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The European Eel
The eel is an ordinary and well known fish but its habits are quite unusual and mysterious. The European eel (Anguilla anguilla) is the only representative of the eel family (Anguillidae) in the eel-shaped (Anguili-formes) order to occur in Poland. Its close relatives, 15 other eel species, in the majority dwell in the warm seas. Among them there are such giants like morays and eel congers which length reaches 3 m and weight nearly 100 kg. Biologically, the closest to our European eel are two other inland species - the American eel (Anguilla rostrata) and the Japanese eel (Anguilla japonica). The last-mentioned is bred in Japan in pond-farms. There is not however any affinity with the electric eel which belongs to a completely different group of fishes.
The characteristic feature of all eel species is slender, eel-shaped form. Our eel, among its exotic relatives, differs in geographical range, variety of colonised habitats and transoceanic migration. It lives in Europe and North Africa and it finds itself well both in icy cold waters of the White Sea and in warm waters, among other things at Nile's estuary. But even individuals which grew in the Arctic migrate for spawning to the tropical Sargasso Sea. Such adaptable abilities are not found in any of our eel' s relatives. The eels are usually resident fishes and their occurence is generally limited to one climatic zone.
The European eel only by the look of it differs greatly from our other fishes. With its alongated, cylindrical form it resembles more a snake than a typical fish. The characteristic trait of its shape is the lack of abdominal fins while long dorsal and anal fins are continuous around the tail tip. Among our freshwater fishes only eel has such a fin „ribbon". Eel' s head is slender and numerous little teeth enable catching a prey. Scales covering the whole body are so small that are hardly seen. Eel's colouring is usually dark with brown or olive tint; during spawning yellowish abdomen becomes silvery. Sometimes black eels occur and rarely albinos.
In its resident phase eel is found in inland and littoral waters almost in every corner of Europe. One of its peculiar habits is the fact that representatives of both sexes dwell in different habitats. Males generally reside in salty waters at rivers' estuaries while females reside in inland fresh waters. They live in ponds, lakes and slowly flowing rivers with muddy bottom and rich vegetation. The European eel, like its relative species, leads a secretive style of life, ravens mainly at night from ambush and spends a day hidden in mud, roots and other aquatic shelters. Its food is based on aquatic animals -invertebrates, fishes and its spawn, tadpoles, crayfishes in time of moulting when they are deprived of carapaces. It is not proven that eel eats carcass - sometimes it looks for a shelter there.
After 7-9 years (sometimes longer) of a resident life eels migrate for spawning covering thousands of kilometres of seas and the Atlantic Ocean. Females living in inland waters float by the rivers to the sea overcoming on the way obstacles like dams, dikes and even short distances on land when it is possible thanks to rain or dew. In the sea females are joined by males and from this moment eels head towards the Atlantic Ocean. Their journey ends up on the western hemisphere. Here in the Sargasso Sea near Bermudas, at the depth of around 1000 m, the European eels spawn. After that eels probably die - their disappearance in deep sea waters is one of mysteries of this fish unsolved so far.
Larvae hatch from spawn and are so totally unlike the adults with their leaflike form that formerly were considered a different species. The smallest larvae known are 7 - 15 mm long. They stay at the depth of 180 to 200 m, where the temperature of water is 20°C. At the beginning they grow very rapidly and in the first year of their life they reach the length of 2.5 cm. Larvae in the western Atlantic Ocean find their way in the zone of Gulf Stream which within three years carries them to the banks of Europe. At the end of this transoceanic migration, when they cover the distance of 6-7 thousand km, they reach the length of about 7 cm and become similar to a small eel but still they remain transparent and colourless.
The next aim of the migration are rivers of the whole Europe where small eels finally get the right colouring. Males usually remain at estuaries while females untiringly continue up the rivers. Some of them reach the waters of the Black Sea's estuary and to Nile's estuary covering the distance of over 10 thousands km from the place of hatch in the Sargasso Sea. Time of resident life before the migration lasts from seven to more than ten years. Growing process is slow but eel can reach in this period the length of 2 m and the weight of 6 - 7 kg or even 9 kg; male is about 55 cm long. In Poland the most usual to find are individuals not exceeding 1 m and 2 kg.
One of eel's peculiarities is its toxic blood which is similar to curare in action. However, the toxin decomposes in the temperature higher than 60°C so after thermal treatment (smoking or frying) eel can be eaten without any fear. Eel is a fish of a considerable economical importance, valued for its meet which has high nutritious and of taste qualities.
Eel in our country is not protected by close-season - it can be angled around the year. There is imposed however minimal size which prevents from angling too small individuals. Depending on the type of a reservoir and other circumstances the size is the following: 40 to 55 cm. Similar regulations are valid in most European countries.
The number of eels in Poland, like in other countries, decreases rapidly. Great demand on the international markets caused extermination of the population by excessive fishing. Also contamination of waters is harmful. However, the biggest threat are dams which render impossible eel's migration for spawning. This is the reason of the disappearance of sturgeon, salmon and bull trout from our waters. May eel not divide their fate.
Prof. Maciej LUNIAK, Museum and Zoology Institute of PAN, Warsaw
Lechosław CZAJKA, Polish Fishing Association