European eel
| European eel | ||||||||||||
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European eel ( Anguilla anguilla ) |
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| Anguilla anguilla | ||||||||||||
| ( Linnaeus , 1758) |
The European eel ( Anguilla anguilla ) is an endangered species of eel and is native to all of Europe , Asia Minor, and North Africa. It has a serpentine, elongated, rotating body. The dorsal, caudal and anal fin form a continuous fin border. Very small round scales are embedded in the thick skin. The European eel has an upper mouth, which means that the lower jaw is slightly longer than the upper jaw. The color of the upper side can vary between black and dark green, the lower side between yellow (young, so-called yellow eel ) and white (adult, so-called silver eel ).
Adult females can grow up to 150 cm long and weigh 6 kg, while males only reach 60 cm in length. However, such sizes are extremely rarely achieved, and a female one meter long is extremely large. From the outside, the European eel can hardly be distinguished from the American eel .
The European eel was named Fish of the Year in Germany in 1995 and 2009 and in Switzerland in 2018 . Since the 1970s, the population in Europe has decreased by 98 percent.
Life cycle and reproduction
Eels hatch in the Atlantic Ocean, in the Sargasso Sea (near the Bahamas). Because of their shape, the eel larvae are called willow leaf larvae ( Leptocephalus larvae ). These larvae need about three years to get from the Sargasso Sea to the European coasts. While it was previously assumed that they let themselves be carried passively by the Gulf Stream , we now know that the larvae actively swim.
The metamorphosis of the willow leaf larvae into the approximately 7 cm long glass eels begins about 100 km off the European coast . In spring they swim in large swarms from the European coasts upriver to the inland waters. During this time they are called "Steigaale", also called "Gelbaale" because of their yellowish belly color. In their home waters they will grow to full size over the next few years. Females become sexually mature at 12 to 15 years of age, males at 6 to 9 years of age. To spawn, the animals migrate from the inland waters in September / October via the rivers back to where they hatched: into the Sargasso Sea. In some cases, distances of more than 5000 kilometers are covered against the Gulf Stream without eating within a year. Results obtained by satellite telemetry on eels to which satellite-recognizable markings were attached showed that the animals spend the day in cool water between 200 and 1000 meters depth during the migration and swim in warmer surface areas at night. They cover the first 1,300 kilometers between Ireland and the Bahamas only 5–25 kilometers per day, much less than the 35 kilometers that would be necessary to cover the distance of 5,000 km within a year. From this it is concluded that the eels later exploit water currents, which then allow them a higher daily speed - for a while it was assumed that European eels did not reach the spawning area at all and all young eels came from American parents and swarmed in both directions.
During the last time in the inland waters and on the way back to the sea, the body characteristics of the animals have changed: their original color changes from green-brown to silvery-gray, the anus is drawn in and the eyes enlarge - the eel becomes a " Silver eel "or" silver eel ". This conversion process takes about four weeks. During this time, food intake is increasingly restricted and finally stopped completely, because the digestive tract recedes completely. Instead, the genital organs develop, which later occupy the entire body cavity. The energy for the "remodeling" of the body and for the long journey to the spawning site is taken by the eels exclusively from their fat reserves, which they have eaten over the years. The fat reservoir is formed in the intestines and under the skin: eels belong to the so-called " fatty fish " because their body mass can consist of up to 30% fat.
The color change is presumably an adaptation to the conditions of the open sea - there a silvery-shiny lower abdomen is less noticeable than a yellow one. The enlarged eyes of the animals could also be a further adaptation to the conditions of the sea.
During the migration, the eels must adapt to the significant changes in ambient osmolarity . This leads to transformations in the gill epithelia of the animals. This process is primarily controlled by prolactin , a hormone that is known in humans primarily for its effect on mammary gland secretion.
The animals spawn and die in the Sargasso Sea.
Eels are able to cover considerable distances over humid land because they can absorb vital oxygen through their skin.
Eels can live up to 50 years of age in the wild. In captivity, they can live to be 80 years old, in individual cases well over 100 years.
Way of life
Eels are particularly active at dusk and at night. They feed mainly on worms, (small) crabs, insect larvae, etc., but also on fish spawn and fish. Small fish are actively hunted in the mean water and on the water surface. The eel proves to be a skilled hunter.
The European eel occurs in two diets in our waters: One is the pointed head eel, with a narrow head and pointed snout, which feeds primarily on crabs and other invertebrates. The other variant is the broad-headed eel, with a broad head and broad snout, a fish hunter.
Both forms also exist side by side in the same bodies of water, the percentage distribution between the two forms exclusively depending on the prevailing food supply. In waters with large populations of small fish and low populations of crabs, up to 90% white eels will be found in relation to black eels and vice versa.
