SIDHAT EROPA - European Eel - Anguilla anguilla - 歐洲鰻鱺 - 欧洲鳗
The European eel, Anguilla anguilla, is a species of eel, a snake-like, catadromous fish. They can reach in exceptional cases a length of 1½ m, but is normally much smaller, about 60–80 cm, and rarely more than 1 m. Much of the European eel’s life history was a mystery for centuries, as fishermen never caught anything they could identify as a young eel. Research in the 19th and 20th centuries shed some light on the subject, though questions remain. They are generally believed to spawn in the Sargasso Sea, after which the adult eels die. The larvae (Leptocephalus) drift towards Europe in a three-hundred-day migration (FAO data). When approaching the European coast the larvae metamorphose into a transparent larval stage called "glass eel", and enter estuaries and start migrating upstream. After entering fresh water, the glass eels metamorphose into elvers, miniature versions of the adult eels. As the eel grows it becomes known as a "yellow eel" due to the brownish-yellow color of their sides and belly. After 5 - 20 years in freshwater the eels become sexually mature, their eyes grow larger, their flanks become silver and belly white in color. In this stage the eels are known as "silver eels", and they begin their migration back to the Sargasso sea to spawn. The European Eel is a critically endangered species. Since the 1970s, the numbers of eels reaching Europe is thought to have declined by around 90% (possibly even 98%). Contributing factors include overfishing, parasites such as Anguillicola crassus, barriers to migration such as hydroelectric plants, and natural changes in the North Atlantic oscillation, Gulf Stream and the North Atlantic drift. Recent work suggests that PCB pollution may be a factor in the decline. Eels have been important sources of food both as adults (including the famous jellied eels of East London) and as glass eels. Glass eel fishing using basket traps has been of significant economic value in many river estuaries on the western seaboard of Europe. In captivity European eels can become very old.
Decreasing population numbers and breeding projectsFor quite some time, the population number of european eels has been falling. For this reason, a research project has been started by Innovatie Netwerk, led by Henk Huizing to see whether it is possible to breed european eels in captivity. The breeding of european eel is very difficult, since eel is generally only able to reproduce after having swum a distance of 6500 kilometers. In the project, this is being similated by means of a hometrainer for the fish. Innovatie Netwerk has also started a breeding project, called InnoFisk Volendam. Sustainable consumptionIn 2010, Greenpeace International has added the European eel to its seafood red list. "The Greenpeace International seafood red list is a list of fish that are commonly sold in supermarkets around the world, and which have a very high risk of being sourced from unsustainable fisheries." References
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In Italy, West Germany and Denmark an extensive culture of European eel in ponds occurred up to the 1970s. However, due to the short summer period (e.g. Denmark) profitable production was impossible. The development of farm systems with heated water was developed in the late 1970s in order to farm the eel at its optimum temperature (24-26 °C). The use of indoor systems started experimentally in Europe in the 1970s. Commercial utilisation started in northern Europe, mainly in the Netherlands, Denmark and Germany in the early 1980s. Nowadays, European eel farming is mainly associated with recirculation systems. These techniques are especially used in Denmark, Italy and the Netherlands, and have resulted in highly intensive farming conditions. European eel production is based on the import of glass eels from France, Portugal, Spain and the United Kingdom. Total production of European eel is expected to be >10 500 tonnes, with the Netherlands being responsible for approximately 50 percent of this total.
Currently, the top three producing countries of farmed European eels are the Netherlands, Italy, and Denmark. Spain, Greece, Sweden and Germany are also centres of eel farming activity and smaller quantities are produced in several other European and North African countries.
The European eel is a catadromous and carnivorous species. Young eels live in freshwater, where they stay for a period of 6-12 years for males and 9-18 years for females. As the eels become sexually mature they migrate to the sea, where they move to the spawning grounds in the Sargasso Sea. During migration the eels do not feed. Once in the Sargasso Sea the eels spawn in late winter and spring. Adult eels do not leave the Sargasso Sea but their progeny, the leaf-shaped larvae (leptocephali) are brought to the continental shelf of Europe by the Gulf Stream, a journey that takes 200-300 days. Before entering coastal zones and estuaries the larvae metamorphose into transparent eels (glass eels). As the eels colonize the freshwater areas of Europe they are known as yellow eels (pigmented eels). During the last summer of their freshwater life, eels become sexually mature and silvery in appearance (silver eels). At this stage their eyes become bigger, their heads broader and the content of body fat increases. The European eel is found in rivers draining into the North Atlantic, Baltic and Mediterranean Seas. It is also distributed along the coast of Europe from the Black Sea to the White Sea.
Production of eels is based on wild catches of glass eels (elvers) used for further ongrowing. Extensive culture of European eels under natural conditions has diminished to a level of no commercial importance. Intensive production by the use of recirculation technology, keeping water temperatures stable around 24 °C, has become the overall production method.
Glass eels are captured around the shores of France, Portugal, Spain and the United Kingdom and either used nationally or exported to eel farmers in other countries. Some glass eel fishing in Spain and Portugal uses scoop nets and traps. In France they are caught by small trawlers using wing nets and trawls. The only legal fishing gear for this purpose in the UK is the scoop net.
