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Eel

 

 


An eel is any of the fishes in the order Anguilliformes.

The flat and transparent larva of the eel is called a leptocephalus. A young eel is called an elver.

Short movie of migrating glasseels: media:glasseelu.mov

The fresh water eels (unagi) and marine eels (Conger eel, anago) are commonly used in Japanese cuisine. Eels are used in Cantonese and Shanghai cuisine too. The European eel and other freshwater eels are eaten in Europe, the United States, and other places around the world. A traditional London food is jellied eels.

See also:

Spiny eels and halosaurs of the order Notacanthiformes are also very eel-like. These deep-diving fish were in the past considered a sub-order of the Albuliformes.

Electric eels are not true eels. They are more closely related to catfish.

Leptocephalus larvae of an ocean eel

The story of the eel was a mystery for a long time and many questions still remain.

Aristotle did the first known research on eels, stating that they are born of "earth worms", which emerged from the mud with no fertilization needed just from the "guts of wet soil". For a long time, nobody could prove Aristotle wrong. Later scientists believed that the eelpout Zoarces viviparus was the "Mother of Eels" (German name "Aalmutter").

In 1777 the Italian Mondini found the gonads and showed that eels are fish.

Until 1893, larval eels a transparent, leaflike 2 inch (5 cm) creature of the open ocean was considered a separate species, Leptocephalus brevirostris (from the Greek leptocephalus meaning "thin- or flat-head"). But Italian zoologist Giovanni Batista Grassi observed the transformation of a Leptocephalus into a round glasseel in the Mediterranean Sea, and French zoologist Yves Delage proved in a laboratory in Roscoff that both leptocephalus and eels were the same species. Despite this discovery, the name lepocephalus is still used for larval eel.

Search for the spawning grounds

Danish professor Johannes Schmidt, from 1904 onwards, directed many expeditions in the Mediterranean Sea and the North Atlantic, largely financed by the Carlsberg Foundation. He postulated from the similarity of all leptocephali he found that they all must originate from the same parent species. The further into the Atlantic Ocean he propelled research ships, the smaller the leptocephali he caught. Finally, in 1922, he ended up south of Bermuda in the Sargasso Sea where he succeeded in catching the smallest leptocephali/eel-larvae ever seen.

However, Schmidt was unable to observe the spawning directly, nor did he find ready-to-spawn adults. From the size distribution, Schmidt formulated this part of the life history of the eel:

The larvae of European eels travel with the Gulf Stream across the ocean and, after three years, reach England at a size of 45 mm. The most famous place for large-scale collection of glasseels (for deli-food and stocking) is Epney at the Severn in England. They migrate up rivers, crossing all kinds of natural challenges, sometimes by piling up their bodies by the tens of thousands to reach even the smallest of creeks (short glasseel migration video). They can wind themselves over wet grass and dig through wet sand underground to reach upstream headwaters and ponds, colonising the continent. In fresh water they develop pigmentation, turn into elvers and feed on creatures like small crustaceans, worms and insects. They grow up in 10 or 14 years to a length of 60 to 80 cm. They are now called yellow eels because of their golden pigmentation.

But then in July their instinct drives them back towards the seas, crossing even wet grasslands during the nights to reach into their rivers. Eel migration out of the Baltic Sea through the Danish belts was the basis of traditional fisheries with characteristic trapnets (Bundgarn).

Whether the adults can ever make the 6,000 km (4,000 mile) open ocean journey back to their spawning grounds north of the Antilles, Haiti, and Puerto Rico remained unknown. By the time they leave the continent their gut dissolves, so they have to rely on stored energy alone. The body undergoes other dramatic changes as well: the eyes start to grow, the eye pigments change for optimal vision in dim blue clear ocean light, and the sides of their bodies turn silvery, best suited to be as invisible as possible during the long open ocean cruise ahead and past many waiting predators. Many now call these migrating eels "Silver Eel" or "Big Eyes".

The German fisheries biologist Friedrich Wilhelm Tesch, an eel expert and author of the book "The Eel" (ISBN 0632063890), equipped many expeditions with high-tech instrumentation to follow eel migration, first down the Baltic, then along the coasts of Norway and England, but finally lost the transmitter signals at the continental shelf when the batteries ran out.

He like Schmidt kept on trying to persuade sponsors to give again. His proposal was to release 50 Silver Eels from Danish waters with probes that will detach from the eels each second day, float up and broadcast position, depth and temperature to satellite receivers, possibly jointly with an equivalent release experiment from the countries of the western coast of the Atlantic. So today our knowledge on the fate of the eels once they leave the continental shelf is based on three eels found in the stomachs of deep sea fish, a whale caught off Ireland and off the Azores and some experiments on fife eels.

There is another Atlantic Eel species: the American eel, Anguilla rostrata. First it was believed European and American eels were of the same origin due to their similar appearance and behavior, but genetic work has distinguished the two species.

The spawning grounds for the two species are believed to be very close together, however, with rostrata probably more westward than anguilla, maybe some even within the Gulf of Mexico. These leptocephali exit the Gulf Stream earlier and reach east coast waterways between February and late April at an age of one year and a length of about 60 mm.

Eel aquaculture

Nobody knows why, but beginning in the mid-1980s, the leptocephalus and glasseel arrival in the spring dropped drastically in Germany to 10% and in France to 14% of their previous levels from even conservative estimates. Data from Maine and other North American coasts showed similar declines, albeit not as drastically.

In 1997 European demand for eels could not be met for the first time ever, and dealers from Asia bought all they could grab. The traditional European stocking programs could not compete any longer: each week the price for a kilogram of glasseel went up another $30 dollars. Even before the '97 generation hit the coasts of Europe, dealers from China alone placed advance orders for more than 250,000 kg, some bidding more than $1,100 per kg.

The demand for adult eels has continued to grow, as of 2003. Germany imported more than $50 million worth of eels in 2002. In Europe 25 million kg are consumed each year, but in Japan alone more than 100 million kg were consumed in 1996. As the European eels become less available, worldwide interest in American eels has increased dramatically.

New high-tech eel aquaculture plants are appearing in Asia with detrimental effects on the native Japanese eel, Anguilla japonica. Traditional eel aquaculture operations rely on wild-caught elvers, but experimental hormone treatments in Japan have led to artificially spawned eels. Eggs from these treated eels have a diameter of about 1 mm, and each female can produce 2 to 10 million eggs.

Threats to eels

There are strong concerns that the European eel population might be devastated by a new threat: Anguillicola crassus, a foreign parasitic nematode that infects the swimbladder of eels. This parasite from East Asia (the original host is Anguilla japonica) appeared in European eel populations in the early 1980s. Since 1995 it also appeared in the United States (Texas and South Carolina), most likely due to uncontrolled aquiculture eel shipments. In Europe, eel populations are infected already from 30% to 100% with the nematode. Recently it was shown that this parasite inhibits the function of the swimbladder as a hydrostatic organ (Wuertz et al. 1996). As an open ocean voyager, eels need the carrying capacity of the swim bladder (which makes up 36% of the eel's bodyweight) to cross the ocean on stored energy alone.

Because the eels are catadromous, dams and other river obstructions can block their ability to reach inland feeding grounds. Since the 1970s an increasing number of eel ladders have been constructed in North America and Europe to help the fish bypass obstructions.

In New Jersey, an ongoing project monitors the glasseel migration with an online in situ microscope. As soon as the communication lines become fast enough to carry video, it will be possible to log into the system via the LEO sites.


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