The European eel has, perhaps, one of the most interesting life history patterns of all European freshwater fish. They are widely distributed within European freshwaters and can be found in a wide variety of freshwater and estuarine habitats.
What makes the eel life-cycle so interesting?
The migration of eels from the freshwater and estuarine waters of Europe to the Sargasso Sea, located to the south-west of Bermuda, is one of the most impressive feats of animal migration observed in nature. Although this migration is accepted as scientific fact, surprisingly little research has been carried out to determine whether this the only site used by European eels to reproduce. No silver eels, the migratory adult form, have ever been caught in the open ocean, and it is unclear whether spawning takes place at only one time of year or over an extended period. It is also unclear how long it takes eels take to get to their spawning grounds, and how many of the eels that leave Europe reach the Sargasso Sea to spawn.
After spawning, it is assumed that the eggs drift eastwards towards Europe with the Gulf Stream. During this time, the eggs hatch and the young larvae, now called leptocephalus, are 'leaf' shaped and it is only on reaching the European continental shelf that they metamorphose into the cylindrical shape that we associate with eels. These fish are virtually transparent and are called 'glass eels'.
On entering freshwater, the glass eels start to change in colour and become darker as pigmentation takes place. They are now known as elvers. During this time, they migrate upstream and occupy a wide variety of freshwater and estuarine habitats, feeding on invertebrates and fish. Eels are also known to move overland, moving over wet terrain to get to some waterbodies. During their freshwater-resident phase they are known as yellow eels and they can remain in these habitats for more than 20 years. When eels are ready to reproduce, their colouration changes and they start to turn silver in preparation for their migration to the Sargasso Sea.
Are eels under threat?
All life stages of the European eel, glass, elver, yellow and silver, have been exploited by man for centuries and throughout Europe they have supported a number of large-scale commercial fisheries. Within Scotland, eels have not been heavily fished, but some limited exploitation has taken place in some localities. Although long-term datasets are few, it is clear that European eel have undergone a significant and drastic decline over the last two decades. It has been estimated that eel numbers have decline by over 90% during this period.
Are there conservation measures in place?
The widespread decline in European eels has led the European Commission has develop an Eel Recovery Plan (Council Regulation No 1100/2007). This plan aims to return the European eel stocks adults and glass eels) to sustainable levels. Each Member State is required to establish national Eel Management Plans. An Eel Management Plan for Scotland was developed by Marine Scotland Science in 2008.
The European eel was also added to the UKBAP Priority Species List in 2007.