South East Rivers Trust

Eel Passage on the Hogsmill


During the summer a collaborative project between Zoological Society of London (ZSL), Surrey Wildlife Trust Kingston Group and the South East Rivers Trust (SERT) was undertaken to improve elver and eel passage on one of the lower weirs of the Hogsmill, the Clattern Bridge weir.

The European eel is a critically endangered species and needs all the help we can give it.  Pollution, overfishing, global warming, disease and habitat loss have all contributed to the demise of this charismatic species. The eel has a fascinating and mysterious life cycle in which it starts life in the Sargasso sea as a larvae, migrates across the oceans via currents to European rivers, metamorphosing a couple of times on the way to become glass eels and then elvers.

Once in rivers, such as the Thames and the Hogsmill, they migrate upstream to find habitat in which to grow and develop into yellow eels. After 5-20 years of life in rivers like as the Hogsmill they metamorphose again into silver eels and travel back to the Sargasso Sea to complete their life cycle.  Weirs and habitat loss in our rivers are factors that cause issues for eels and stop their upstream migration to suitable habitat.

This project involved installing plastic tiles covered in regularly spaced plastic protrusions onto the weir face.  The weir at Clattern Bridge is smooth and has shallow fast water flowing over it.  The tiles allow eels to wriggle up the weir and into the river upstream enabling them to carry on their migration. Eels are not very good swimmers compared to other fish and prefer to ‘wriggle’ so increasing friction in this way is ideal for them!

Below is a great video of eels using a similar design on the Wandle:

Armed with a couple of battery drills, some long drill bits and various stainless steel fixings we attached a continuous line of tiles to the weir surface.

eels1

It was a lovely sunny day and perfect for a day in the river!

eel2

A big thanks to the Kingston local branch of SWT who provided funding for materials, drill bits and fixings and to ZSL for supplying manpower and eel tiles.

Elvers migrate upstream between April-September and so we are hoping they will appreciate our efforts when they arrive in 2017!

Author: Tim Longstaff

clattern-bridge-23-september-2016-3

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