At first the glass eels (~0.33 g each) are kept in smaller tanks of 3-4 m² for quarantine purposes. The density at this stage is 10-15 kg/m². The eels are examined for diseases and, following diagnosis, treated. The eels are also weaned to artificial diets with cod roe and, later on, dry starter feed. When the eels reach approximately 5 g they are transferred to a juvenile production unit with larger tanks (6-8 m²) and stocking densities (50-75 kg/m²). At this point the eels can digest dry feed pellets (1 mm).
Extensive pond systemsThe traditional form of eel culture in Europe is in ponds of about 100-350 m². When eels reach marketable size they are transferred to larger ponds (1000-1500 m²). The ponds may be static or flow-through. The best temperature range in ponds is 18-25 °C. Intensive culture in recirculation systemsThese systems consist of square or circular tanks from 25-100 m², usually built of cement or fibreglass. The eels are stocked at a size of 50 g. Densities reach up to 100-150 kg/m². Extruded dry feed (1.5-3 mm) is fed automatically several times a day. Individual growth rates are very different, and grading every 6 weeks is necessary in order to reach a high overall growth performance. VallicultureEels are also extensively cultured in marine and brackish waters within a form of aquaculture known as valliculture. In these Mediterranean systems, mainly in Italy, in the north Adriatic, elvers of 15-35 g are stocked at the rate of 4-15 kg/ha. The elvers are mainly imported from France but also from Denmark, the Netherlands and Sweden.
Almost all forms of intensive culture use formulated feeds in the form of a moist paste for glass eels and steam-pressed or extruded pellets for the later stages. Several European aquafeed manufacturers produce eel feeds.
Partial harvesting and grading are important facets of successful eel farming. The actual technique used is simple but care is necessary to minimize stress. Feeding is stopped minimum 1-2 days before grading or harvesting. Eels are led passively through a pipe from the rearing tank to a grading machine. Eels large enough for the market are dealt with as indicated in the following section of this fact sheet. Eels that have not reached market size are returned to the rearing tanks for further ongrowing. During harvesting, care is taken to keep oxygen levels high.
After harvesting, eels are rapidly sorted into different sizes, using grading systems. They are kept in holding tanks without feed for several days to purge their stomachs and eliminate possible off-tastes. If they are to be consumed fresh, they are chilled, packed into strong oxygenated plastic bags with just enough water to ensure that the skin remains moist, and transported to market. If they are destined for processing they are transported live to the processing plant. Most Danish farmed eels are smoked at Dutch processing plants, while a smaller part is skinned in Denmark for frying.
The cost of elvers varies significantly depending on annual catches and the interest from Asian eel producers, who buy European eels for farming in their home countries. Prices during 2004 varied between 300.00 and 750.00 EUR/kg. General over all production costs in recirculation systems of about 6.00 EUR/kg have been reported (2003) from Denmark.
European eels are susceptible to numerous parasites, fungi, bacteria, and viruses that can cause disease outbreaks. However, in aquaculture only a few disease agents result in disease outbreaks that decrease growth or increase mortality. Commonly seen disease agents are listed in the following table. In some cases antibiotics and other pharmaceuticals have been used in treatment but their inclusion in this table does not imply an FAO recommendation.
Suppliers of pathology expertise Assistance can be obtained from the following sources:
Eels are normally sold at two size ranges for smoking; either at 120-180 g, which is the normal Dutch smoking size, or at 300-600 g (large smoking size) in Germany. A smaller amount is fried or jellied or stewed. More specialised processed products include the Japanese specialty "kabayaki" [a style of serving, in which eels of around 150-200 g are butterflied, placed on skewers, basted and marinated in a thick soy-based sauce, and steamed or grilled; over 90 percent of all global production of eels (all Anguilla spp.) is destined for this form of consumption].
European aquaculture can currently be considered a mature technology. It seems likely that the industry will see little further expansion, and few new production units are expected to be constructed. There is a shortage of elvers and considerable resistance to their extraction from the wild for farming purposes. Research and development efforts for the European eel are required in the following areas:
Eel farmers are primarily concerned about the future access to elvers as these catches may be regulated heavily in order to protect the wild eel population. Ongrowing of elvers for restocking of small eels is, however, used in the effort to support the wild stocks. Although some claims to success on a laboratory scale have been made, commercial reproduction of the European eel in captivity has not yet been successful, and is unlikely to become so in near future.
Eel culture should follow the tenets of the following codes: European Aquaculture Code of Conduct (FEAP). Code of Conduct for Responsible Fisheries (FAO).
Ottolenghi, F., Silvestri, C., Giordano, P., Lovatelli, A. & New, M.B. 2004. Eels. In: Capture-based aquaculture – the fattening of eels, groupers, tunas and yellowtails, pp. 21-67. FAO, Rome, Italy. Tesch, F.W. (Thorpe, J.E., ed.). 2003. The eel, 5th ed. Blackwell Science, Oxford, England. 408 pp. Walton, I. 1897. Observations of the Eel, and other Fish that want Scales; and how to fish for them. In: The Compleat Angler, pp. 195-204. John Lane the Bodley Head, London, England.
