Eel Life History - Search For The Spawning Grounds

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 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 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 they 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.

How the adults make the 6,000 km (3,700 mi) 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 their gut dissolves making feeding impossible, so they have to rely on stored energy alone. The external features undergo 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 spawning by the American eel possibly even occurring off the Yucatán 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 and their leptocephali are then transported to the west to East Asia by the North Equatorial Current. Furthermore, in June and August 2008, Japanese scientists discovered and caught matured adult eels of A. japonica and A. marmorata in the West Mariana Ridge.