The often widespread statement that eels are scavengers results from their hiding behavior, which was previously used to catch animals by laying out animal skulls. This fishing method was described beautifully in literature by Günter Grass in the tin drum , but technically incorrectly. Fishermen know that at best, eels will eat freshly killed bait, but never decay. The reason lies not least in the eel’s extremely finely developed sense of smell. He is able to perceive individual smell or taste molecules. His tubular nostrils also enable him to perceive and follow a scent trail in all three dimensions ( stereoscopic smelling).
Hike
Eels, as indicated, are catadromous migratory fish, meaning that they migrate from freshwater into the ocean to spawn. The journey to the Sargasso Sea takes one to a year and a half, and it begins in the eels' living waters. Between October and November, and even in December if the weather is mild, the eels get restless and set off. The train time is in the evening and night hours. Especially in very bad weather, when it is stormy and raining, the eels seem to increase their "wanderlust". At first they are still very active, meandering from the smallest trenches into larger streams or from standing, closed bodies of water through damp grass into the next stream or river. In the major rivers such as the Rhine, Weser, Ems, Elbe and Oder, however, they can then be largely driven by the current, saving energy. They float, curved in an S-shape, in the mean water. Once in the estuary, they swim actively again and immediately go to depth.
During their migration in the sea, the silver eels perform diurnal vertical migrations, i.e. H. During the day they swim at depths of up to 1000 m and at night they almost rise to the surface of the water. In the following year, they arrive in the Sargasso Sea, where they probably spawn at depths of up to 2000 m. This last act of life robs them of the very last energy reserves - after mating and releasing the sex products they die.
History of science
The life cycle of the European eel has puzzled man for many centuries. Aristotle was still convinced that eels either formed spontaneously in the mud, formed from dust or were born from earthworms. The viviparous eel mother ( Zoarces viviparus ), a small to medium-sized marine fish with an elongated body, got its name because it was said to give birth to small eels. In the Middle Ages, the eel was often assigned to snakes, or at least claimed that eels and snakes would mate. Numerous components of the eel have been assigned healing powers in folk medicine .
It was not until the end of the 19th century that the transparent fish shaped like willow leaves, which had previously been scientifically described as Leptocephalus brevirostris , were recognized as the larval form of eels. In 1922 the Danish zoologist Johannes Schmidt discovered the smallest larvae north of Bermuda to this day . The exact mating process has never been observed in the wild.
The European and American eel spawn in the Sargasso Sea south of the Bermuda Islands between 20 ° and 30 ° north latitude and 80 ° and 50 ° west longitude, the Japanese eel in the western north Pacific south of Japan near Guam [1] and the Australian short fins Eel and the New Zealand eel in the central Pacific between the Bismarck Archipelago and Fiji.
In 2013 and 2017, experimental evidence was published that European eels can orient themselves in the earth's magnetic field and therefore have a magnetic sense .
Danger
The European eel is now considered to be critically endangered, the IUCN lists the species as Critically Endangered , the signatory states to the Washington Convention on Endangered Species (CITES) decided in 2007 to include the European eel in Appendix II (species in need of protection) of the Convention, what was implemented by the European Union in 2009.
Fishing
Glass eels are caught in large quantities off the European coasts for direct consumption or for fattening in aquaculture. In recent years the number of catches has decreased dramatically (according to Greenpeace by 99% in the last 20 years). In addition, through publicly funded eel stocking measures, glass eels are caught in the estuaries of large rivers and used in inland waters to improve the eel stocks. This practice is criticized by the fisheries ecologist Reinhold Hanel, as many animals (40% according to a French study) die while being caught and transported. In the meantime, there has been a lucrative illegal trade in glass eels from Europe to Asia.
Swim bladder worm
The swim bladder worm, which is brought in from Asia, lives as a larva in hoppers and is ingested with them by the eating eel. The worm develops in the eel and migrates into the swim bladder , where it lives on epithelial and blood cells. The swim bladder is damaged by the worm and can no longer fulfill its function of balancing the fish in open water. As long as the eel lives in fresh water, it is a bottom fish that is only slightly dependent on its swim bladder. As soon as it wanders into the sea as a silver eel, the swim bladder becomes its most important pressure compensation organ. A damaged swim bladder can no longer allow the eel to float freely in the water, so that the eel has to invest more energy in swimming. This energy, which he draws exclusively from his fat reserves, may then no longer be sufficient for the entire journey or may be missing in the subsequent spawning business. This means that the eel starves to death during the journey or does not spawn later.
environmental pollution
Many toxins that have got into the rivers are fat soluble. The eel takes them in with its food and thereby enriches it in its fat stores. When his body is restructured - the digestive organs are broken down and the sexual organs are built up - these toxins get into the gonads and can prevent successful reproduction.
Water management
Although the eel is characterized by an extremely tough migration behavior, which enables it to go ashore or even allows it to overcome smooth concrete weirs, large numbers of the silver eels become victims of the hydropower plants when they migrate . They follow the flow and get into the turbines of the power plants.
