The eel is a long, thin bony fish of the order Anguilliformes. Because fishermen never caught anything they recognized as young eels, the life cycle of the eel was a mystery for a very long period of scientific history. Although there have been more than 6500 publications about eels, much of its life history remains enigmatic.
The European eel (Anguilla anguilla) was the one most familiar to Western scientists, beginning with Aristotle who did the first known research on eels. He stated that they are born of "earth worms", which emerged from the mud with no fertilization needed — they grew 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" (the translation of the German name "Aalmutter").
In 1777, the Italian Carlo Mondini found the creature's gonads and proved that eels are fish. In 1876, the young Austrian student Sigmund Freud dissected hundreds of eels in search for the male sex organs. He had to concede failure in his first published research paper, and turned to other issues in frustration.
Until 1893, larval eels — transparent, leaflike two-inch (five cm) creatures of the open ocean — were considered a separate species, Leptocephalus brevirostris (from the Greek leptocephalus meaning "thin- or flat-head"). In 1886, French zoologist Yves Delage kept leptocephali alive in a laboratory tank in Roscoff until they matured into eels, and in 1896 Italian zoologist Giovanni Battista Grassi observed the transformation of a Leptocephalus into a round glass eel in the Mediterranean Sea, and recognized the importance of salt water to the process. Despite this discovery, the name leptocephalus is still used for larval eel.
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 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 of the leptocephali he collected, 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 one to three years, their leptocephali reach a size of 75 - 90 mm before they reach the coasts of England and Europe. The common name for this recruiment stage of eels is glass eel, based on the transparency of the body. One famous place for large-scale collection of glass eels (for deli-food and stocking) is Epney at the Severn in England. Glass eels are also eaten as food in Spain. Once they recruit to coastal areas the migrate up rivers and streams, overcoming all sorts of natural challenges — sometimes by piling up their bodies by the tens of thousands to climb over obstacles — and they reach even the smallest of creeks ().
They can move themselves over wet grass and dig through wet sand to reach upstream headwaters and ponds, thus colonising the continent. In freshwater they develop pigmentation, turn into elvers (young eels) 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. In this stage they are now called yellow eels because of their golden pigmentation.
In July some individuals mature and then they migrate back towards the sea, crossing even wet grasslands at night to reach rivers that lead to the sea. Eel migration out of their freshwater growth habitats from various parts of Europe, or through the Baltic Sea in the Danish belts have been the basis of traditional fisheries with characteristic trapnets (Bundgarn).
How the adults make the 6,000 km (4,000 mile) open ocean journey back to their spawning grounds north of the Antilles, Haiti, and Puerto Rico remains unknown. By the time they leave the continent they stop feeding, so they have to rely on stored energy alone. The body undergoes other dramatic changes as well: the eyes start to enlarge in size, the eye pigments change for optimal vision in dim blue clear ocean light, and the sides of their bodies turn silvery, to create a countershading pattern to make them difficult to see by predators during their long open ocean migration. These migrating eels are typically called "Silver Eels" or "Big Eyes".
The German fisheries biologist Friedrich Wilhelm Tesch, an eel expert and author of the book "The Eel" (ISBN 0-632-06389-0), conducted many expeditions with high-tech instrumentation to follow eel migration, first down the Baltic, then along the coasts of Norway and England, but finally the transmitter signals were lost at the continental shelf when the batteries ran out. According to Schmidt a travel speed in the ocean of 15 km per day can be assumed, so a silver eel would need 140 to 150 days to reach the Sargasso Sea from around Scotland and in about 165 to 175 when leaving from the English Channel.
Tesch — like Schmidt — kept on trying to persuade sponsors to give more funding for expeditions. His proposal was to release fifty Silver Eels from Danish waters with transmitters 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. However, only preliminary experiments such as these have so far been performed.
Today our sum of the only knowledge about the fate of individual silver eels once they leave the continental shelf is based on three eels found in the stomachs of deep sea fishes, that include whales caught off Ireland and off the Azores and some experiments on the physiology of eels in the laboratory.
There is another Atlantic Eel species: the American eel, Anguilla rostrata. First it was believed European and American eels were the same species due to their similar appearance and behavior, but research has shown that they differ in chromosome count and various molecular genetic markers, and in the number of vertebrae, Anguilla anguilla counting 110 to 119 and Anguilla rostrata 103 to 110.
The spawning grounds for the two species are in an overlapping area of the southern Sargasso Sea, with A. rostrata apparently being more westward than A. anguilla, and with some some spawning by the American eel possibly even occurring off the Yucatan Peninsuala outside of the Gulf of Mexico, but this has not been confirmed. After spawning in the Sargasso Sea and moving to the west, the leptocephali of the American eel exit the Gulf Stream earlier than the European eel and begin migrating into the estuaries along the east coast of North America between February and late April at an age of about one year and a length of about 60 mm.
The spawning area of the Japanese eel, Anguilla japonica, has also been precisely located to be to the west of the Suruga seamount (Oceanic biology: spawning of eels near a seamount - K. Tsukamoto (23 February 2006); Nature 439(7079):929) and their leptocephali are then transported to the west to East Asia by the North Equatorial Current.
No one yet knows the reasons, but beginning in the mid-1980s, 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, although not as drastic.
In 1997 European demand for eels could not be met for the first time ever, and dealers from Asia bought all they could. The traditional European stocking programs could not compete any longer: each week the price for a kilogram of glasseel went up another US$30. Even before the 1997 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. Asian elvers have sold in Hong Kong for as much as $5,000 to $6,000 a kilogram at times when $1,000 would buy the same amount of American glasseels with gunfights at their catching sites Such a kilogram, consisting of 5000 glasseels, may bring at least $60,000 and as much as $150,000 after they leave an Asian fish farm. In New Jersey over 2000 licences for glasseel catch were issued and reports of 38 kg per night and fisherman have been made, although the average catch is closer to 1 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.
Because the eels are catadromous (living in fresh water but spawning in the sea), 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 more funding becomes available, it will be possible to log into the system via a Longterm Ecological Observatory (LEO) site.