Eel life history
Of particular interest has been the search for the spawning grounds for the various species of eels, and identifying the population impacts of different stages of the life cycle.
He speculated that they were born of "earth worms", which he believed were formed of mud, growing from the "guts of wet soil" rather than through sexual reproduction.
[1] In 1876, as a young student in Austria, Sigmund Freud dissected hundreds of eels in search of the male sex organs.
[6] Once the glass eels arrive at coastal areas, they migrate up rivers and streams, overcoming various natural and man-made challenges — sometimes by piling up their bodies by the tens of thousands to climb over obstacles[citation needed] — and they reach even the smallest of creeks.
[7] In fresh water they develop pigmentation, turn into elvers (young eels), and feed on creatures such as small crustaceans, worms, and insects.
Eel migrations 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.
Details of the adults' migration across 6,000 km (3,700 mi) open ocean journey back to their spawning grounds north of the Antilles, Haiti, and Puerto Rico remain poorly understood.
[8] 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.
German fisheries biologist Friedrich Wilhelm Tesch 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 [when?]
In December 2018 researchers in the Azores, (about 1,400 km (870 mi) west of the Iberian coast—the furthest point on the migration route identified in previous experiments) fitted 26 large female European eels with satellite tags and released them into the Atlantic Ocean.
In June and August 2008, Japanese scientists discovered and caught matured adult eels of A. japonica and A. marmorata in the West Mariana Ridge.
[12] 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.
[16] 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.
[14] For unknown reasons, beginning in the mid-1980s, glass eel arrival in the spring dropped drastically—in Germany to 10% and in France to 14% of their previous levels—from even conservative estimates.
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 glass eels at their catching sites.
[22] Strong concerns exist that the European eel population might be devastated by a new threat: Anguillicola crassus, a foreign parasitic nematode.
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.
As of the early 2000s, in New Jersey, the Long-term Ecosystem Observatory was monitoring glass eel migration with a planned online in situ microscope.
