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European eel, Anguilla anguilla (Description, feeding, reproduction, threats and conservation)

Credit of the photos: Glenda Vélez Calabria (Colombia)

Technical review: Abdel Rahman El Gamal

These photos have been taken in the recovery plan of European eel (Spain)

 

Introduction: The European eel, Anguilla anguilla, is a catadromous snake-like species of eels, that belongs to the family “Anguillidae”. This species is a popular food fish of high economic value whether in food fish and/or in the trade of young eels (glass eel).

The European eel is found in water bodies that are connected to the sea in all European rivers draining to the Mediterranean, North and Baltic seas, in the Atlantic south to Canary Islands and parts of Mediterranean North Africa and Asia. The spawning area of the species is in western Atlantic (Sargasso Sea) in the West Central Atlantic. The species has introduced from its native range and stocked in most inland waters of Europe as well as in Asia and South and Central America.

Even though the average life span of the European eels is about 15-20 years, so individuals can live more than 50 years. In general, females live longer than males and grow to be about twice the size.  During the cold months, eels are capable to hibernate.

Description: The European eel, Anguilla anguilla has an elongated shape of an average length that ranges from 60–80 cm. However, they may reach a length of 1 m and even 1.5 m in exceptional cases. They have a tough and slimy skin with small, minute and elliptical scales embedded in the skin. Their gill openings are small. The eel has one pair of small and rounded pectoral fins but no pelvic fins. They have a visible lateral line.

Feeding: European eels are nocturnal opportunist carnivores. In nature, they eat a variety of fish and invertebrates especially molluscs and crustaceans, and will also scavenge on dead fishes. As they migrate for spawning, its food are obtained from the aquatic fauna whether in freshwater or in marine. The ability to stay out of water –as conditions permits- enhances their feeding ability and allows them to feed on other organisms such as worms. Small eels feed on insect larvae, molluscs, worms, and crustaceans.

In eel farms whereas glass eels are stocked in farming operations, the transition from natural food such as worms to artificial feed (i.e. paste and/or pellets) is important for replacing the natural food with a nutritionally rich dry/artificial diet. In general, eels demand extraordinary high quality of feeds in regard to nutrient specifications and digestibility.

Reproduction and life cycle: The life history of the European eel’s was a mystery for centuries especially in regard to their trip to spawn and the back trip of their progeny to parent’s home.

It is documented that the Danish researcher Johannes Schmidt was the first who pointed out in the early 1900s that the Sargasso Sea is the spawning grounds for the European eels.

Eels reach their sexual maturation in freshwater after 5–20 years. Sexually mature eels are characterized by their larger eyes, their silvery flanks and white belly and are known in this stage as “silver eels”. Mature eels migrate from European rivers and estuaries to the Sargasso Sea in the West Atlantic for spawning. This trip covers great distances which could range from 5,000 – 6,000 km.

Mature females may spawn up to 3 million eggs per/kg of her body. Adults die after spawning and egg fertilization whereas fertilized eggs hatch into planktonic, transparent ribbon-like larvae known as “Leptocephalus” which drifts from the sea back to the continental waters. When the “leptocephali” are just approaching the European coast in about 200-300 days, they metamorphose into a transparent larval stage (6-8 cm length) called “glass eel” that results in a shortening of the body and formation into a more cylindrical shape.  As “glass eels” continue their trip in estuaries and lakes and gain pigmentation, they metamorphose again into pigmented “elvers” which move into freshwaters. The elvers are miniature versions of the adult eels. As elvers grow, they turn into “yellow eels” based on the brownish-yellow color of their sides and belly. Afterwards, yellow eels in freshwater undergo a long feeding period and so growth for a period of 6-12 years in males and 9-20 years in females ending by sexual maturation, changing color into silvery (silver eels), developing bigger eyes and accumulating higher body fat contents. This phase is their last summer in freshwater before being stimulated by lower temperature and higher salinities and hence performing their spawning trip to the Saragossa Sea. During the spawning trip, the gametogenesis occurs.

Threats and conservation efforts: Overfishing of glass eels in specific European waters as well as on downstream migrating eels (silver eels) across Europe is considered a significant threat to the species. Since there is no artificial reproduction of eels, the shortage in glass eels will lead to a subsequent decline in the catch of adult eels. The reports show that the recruitment of glass eels has drastically declined from 1980 onward, showing in 2000 a 95 to 99% decline compared to prior 1980. This decline in recruitment will translate into a future decline in adult stock, at least for the coming two decades.

Noting the longevity of the species, and the extremely depleted stocks, the restoration of the stocks is expected to take several generations depending on the protection level especially in the absence of artificial reproduction.

The sources of threats include overfishing (to adults or glass eels), dams (may block the migration routes), disease (e.g. nematodes), pollution and loss/degradation of spawning grounds.

Because the decline of eel stocks is likely to continue, the protection and conservation of eels became necessary. The conservation strategy is being implemented in concerned countries through governing regulations and recovery plans.

An example to the actions taken at the international level, is listing the species in the Appendix of CITES in 2007 which implies some actions like the exporting permit to ensure that the export of eels will not be detrimental to the survival of the species. Some states issued their own regulations such as banning the importation, sale, or culture of live eels in California (USA).

The recovery of the European Eel stocks targeted to permit the escapement to the sea of at least 60% of eels less than 12 cm in length; caught eels of this length category should be reserved for restocking and not for aquaculture. The conservation measures include reducing commercial and recreational fisheries, improving habitats and making rivers passable.

References: IUCN Red List, Encyclopedia of Life, FAO, Wikipedia

European eels (adult) European eels (glass)

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