One of the most mysterious and endangered fish in Europe is the European eel. Eels live long and complex lives and during their lifetime will travel thousands of miles, transforming themselves as they go. They can grow up to one metre long and have long, snake-like bodies with one pair of small fins at the sides. They are secretive creatures; most of their early life at sea is a mystery and when they come inland during their adult lives, they are nocturnal, living under stones and burrowing into mud during the day. They can live between seven and 85 years, with an average lifespan of 55 years, and much of this time is spent out of sight.
The European eel is a ‘catadromous’ fish: it is born at sea and spawns or reproduces there; then it migrates to inland waters to eat and grow. European eels can be found from Russia and Finland to as far south as the coasts of Morocco, Egypt and the countries around the Black Sea. They spend most of their adult lives in freshwater rivers, streams and estuaries before returning to the open ocean to lay eggs. We know quite a lot about the eels’ adult lives but big questions remain about their migration. It was only very recently that scientists following eels with electronic tracking devices along part of their route discovered that the adults do not migrate directly to their breeding grounds. Instead, they follow ocean currents far south of their destination and then catch another current to go north later. They are a solitary species for most of their life and do not join other eels in schools or groups.
In the course of their life, eels pass through a number of very different stages, marked by changes in size, shape and colour. The European eels’ life begins near Bermuda in the deep waters of the Sargasso Sea, in the middle of the North Atlantic, where the eggs hatch into transparent larvae called leptocephalus. The Sargasso is noted for its calm blue waters and the Sargassum seaweed that grows there, which the European eels use as cover, drifting with the Gulf Stream as it moves north-eastwards across the Atlantic. This 4,000-mile journey lasts over a year before the eels reach European and North African shores. They gather in the estuaries of rivers in the form of transparent miniature eels called glass eels to continue their migration inland. As glass eels leave the open ocean to enter the estuaries and ascend rivers, they gain colour and are known as elvers. Their migration occurs in late winter, early spring and through the summer months. For the next six to 20 years, the eels grow, putting on weight, becoming longer and developing yellow undersides. This is their yellow eel stage, when they travel continuously upstream toward lower temperatures and less salty waters, possibly also in reaction to crowded waters downstream. During their upriver journey, eels have been observed climbing obstacles such as dams to reach their breeding grounds and also leaving the water altogether and entering fields to eat slugs and worms. After migrating upstream, the male eels grow and feed for six to twelve years, the females for nine to 20 years, before returning to the sea to reproduce. Moving back to the sea, their stomachs dissolve, their skin turns silver and they are called silver eels. They seem to be stimulated to move downstream at night when there is a new moon or when the river is flooding.
Eels were once plentiful in Europe and were a source of food for many inhabitants. In the city of London, which is located on the river Thames, eel pie and jellied eels were a speciality in the poorer East End of the city. But now European eels are on the list of critically endangered species and their numbers have declined by around 90% since the 1970s. Researchers believe that this decrease happens at the glass eel stage, which is when the species is most at risk. In the past 40 years, the number of glass eels arriving in Europe has fallen by around 95%. Several reasons seem to be responsible for the decline. Artificial blocks to their natural migration routes, like weirs and dams, hydropower and water-pumping stations could be preventing the eels reaching their freshwater breeding grounds. Overfishing, pesticides and parasites are also believed to be part of the problem. Researchers in Portugal have found that rising temperatures and acidic waters are posing yet another threat. Dr Reinhold Hanel at the Thunen Institute in Germany also believes that climate change is harming eels by changing the ocean currents along their migration routes to and from the Sargasso Sea so that fewer glass eels are able to drift across to European shores. He stated that for fish that cross oceans and move between fresh and saltwater as eels do, climate change is yet another pressure. These vulnerable fish face so many dangers on their long journeys that the impact of man-made risks is proving too much for their survival.