Jumat, 06 Mei 2011

SIDHAT EROPA - European Eel - Anguilla anguilla - 歐洲鰻鱺 - 欧洲鳗

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.

Contents

 Decreasing population numbers and breeding projects

For 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 consumption

In 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

 External links

 




  1. Identity
    1. Biological features
    2. Images gallery
  2. Profile
    1. Historical background
    2. Main producer countries
    3. Habitat and biology
  3. Production
    1. Production cycle
    2. Production systems
    3. Diseases and control measures
  4. Statistics
    1. Production statistics
    2. Market and trade
  1. Status and trends
  2. Main issues
    1. Responsible aquaculture practices
  3. References
    1. Related links
Identity



Anguilla anguilla  Linnaeus, 1758 [Anguillidae]
FAO Names:  En - European eel,   Fr - Anguille d'Europe,  Es - Anguila europea



Biological features

Body elongate, cylindrical anteriorly, somewhat compressed posteriorly. Head rather long.  Eye always rounded, small in young and yellow eels, large in silver eels. Lower jaw longer than the upper and protruding.  Teeth minute, set in bands in both jaws and in a patch on vomer.  Gill openings small and vertical, restricted to sides.  Dorsal and anal fins confluent with caudal fin; the dorsal fin originates far behind the pectorals; anal fin origin slightly behind anus, well back from origin of dorsal fin. Pectoral fins small and rounded. Pelvic fins absent. D:245-275; A:205-255;  Vertebrae: 110-119.  Lateral line conspicuous. It has minute, elliptical scales embedded in the skin.  Adults in freshwater are greenish-brown on black, yellowish on belly (yellow eel stage), changing to blackish on back and bright silvery on sides and belly (silver-eel stage during spawning migration). Leptocephali and glass-eel stage transparent, elvers greenish-brown; very rarely orange coloured specimens are reported. 

Images gallery

Anguilla anguillaAnguilla anguilla Glass eelsGlass eels (Tesch, 2003).

Profile

Historical background

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.

Main producer countries

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.

Main producer countries of Anguilla anguilla (FAO Fishery Statistics, 2006)

Habitat and biology

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

Production cycle

 
Production cycle of Anguilla anguilla

Production systems

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.

Seed supply 

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.

Nursery 

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).

Ongrowing techniques 

Extensive pond systems



The 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 systems



These 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.

Valliculture



Eels 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.

Feed supply 

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.

Harvesting techniques 

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.

Handling and processing 

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.

Production costs 

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.

Diseases and control measures

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.

DISEASE AGENT TYPE SYNDROME MEASURES
Red fin disease Aeromonas hydrophila Bacterium Tail rot; fin rot; haemorrhagic septicaemia Improved water quality; NaCl (0.5-0.9%)
Red eel pest Vibrio anguillarum Bacterium Red spots on ventral & lateral areas; swollen & dark skin lesions that ulcerate Antibiotics
Viral diseases (Red head) Herpes & Rhabdoviruses Viruses Haemorrhages around the head and operculum initially; later spread to the whole body Vaccination with infected fingerlings from preceding year; decreased temperature (18-20 °C); NaCl (0.1%)
Fungal infections Saprolegnia spp. Fungi White to brown cottony or hairy patches on the skin, fins and gills; death may occur if gills obstructed; usually secondary infection. NaCl (0.1%)
Dermocystidium anguillae Sometimes classed as a protozoan but probably a fungus Swellings on gills, fins or body None - removal of infected eel
Parasitic infections Pseudodactylogyrus anguillae; P. bini Monogeneans Invade the gills; respiratory distress Formaldehyde (24%) - 60 ppm; Mebendazole
Trichodina spp. Ciliates Flashing; lethargy; increased mucus production; sometimes ulcers and frayed fins; respiratory distress if gills affected Formaldehyde (24%) - 60 ppm
Ichthyophthirius multifiliis Protozoan White patches on body; becoming lethargic; attempt to remove parasites by rubbing on enclosure surfaces NaCl (1%)


Suppliers of pathology expertise

Assistance can be obtained from the following sources:
  1. Professor Kurt Buchmann, Royal Veterinary and Agricultural University, Stigbøjlen 4, Dk-1870 Frederiksberg C, Denmark.
  2. Dr. Inger Dalsgaard, Danish Institute for Fisheries Research, Fish Disease Laboratory, Stigbøjlen 4, Dk-1870 Frederiksberg C, Denmark.

Statistics

Production statistics




Global aquaculture production of Anguilla anguilla
(FAO Fishery Statistic)

In 2002, the top three producing countries of farmed European eels were the Netherlands, Italy, and Denmark. Substantial quantities are also farmed in Spain, Greece, Sweden and Germany, together with smaller quantities in several other European and North African countries, including Algeria, Hungary, Macedonia, and Morocco.

Market and trade

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].

Status and trends

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:
  • Enhancing the sustainability of the wild population.
  • Hatchery production of larvae, aimed at producing a stable and high quality supply.
  • Improvements in culture systems and automation, as well as improved management.
  • Disease prevention and control.
  • Improved marketing, with the consolidation of existing markets and development of new markets.
  • Training technical personnel with combined skills in aquaculture technology and business management.

Main issues

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.

 

Responsible aquaculture practices

References

Bibliography 

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.

Related links







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。 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

目录

 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、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

幼體或成年(包括倫敦東區(East End of London)著名的鱔魚凍(jellied eels))歐洲鰻鱺為重要的食物來源。利用籃的陷阱捕捉幼體鰻魚在歐洲西面的海岸線有顯著的經濟價值。

 參見

 參考資料

  1. ^ (英文) Freyhof, J. & Kottelat, M. (2008). Anguilla anguilla. 2008 IUCN Red List of Threatened Species. IUCN 2008。擷取於2009-03-17
  2. ^ Anguilla anguilla. Integrated Taxonomic Information System [11 March, 2006].
  3. ^ PCBs are killing off eels. New Scientist. 2006, 2452: 6.

 外部連結

 






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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