European eel
The European eel is a species of eel. Their life history was a mystery for thousands of years, and mating in the wild has not yet been observed. The five stages of their development were originally thought to be different species. They are critically endangered due to hydroelectric dams, overfishing by fisheries on coasts for human consumption, and parasites.
Description
European eels undergo five stages of development in their lifecycle: larva, glass eel, elver, yellow eel, and silver eel. Adults in the yellow phase are typically around and rarely reach more than, but they can reach a length of up to in exceptional cases. They have 110 to 120 vertebrae. They tend to live approximately 15–20 years in the wild, although some captive specimens have lived for over 80 years. One such specimen known as "the Brantevik Eel" lived for 155 years in the well of a family home in Brantevik, a fishing village in southern Sweden.Ecology
Eels tend to range underwater. After spawning in the Sargasso Sea, they disperse northwards throughout the Atlantic Ocean, its coasts, and the rivers that empty into it. Feeding occurs mainly at night via scent, with prey consisting of worms, fish, mollusks such as slugs, crustaceans such as crayfish, and occasionally plankton, when available in large quantities. European eels are preyed upon by larger eels, herons, cormorants, and pike. Seagulls also prey on elvers. Eels usually find and compete for shelter by hiding in plants or in tube-shaped crevices in rocks. They also hide in muddy fields when inland.Conservation status
The European eel is a critically endangered species. Numbers of eels reaching Europe is thought to have declined by around 90% since the 1970s. Contributing factors include overfishing, parasites such as Anguillicola crassus, barriers to migration such as hydroelectric dams, and natural changes in the North Atlantic oscillation, Gulf Stream, and North Atlantic drift. Recent work suggests that polychlorinated biphenyl pollution may be a factor in the decline. The TRAFFIC program is introducing traceability and legality systems throughout trade change to control and reverse the decline of the species. The species is listed in Appendix II of the CITES Convention. Hydroelectric dams have been shown to have a significant negative impact on eel populations. Over an 80 year period, waters with large dams have experienced almost twice the reduction of eel numbers as dam-free waters.Sustainable consumption
Eels have been important sources of food both as adults and as glass eels. Glass-eel fishing using basket traps has been of significant economic value in many river estuaries on the western seaboard of Europe. In addition, the United States imports 11 million pounds of eel every year to support its sushi industry, including European eels. In order to make eel consumption sustainable, in 2010, Greenpeace International added the European eel to its "seafood red list", and the Sustainable Eel Group launched the Sustainable Eel Standard.Breeding projects
As the European eel population has been declining for some time, several projects have been initiated. In 1997, Innovatie Netwerk in the Netherlands began a project in which they attempted to get European eels to breed in captivity by using a swimming machine to simulate the journey from Europe to the Sargasso Sea.The first to achieve some success was DTU Aqua, a part of the Technical University of Denmark. Through a combination of fresh and salt water, as well as hormones, they were able to breed it in captivity in 2006 and make the larvae survive for 4.5 days after hatching. By 2007, DTU Aqua scientists were able to set a new record where the larvae survived for 12 days by feeding the mother eel with a special arginine-enriched diet. At this age the content of the larval yolk sac has been used, the mouth and digestive channel have developed, and it requires feeding. Attempts with various substances failed. Deep water sampling of the presumed habitat of larval European eel in the Sargasso Sea was performed by the Galathea 3 expedition in 2006–07, in the hope of revealing the likely feeding preference at the early stage. Their results indicated that they feed on various planktonic organisms, but especially microscopic jellyfish. A follow-up expedition was performed by DTU's own research ship to the Sargasso Sea region in 2014.
To further the research, the PRO-EEL project, led by DTU Aqua and involving several research institutes elsewhere in Denmark, Norway, the Netherlands, Belgium, France, Spain and Tunisia, was started in 2010. By 2014, the eel larvae at their facilities typically survived 20–22 days, and by 2022 they were surviving up to around 140 days, well into the leptocephalus stage, but the full life cycle has still not been completed in captivity.
Life history
Much of the European eel life history was shrouded in mystery for centuries, as fishermen never caught anything they could identify as a juvenile eel. Unlike many other migratory fish, eels begin their life cycle in the ocean, spending most of their lives in fresh inland water, or brackish coastal water before returning to the ocean to spawn and then die. In the early 1900s, Danish researcher Johannes Schmidt identified the Sargasso Sea as the most likely spawning grounds for European eels. The larvae drift towards Europe in a 300-day migration.When approaching the European coast, the larvae metamorphose into a transparent larval stage called "glass eel", enter estuaries, and many start migrating upstream. After entering their continental habitat, the glass eels metamorphose into elvers, miniature versions of the adult eels. As the eel grows, it becomes known as a "yellow eel" due to the brownish-yellow colour of their sides and belly. After 5–20 years in fresh or brackish water, the eels become sexually mature, their eyes grow larger, their flanks become silver, and their bellies white in colour. In this stage, the eels are known as "silver eels", and they begin their migration back to the Sargasso Sea to spawn. Silvering is important in an eel's development because it allows for increased levels of the steroid hormone cortisol, which is needed for their migration from fresh water back to the sea. Cortisol plays a role in the long migration because it allows for the mobilisation of energy during migration. Also playing a key role in silvering is the production of the steroid 11-Ketotestosterone, which prepares the eel for structural changes to the skin to endure the migration from fresh water to saltwater.
Sometimes eels will never enter freshwater and will remain in a marine environment throughout their life. Others grow up in brackish water, or migrate between saltwater, brackish water and freshwater several times during their lifetime.
Magnetoreception has also been reported in the European eel by at least one study, and may be used for navigation.