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Dieren en planten

Fish   Eel family   Eel   Conger   Fish biology   Fish communities   Reproduction of fish   Nurseries   Fish diseases   

Eel

size:

maximum 150 centimeters

gewicht:

maximum 10 kilograms

color:

Larva: transparent
young eel:olive green to brown and yellow-white belly
adult eel: dark gray, lighter by the belly

age:

up to around 30 years

food:

larva: zooplankton
young eel: water fleas, small crustaceans, worms and insect larva
large eel: also fish

enemies:

birds such as cormorants and herons; humans

reproduction:

lays eggs
maturity: between 8-13 years old
number: millions of eggs

  • Dut: Paling (aal, bamisaal, blankaal, biezenbijter, blinker, breedbek, dikkop, happer, koppe, platkop, robber, schieraal, schoenveter, slokker, spitskop, zilverpaling, tetting (juveniel), tochtaal (juveniel), gele aal (juveniel), rode aal (juveniel), glasaal (larve))
  • Lat: Anguilla anguilla
  • Eng: European eel
  • Ger: Flussaal
  • Fren: Anguille
  • Dan: Ål
print print
Eel, foto fitis, sytske dijksen

Eel

Eels are benthic fish that live in fresh as well as salt water. They are nocturnal and stay hidden in crevices or burrows during the day. They migrate to the sea to spawn, traveling long distances. In the winter, eel look for frost-free water where they dig themselves into the muddy bottom. Eel are threatened by overfishing. In 2007, eel was declared as a protected animal species and an European recovery plan has been developed.

  • Life cycle

    The smallest larvae of the eel have been found in the Sargasso Sea (in the western Atlantic Ocean) and therefore it is assumed that the place of reproduction is also in the vicinity. The exact place has not yet been found because the Sargasso Sea is larger than all of Europe together. One assumes that adult eel die after spawning.
    In the winter and spring, the glass eels enter the estuaries of the rivers. By allowing themselves to drift into the rivers with flood and hiding in the bottom during ebb, they are able to migrate upriver without swimming. This is called selective tidal transportation. The eels can only swim at a temperature of 10 to 12 degrees Celsius. In the winter, when the water is not warm enough, large concentrations of glass eels group in the river mouths and in front of sluice doors. Glass eels are commercially fished in England and a few countries around the Mediterranean Sea at such places.
    After the following growth stage, they are called elvers and feed upon insect larvae, crustaceans, worms and water fleas. After reaching a length of 25 centimeters, the eels will also eat fish. In the spring, most of the young eels swim upriver. They remain in this freshwater until reaching adulthood. However, some of the young eel stay in brackish or saline water their entire life.
    The eels grow slowly due to the low temperatures in the European waters. A 30-centimeter long eel bought on the market can be eight to ten years old. The migration trek to reproduce begins when the males reach a length between 35 and 45 centimeters and the females at least 45 centimeters. The animals are not totally sexually mature: they become that during the trek, underway to the spawning grounds.
    The trek begins late in spring and summer. During this time, the animals change into silver eel. The color of their back changes from green-brown to black, while the belly side changes from yellow to a metallic silver. The eyes also grow larger, their pectoral fins become elongated, their face narrower and they stop eating. It is known that eel can crawl over (damp) fields during their trek from ditches. It then closes its gills and makes use of skin inhalation. In the autumn, millions of these silver eels migrate through the North Sea.

  • Studies of migration and reproduction

    An experimental situation has been built at the University of Leiden to study the migration habit of eel. One can simulate the entire migration trek from the Netherlands to the Sargasso Sea in a complex of aquaria and tunnels. Light, air pressure, temperature and water currents are imitated as closely as possible. In December 1997, 22 adult eel from the Grevelingen began their trip to the Sargasso Sea in this system. The researchers hope that this experiment will supply more clarity about the migration of eel and the mystery behind their reproduction.
    In January 2000, the study in Leiden showed that the eel contain sufficient fatty reserves to make the long trip to the Sargasso Sea. They can swim very efficiently and burn very little fat. The fish need these reserves during the trip because they do not eat then. There is a danger hidden in the fat: fatty eel contain relatively extra amounts of PCBs, which can have a disasterous effect on the reproduction.

  • High tech eel

    Genetic studies have shown that there must be other populations with different spawning grounds. Iceland eel differ strongly from Morocco eel and eel from the European mainland. The scientists placed extremely light-weight transmitters on a number of different populations at the end of 2000 in order to follow them.
    Another joint study is being done on migrating eels with transmittors in the Netherlands and Germany. The study uses a network of detection wires laid over the bottom of large rivers. The system was once constructed for studying salmoniformes migration. The fish, 130 specimen in 2005, were equipped with a transmittor in Germany and followed during their trip towards the sea.

  • Threatened eel

    The eel stock in the Netherlands is only a quarter of what it was in the 1950s. The exact cause of the decline is unknown. Overfishing, land reclamation and the closure of rivers with sluices and dams could have contributed to a drastic decline of the eel stock. Climate changes and/or pollution may also play a role.
    According to IMARES biologist Dekker, also chairman of the eel workgroup of ICES, it is possible that the real cause is the release of French eel after the war. This could have changed the ecology of the indigenous eel too much. According to him, a general ban on catching eel is the only solution, and even then it could take two hundred years before the population has recovered.
    The eel nursery in the Netherlands and other European countries have problems with a shortage of young eel. There have been much fewer glasseel since the 1980s. In the Netherlands, the decline was at least 99% in twenty years. In addition to natural immigration, fishermen als release glasseel. They have been releasing glasseel in the polders for over a century, where they grow into decent size elvers in four to ten years time. However, the price of glasseel has gone from ten euros per kilogram in 1980 to 250 euros per kilogram 25 years later. What's more, Chinese eel nurseries offer 600 euros per kilogram.
    Glasseel is fished commercially in England and a few countries around the Mediterranean Sea. The fisheries take place in the winter and spring in saline-freshwater transition zones, such as estuaries, river mouths and in front of dams. A large part of the catch is farmed further in Asia in eel nurseries and part of the EC. The nurseries need glasseel because artificial reproduction of eel has not yet succeeded. Some of the eel caught are released or consumed, particularly in Spain.

  • Eel plans

    The problem with the eel fisheries must be solved at a European level. The solution probably lies in a better division of the glasseel catch over the European inland waters, combined with a better controlled fishing of adult eel, so that enough elvers can migrate over the ocean to spawn. The international council for fishery research (ICES) concluded in 1998 that the eel stock was already below the safe biological minimum.
    In 2004, the European Commission announce regulations to limit the exploitation of the eel stock to the lowest possible level. The goal is bring the eel stock back to an acceptable level in the entire continental distribuation area. The Dutch freshwater fishermen that are largely or totally dependent upon the eel fisheries are very worried about their future.

  • Golden eel
    Partially golden eel from Lake Grevelingen, Jaap de Ronde, Visserijnieuws

    Sometimes, very unusually colored eel are found. Two brothers caught a bright yellow eel with black spots in 2004 in Lake Grevelingen. They brought it to a biogist at IMARES in IJmuiden, who identified it as a 'golden eel', a kind of albino. The yellow color becomes visible (xanthochromatism) when there is too little dark colors available.

  • Distribution of the eel
    Distribution of eel, Ecomare

    In addition to its distribution, the map above shows the migration route to the Sargasso Sea.