Common freshwater eels and their migration

In the autumn last year, adult common eels migrated down the river Ouse and other Sussex rivers to the sea. Their return to the sea was the first stage of a very long migration when thousands of common eels swim to the Sargasso Sea where they are believed to breed. The adult eels live in the freshwater rivers where they grow and mature. This can be affected by temperature, availability of food and other factors. A male eel may be between 7 and 12 years old and a female between 9 and 16 years old.

When they reach the Sargasso sea, breeding takes place in the dark depths and a female eel may lay millions of eggs but many will be eaten by other animals. However the exact location has so far not been found and it is assumed that the adults die after spawning. The fish larvae that hatch from the eggs are leaf-like in shape and these larva eventually travel 3,400 miles back to the UK, Sussex and other parts of Europe.

These eel larvae (called leptocephalli ) follow the Gulf stream back to the UK. They live in plankton and some can travel 5 miles an hour riding the ocean currents. When the larvae reach the continental shelf they change into what is called the “Glass Eel” stage before continuing with their migration. The continental shelf is the shallow part of the seabed that extends out from a continent before the seabed drops into deeper water. They reach the UK during the spring and when the sea temperatures have reached 9 degrees centigrade, the glass eels make their migration through the estuaries and into freshwater. This usually happens from March to May.

In freshwater the young eels begin to darken and are then known as elvers and look like tiny versions of the adult and are about 5cm in length. To help their journey through the estuary and river they take advantage of the use the tidal currents, migrating upstream on the flood tide while the tide is carrying them upstream. During the ebb tide, the river direction changes and the river again flows more strongly back to the sea. The tiny eels move out of this current towards the river bank to prevent them from being washed out to sea. The tiny eels face many dangers including predators. However one of the biggest dangers comes from human development of the river where man-made obstacles block the way. This would prevent the eels from continuing their migration. Many made eel passes have been put in place allowing the eels to continue on their migration up river, such as the one at Barcombe Mills many miles upriver from Lewes.

The elvers continue to grow and are then know as brown (or yellow) eels. After an average of six years for males and nine years for females, the eels begin to mature, stop feeding and become silver in colouration ready to return to the ocean, but some eels may be much older.

Common eels are critically endangered because of river obstructions, pollution and drought and eel management plans have been set up to help protect and restore the number of eels.
We often see small elvers in the ponds and ditches on the reserve. Eels are even able to travel short distances across the land over damp ground to reach another body of water.