), averaging 2,600 tonnes per year. From 1987 onwards catches gradually declined by a 50-100 tonnes on average per year, and in 2011 they were only 300 tonnes. Over 90% of these catches were reported by Japan, with Taiwan and Korea only reporting small quantities, and mainland China none. These catches include all life stages, except the oceanic larval stage (FAO 2013).
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 to this species include overfishing, loss of habitat and changes to oceanic conditions among other threats and as a result the FAO have suggested that Japanese eel stocks are outside safe biological limits and in recent years the fishery has not been sustainable (FAO 2013). These declines have, in the past, driven culture facilities to source glass eels of other species from elsewhere in Asia (e.g. A. bicolor, A. marmorata), Europe (A. anguilla) and America (A. rostrata) (Han et al. 2002).
It is argued that fishing of A. japonica glass eels to stock farming facilities on a national/international scale likely constitutes a major threat to the population. Indeed, the severe decline of Japanese Eel in freshwater habitats in Taiwan may be a result of overfishing of glass eels for farming and degradation of suitable growth habitat. Restocking does occur in Japan and although some studies are beginning to address the issue (Lin et al. 2010) it is still very much unknown what percentage of the yellow and silver eel fisheries data are made up of stocked eels. In 2003, a study was published documenting the first successful captive production of glass eels in Japan from reared larvae and the life cycle was fully closed in 2010 (Masuda et al. 2012). For mass production of eels in Japan to become biologically and economically viable, however, better feed and a simplification and shortening of the rearing process is still required. Although other target species of freshwater eels are now also being used to stock farms, the threat from overfishing persists in many parts of its range because A. japonica remains a premium eel species and the species of choice for consumption in Japan.
Environmental phenomena, such as El Niǹo and sunspots, have been proposed to influence larval transport and survival, and as a consequence impact upon recruitment of glass eels to coastal and freshwater habitats (Kimura et al. 2001, Bonhommeau et al. 2008, Miller et al. 2009, Tzeng et al. 2012). Such environmental factors are proposed to have fluctuating effects on recruitment of glass eels, while exploitation and river habitat modification and degradation have a more consistent effect. However, the role that changing ocean currents play in driving population level changes such as recruitment remains unclear. Additionally, like other eel species, A. japonica is host to the nematode (Anguillicola crassus) that parasitizes the swim bladder which has been proposed to affect survivorship and migration behaviour. Japanese Eels, however, appear to show more evidence of acquired immunity and significantly reduced pathological effects than the European Eel (Knopf 2006) possibly because of longer historical exposure to this parasite (Münderle et al. 2006).
As with other anguillid eels, the upstream and downstream migration of the Japanese Eel has been impacted due to the presence of barriers in waterways. In Japan, construction of dams has reduced eel catches in some areas (Tatsukawa 2003). The reduction rates of yellow and silver eel catches (216 rivers) were significantly positively correlated to the numbers of barriers preventing the upstream migration of fishes, indicating that fragmented rivers lose their carrying capacities for eels (Hakoyama unpub. data). Annual eel (yellow and silver) catch data during 1953 to 2009 were obtained from the database of the Ministry of Agriculture, Forestry and Fisheries (MAFF), Japan and Yoshimura et al. (2005) report that in Japan alone there are 2,675 dams that are higher than 15 m and thus are considered impassable to eels.
Other threats include the loss of river habitat due to agricultural, urban and industrial development, which in Japan has resulted in the extensive revetment of the shorelines of rivers and lakes for flood control purposes (Yoshimura et al. 2005, Itakura et al. 2014). This is now thought to have stabilised in Japan but continues to occur in other parts of this species’ range. Development has also increased the amounts of pollution caused by industrial effluent/agricultural runoff/herbicides/pesticides, all of which are known to negatively impact eel numbers through displacement, reduced reproductive success and direct mortality (e.g. Tzeng et al. 2006).Given the relative lack of understanding of the threats we have attempted to quantify this using the IUCN ‘Threat Classification Scheme’, however, this is by no means definitive.
Although the larval/oceanic phases of A. japonica life histories are by far the most well studied of all the anguillids, there are still significant gaps in our knowledge of this species. Coupled with their broad range and multiple life stages, this makes conservation measures difficult to implement. In 2009 however, the Eel River Project was set up by the East Asia Eel Resource Consortium (EASEC) and was designed to sample and monitor glass eel recruitment year round across a number of key localities in Japan, mainland China and Taiwan. At the time of writing however, the EASEC is still without official support of the Japanese government which limits it power to make changes. Current resources appear to be focused on the aquaculture industry, although some recent studies (e.g. Lin et al. 2010) have begun to address the survival rates and ecology of reared eels back into their natural habitat (i.e. restocking).
In Japan, catch of juvenile eels is prohibited and a special license for catch is issued in order to capture seed for aquaculture or research within a limited fishing period. Illegal fishing still remains a big problem, with catch potentially representing at least double that of the licensed fishery nationally. Since 2013, the capture of silver eel is prohibited or restricted in three of the principal glass eel-fishing prefectures (Miyazaki, Kumamoto and Kagoshima), with the aim of preserving spawning eels in these productive areas. Restocking rivers and lakes with young eels for stock enhancement has also been carried out for many years in Japan and studies on its effectiveness are currently being conducted. However, non-native species have been used (Miyai et al. 2004) and the quality of the restocking material has been suggested as being variable (J. Aoyama and K. Kaifu, 2014 pers. comm.).
In China, the number of licenses is restricted. In Taiwan, closed seasons (1st Mar – 31st Oct) have been imposed on glass eel fishing since 2013 and the capture of eels of TL >8 cm is now prohibited in all major rivers except rivers in Taitung and Hu-Lien prefectures, eastern Taiwan. In these rivers catch is dominated by A. marmorata not A. japonica and restocking of yellow and hormone-stimulated silver eels has been conducted since 1976. Also in Taiwan, year-round, systematic, fisheries independent monitoring occurs at the Yilan River and has been monitoring recruitment since July 2010. At the international level, co-operation among Japan, China and Taiwan was started in 2012 for the conservation of Japanese eel, which is aimed at establishing proper management of eel fisheries as well as introduction of traceability schemes. In summary, there has been a heavy focus to date on fisheries regulation and conservation relating to the many other threats facing A. japonica is essential.
|Citation:||Jacoby, D. & Gollock, M. 2014. Anguilla japonica. The IUCN Red List of Threatened Species 2014: e.T166184A1117791.Downloaded on 30 September 2016.|
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