The European eel has 'critically endangered' status. It is estimated that this fish will no longer be found in European waters in 20 to 30 years. This assessment is based on the sharp decline in the glass eel population on European coasts since the late 1970s (Moriarty & Dekker 1997). The eel can still be found in almost all waters that drain into the Atlantic.
Fish of the year
The Association of German Sport Fishermen (VDSF), the Austrian Board of Trustees for Fisheries and Water Protection (ÖKF), the Federal Agency for Nature Conservation (BfN), the Association of German Sport Divers (VDST) and the Swiss Fisheries Association (SFV) have the eel as fish of the year 2009 and Elected in 2018. This choice is intended to indicate the risk and to promote improved protection.
Eel catch
→ Main article: Eel fishing
The eel migration in the Baltic Sea through the Danish Belte was the basis for an important traditional fishery with characteristic nets (bundle yarn). Most eels are caught in eel traps, which are special trap constructions.
Importance in the kitchen
The eel is a popular food fish , which is characterized by its extremely high-fat meat. Due to its fat content, it is particularly suitable for smoking. But it is also fried or boiled. In addition to smoked eel, the forms of administration are eel skewers and eel soup . A specialty in the Hanoverian area is above all Steinhuder Rauchaal also prepared as “eel in jelly” and “eel green”. The Hamburg eel soup contains smoked eel as an ingredient.
Eel bridges are small eels that are processed into roast marinades .
The first eel eaters were probably the Scandinavians, because eel skeletons were found in their kitchen waste from the period between the Late Paleolithic and the Iron Age.
The blood of the eel contains a haemolytic poison ( ichthyotoxin ) which, however, is neutralized during cooking, roasting or smoking. Eel blood should therefore not come into contact with the eyes or mucous membranes. It can lead to paralysis or vomiting.
Individual evidence
- ↑ Overview "Fish of the Year" in Germany. German Fishing Association, accessed on February 26, 2018 .
- ↑ Fish of the year 2017 in Switzerland. (No longer available online.) Swiss Fisheries Association, archived from the original on February 27, 2018 ; accessed on February 26, 2018 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ a b K. Aarestrup, F. Okland, MM Hansen, D. Righton, P. Gargan, M. Castonguay, L. Bernatchez, P. Howey, H. Sparholt, M. Pedersen, RS McKinley: Oceanic spawning migration of the European Eal (Anguilla anguilla) . Science, Sep 25, 2009; 325: 1660.
- ↑ Background information: European river eel (Anguilla anguilla) , WWF Germany and TRAFFIC Germany, Frankfurt am Main 2010.
- ↑ ORF.at : Probably the oldest eel in the world perished on August 8, 2014, loaded on May 13, 2015.
- ↑ Kathrin Passig, Aleks Scholz: Lexicon of ignorance. Rowohlt, 2007, ISBN 3-87134-569-5 , pp. 19-24.
- ^ Caroline MF Durif et al .: Magnetic Compass Orientation in the European Eel. In: PLoS ONE. Volume 8, No. 3, 2013, e59212, doi: 10.1371 / journal.pone.0059212 .
- ↑ Lewis C. Naisbett-Jones, Nathan F. Putman et al .: A Magnetic Map Leads Juvenile European Eels to the Gulf Stream. In: Current Biology. Volume 27, No. 8, 2017, pp. 1236-1240, doi: 10.1016 / j.cub.2017.03.015 .
- ↑ Anguilla anguilla in the IUCN Red List of Threatened Species 2015.4. Posted by: Jacoby, D. & Gollock, M., 2013. Retrieved March 1, 2016.
- ↑ Import and export of the European eel (Anguilla anguilla) and its products. Federal Agency for Nature Conservation, accessed on October 4, 2018 .
- ↑ Greenpeace study 2006. Summary https://www.greenpeace.de/themen/endlager-umwelt/swimming-chemikals-greenpeace-studie-zur-schadstoffverbindung-von-aalen and https://www.greenpeace.de/sites /www.greenpeace.de/files/greenpeace_chemie_im_aal_0.pdf Note: Full text of the study not found; here only secondary information without reference to the original study on population decline.
- ↑ Weser-Kurier of Tuesday, April 21, 2015.
- ↑ The European eel - an endangered species of fish. (PDF; 286 KB) Federal Food Safety and Veterinary Office , February 6, 2019, accessed on December 1, 2019 .
- ↑ Death trap hydroelectric power plant In: srf.ch , January 31, 2018, accessed on February 1, 2018.
- ↑ Waterworks turbines shred thousands of eels In: srf.ch , January 31, 2017, accessed on February 1, 2018.
Web links
- European eel on Fishbase.org (English)
- Supplement to the eel migration
- European eel on Pivi.de
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