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Anguilla anguilla
Glass eels (Tesch, 2003).
歐洲鰻鱺（學名：Anguilla anguilla）[2] 為一種蛇狀、兼性的降河迴游產卵（catadromous）魚類，特殊的例子可以長達140公分，但普遍為60-80公分，極少長於1公尺。為輻鰭魚綱鰻鱺目鰻鱺亞目鰻鱺科的其中一種。 各種原因令歐洲鰻鱺的數目大幅減少，在IUCN紅色名錄內列為極危物種。
分布深度水深0~700公尺。特徵 本魚體圓細長，下頷比上頷長；鱗片隱藏魚皮膚之下；鰓孔在圓形的胸鰭前；幼魚背部體色為橄欖色或灰褐色，腹部為銀色或銀黃色；成魚的背部黑灰綠色，腹部為銀色；延長的背鰭與臀鰭、尾鰭匯和鰭，形成一個獨特的鰭從肛門到背部中央最少有500個軟鰭條。背鰭起點在胸鰭後方遠處；臀鰭起源些微地在肛門後面體長可達140公分。 生態 幼、成鰻棲息河川、河口、潟湖，喜鑽洞潛居，以蝦、蟹、貝、海蟲維生的肉食者。每年秋季成熟的鰻魚，其眼徑變大，內臟萎縮，生殖腺肥大，體色由黃褐變銀灰色，將開始作長途的產卵洄游作準備，經常選擇一個沒有月亮的夜晚，由河川降海在到大西洋的馬尾藻海產卵，其受精卵會在春季和夏初被發現，幼體時期（柳葉鰻 Leptocephalus）則利用三年時間向歐洲遷移。有如玻璃鰻（glass eels），歐洲鰻鱺可以到達歐洲沿岸及進入河口。進入淡水前歐洲鰻鱺變成鰻線（elvers）。她們把大半生花在淡水中。雖然近期與歐洲鰻鱺有關的日本鰻鱺（Japanese eel,Anguilla japonica）研究顯示部分物種族群從未遷移至淡水中生活，而生活在海洋或入海的棲息處。歐洲鰻鱺進入淡水後色素產生變化，其腹部會變成黃色。有假設認為黃色有助成為保護色，可以令獵食者較難注意。其黏滑外層認為可以抵抗鹽度改變。 近期的巨大減少數目自從1970年代開始，到達歐洲的歐洲鰻鱺數目下降了約90%（可能甚至為98%）。現在並不清楚此現象為正常的長期周期或是反映鰻魚數量的普遍下降。潛在可能原因包括過度捕魚（Overfishing）、寄生蟲如粗厚鰾線蟲（Anguillicola crassus）、河流障礙物如水力發電廠，與及北大西洋振盪（North Atlantic oscillation）、墨西哥灣暖流（Gulf Stream）、北大西洋漂流（North Atlantic drift）的天然轉變。近期研究顯示多氯聯苯（Polychlorinated biphenyl, PCB）污染可能是數目下降的主要原因[3]。 經濟參見參考資料
外部連結
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 NOME LATINO: Anguilla anguilla (Linnaeus 1758) FAMIGLIA: Anguillidae ORDINE: Anguilliformes NOME INGLESE: Eel NOMI DIALETTALI: Bisat (Ven.); Ancidda (Sicil.) MORFOLOGIA: corpo molto allungato a sezione cilindrica anteriormente e compressa lateralmente nella regione della coda; colorazione molto scura sul dorso, giallastra o grigia sul ventre; pinna dorsale, caudale ed anale fuse in un'unica pinna; mancano le pinne ventrali; squame piccolissime e cute molto scivolosa per l'abbondanza di muco prodotto da apposite cellule cutanee. TAGLIA: può raggiungere 150 cm di lunghezza. DISTRIBUZIONE: nelle acque salmastre costiere ed in ogni tipo di acque interne. HABITAT: specie a migrazione catadroma (dai fiumi scende al mare per riprodursi), predilige gli ambienti con fondali melmosi pur adattandosi a qualunque ambiente d'acqua dolce. ALIMENTAZIONE: invertebrati acquatici, piccoli pesci ed uova di pesce in attesa di schiusa, rane, girini, vermi. RIPRODUZIONE: la riproduzione avviene nell'Oceano Atlantico (Mar dei Sargassi). Le giovani larve (leptocefali) migrano verso le acque dolci delle coste europee nel corso dei primi tre anni di vita. Raggiungono le foci dei grandi fiumi quando, nel terzo anno, misurano circa 6-7 cm (ceche). Risalgono lungo i fiumi popolando le acque interne di ogni genere. Nelle acque dolci si accrescono fino al raggiungimento della maturità sessuale (8-10 anni), raggiunta la quale cessano di alimentarsi ed iniziano la migrazione inversa verso il mare. Gli adulti muoiono a riproduzione avvenuta. VAL. ECONOMICO: notevole. |
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