The Danish professor Johannes Schmidt, beginning in 1904, led a series of expeditions into the Mediterranean Sea and the North Atlantic (the Dana expeditions) to investigate eels. The expeditions were largely financed by the Carlsberg Foundation. He noted that all the leptocephali he found were very similar, and hypothesized that they all must have descended from a common ancestor species. He also observed that the farther out to sea in the Atlantic Ocean he went, the smaller the leptocephali he caught were. In a 1922 expedition, he sailed as far as the Sargasso Sea, south of Bermuda, where he caught the smallest eel-larvae that had ever been seen.
Although Schmidt did not directly observe eel spawning, or even find ready-to-spawn adult eels, he was able to deduce the following about the life history of the eel, based on the size distribution of the leptocephali he collected: The larvae of European eels travel with the Gulf Stream across the Atlantic Ocean, and grow to 75–90 mm within one to three years, before they reach the coasts of Europe. Eels in this so-called "recruitment" developmental stage are known as glass eels because of the transparency of their bodies. Glass eels typically refers to an intermediary stage in the eel's complex life history between the leptocephalus stage and the juvenile (elver) stage. Glass eels are defined as "all developmental stages from completion of leptocephalus metamorphosis until full pigmentation". The term typically refers to a transparent glass eel of the family Anguillidae.
One well-known place where glass eels are collected large-scale (for deli food and stocking is Epney, on the Severn, in England. (Glass eels are a food item in Spain.) Once they recruit to coastal areas, they migrate up rivers and streams, overcoming various 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. The eels can propel themselves over wet grass and dig through wet sand to reach upstream headwaters and ponds, thus colonizing the continent. In fresh water they develop pigmentation, turn into elvers (young eels), and feed on creatures such as small crustaceans, worms, and insects. For 10 to 14 years they mature, growing to a length of 60 to 80 cm. During this stage they are called yellow eels because of their golden pigmentation. Marine eels of the order Anguilliformes also have a leptocephalus stage, and likely pass through a stage similar to the anguillid glass eels, but they are rarely seen in the ocean. In July, some mature individuals migrate back towards the sea, crossing 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 straits, 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, 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 which makes them difficult to see by predators during their long open-ocean migration. These migrating eels are typically called "silver eels" or "big eyes".
German fisheries biologist Friedrich Wilhelm Tesch, an eel expert and author, 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 around 140 to 150 days to reach the Sargasso Sea from Scotland and about 165 to 175 days when leaving from the English Channel.
Tesch — like Schmidt — kept trying to persuade sponsors to provide more funding for expeditions. His proposal was to release 50 silver eels from Danish waters, with transmitters that would detach from the eels each second day, float up toward the surface, and broadcast their position, depth, and temperature to satellite receivers. He also suggested that countries on the western side of the Atlantic could perform a similar release experiment at the same time. However, even today, only preliminary experiments along these lines have ever been performed. Migration was mapped in 2016. Knowledge of what happens to individual silver eels after they leave the continental shelf is based solely on the study of three eels found in the stomachs of deep sea fishes and whales — caught off the coasts of Ireland and the Azores — and on laboratory research into the physiology of eels.
American eel
Another Atlantic eel species is known: 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 they differ in chromosome count and various molecular genetic markers, and in the number of vertebrae, A. anguilla counting 110 to 119 and A. 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 Peninsula off 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 around one year and a length around 60 mm.
Japanese eel
The spawning area of the Japanese eel, Anguilla japonica, has also been found. Their breeding site is to the west of the Suruga seamount (14–17°N, 142–143°E), near the Mariana Islands. Their leptocephali are then transported to the west to East Asia by the North Equatorial Current.
In June and August 2008, Japanese scientists discovered and caught mature adult eels of A. japonica and A. marmorata in the West Mariana Ridge.
Southern African eels
Southern Africa's four species of freshwater eels (A. mossambica, A. bicolor bicolor, A. bengalensis labiata, and A. marmorata) have an interesting migratory pattern: It takes them on a long journey from their spawning grounds in the Indian Ocean north of Madagascar to high up in some of the Southern African river systems and then back again to the ocean off Madagascar.
New Zealand longfin eels
New Zealand longfin eels breed only once at the end of their lives, making a journey of thousands of kilometres from New Zealand to their spawning grounds near Tonga.Their eggs (of which each female eel produces between 1 and 20 million) are fertilized in an unknown manner, but probably in deep tropical water. The mature eels then die, their eggs floating to the surface to hatch into very flat leaf-like larvae (called leptocephalus) that then drift along large oceanic currents back to New Zealand. This drifting is thought to take up to 15 months.There have been no recorded captures of either the eggs or larvae of longfin eels.