Eels are fascinating – and slippery – subjects. When lake dipping with a beaver group last week we discovered an elver.
This was the first eel I had ever seen and so I decided to find out more!
Firstly, eels have a fascinating life cycle. The story starts in the Sargasso Sea (interestingly the only sea in the world with no coastline, defined entirely by ocean currents), halfway between Bermuda and the West Indies. Here, eggs hatch into transparent larvae called leptocephalus. By floating on oceanic currents, these tiny creatures make their way across the ocean to Europe, arriving at the Continental Shelf in early spring where they become glass eels.
Now cylindrical in shape (instead of flat), they are washed into our rivers by the tide. Some eels stay in the estuaries, others work their way upstream. As they grow they develop into elvers, around 7 – 8cm long. As they gain in size, they start to develop colour. Eventually the elvers reach a location where they will stay as yellow eels for up to 40 years. Here they will feed, grow and continue to develop the characteristic dark eel colour.
The next part is all a bit mysterious. One night, and no one seems to know why, the eels start to change. They turn mottled green-black on top and silver underneath. They head downstream and swim back to the Sargasso Sea on an epic 3000 mile journey. Here they will spawn and the cycle begins again.
Eels were once so numerous they were used as a form of currency. Records show that in 1087 the river Ouse alone yielded an incredible 52,000 eels! However, European eels are classified as critically endangered by the IUCN. Across Europe, elver numbers have crashed to less than 10% of their 1980’s levels.
So what has happened? There are so many barriers (both literally and metaphorically) to eel populations.
Habitat loss 50% of wetlands have disappeared across Europe in the last century. This declining freshwater habitat is vital from the growth stage of eels’ life cycles.
Barriers to migration Much of the habitat that does still remain is inaccessible to eels. Flood defence engineering and weir construction have created barriers which prevent eels from moving upstream..
Climate change Changes in climate affect ocean currents and it may be that the larvae just aren’t getting here
Overfishing Overexploitation and illegal fishing has led to a decline in eel numbers at all stages of their life cycle
Parasites An invasive parasite, Anguillicola crassus, is now widespread across European waters. This parasite infects the eels’ swim bladders, affecting spawning migration.
Hopefully, as the summer continues we will find more eels, as well as other fascinating creatures. If you would like to have a go at lake dipping, we will be running sessions every Tuesday during the summer holidays.