The Colne Valley Regional Park are working with Affinity Water to improve migratory routes for the critically endangered European Eel across the lower Colne Catchment. 

European Eel Life Cycle and Migration

European Eels have the longest migration of any freshwater fish in the UK. After hatching in the Sargasso Sea, in the Western Atlantic, they travel over 6000km across the Atlantic Ocean to Continental Europe and up the River Thames to reach the rivers and wetlands of the Colne Catchment.    

Upon reaching freshwater, juvenile eels, known as glass eels, undergo a series of metamorphoses to become recognized firstly as elvers (small bootlace sized eels with the instinct to swim upstream) and then as yellow eels (adult eels) as they enter suitable freshwater habitat. 

Eels can spend up to 20 years living in their freshwater homes, before making the return trip to the Sargasso to spawn. As eels ready themselves for the journey, they turn silver in coloration and adapt their internal organs to tolerate saltwater. 

What Obstructs Eel Migration and How can this be Overcome?

Barriers to eel migration, such as weirs and culverts, obstruct the course of the river and prevent eels from reaching the habitats they require to complete their adult life stage. 

Fortunately, methods exist to help elvers get past even the largest of barriers. In most instances, a bespoke eel pass can be fitted to provide elvers with suitable substrate to climb over obstructions. In some cases the total removal of a weir may be considered in order to provide free passage for all fish species. 

How does an Eel Pass Work?

An eel pass is a shoot with water running down with special climbing substrate. This could be made of eel tiles which have a surface similar to the bottom of football boots or an eel brush. 


An Eel Brush Pass, Photo: Aquatic Control Engineering 


Project Funding and Partnership

Affinity Water has provided the CVRP with funding to improve eel passage at key sites where there are barriers to their migration.
Where screening at a surface water intake is found not to be compliant with The Eel Regulations (England and Wales) 2009, the organisation must make the intake compliant, if feasible to do so, or make improvements via alternative measures by other means which includes improving eel passage within the same catchment.

Eel Passage Project Plan

The mitigation works will be delivered as part of the Colne Valley Regional Park’s Fish Passage improvement program, which works to rectify barriers to the passage of all fish species across the Colne Valley. You can find more information on fish passage on the CVRP website, here

fish highways | fish passage

The Colne Valley Regional Park are in the process of approaching nine landowners of sites where key barriers to the migration of eels exist, covering the River Colne, Wraysbury River and Frays River. Eel passes will be installed at the sites where the landowners are happy for work to proceed and where it is feasible to do so.

Over the next year we will work alongside a specialist contractor to produce bespoke designs for eel passes at each site; to obtain the relevant permissions to carry out works and finally to construct each pass next winter. 

Project timeline: 

  • Feb – March: Initial feasibility assessment of all sites 
  • March – July: Design phase for all viable passes 
  • Sept – March: Construction of all permitted passes 
  • GlassElversInPassCreditCliffCarson

Elvers in a brush pass, Photo: Cliff Carson.


Slippery Eel Facts

Some questions and answers on this fascinating topic:

How do eels migrate when they are so tiny?
Eels rely on transatlantic currents such as the Gulf Stream to carry them across the Atlantic Ocean to our shores. Once they reach our rivers, they will use tides to carry them upstream. Once they reach non tidal sections it’s up to them to swim, but by this stage they have changed in shape, appearance and strength and are better suited to swimming. 

How do we know they spend 20 years here? Have they been tagged?
Fish can be aged via analysing a scale sample or otolith (ear bone) under a microscope. They contain age rings, much like the trunk of a tree, and when counted will tell you how old a fish is and how long it has spent in sea or freshwater. 

Do the eel passes work both ways or is it easier to get downstream? Presumably the pass is designed for the elvers?
Eel passes are designed for upstream migration. Providing a barrier does not cause damage to fish passing downstream, eels are generally able to plop over the crest of a weir and into the pool downstream. No pass required unless there are features that may damage the eel as it moves over the weir. 

What is it about the CVP that makes it ideal habitat for an eel? 
Bountiful wetland habitat with abundant sources of food (freshwater invertebrates and fish). Eels will make use of the rivers in the CVRP and all the waterbodies connected to them (canals, gravel pit wetlands, ditches, ponds). Plus eels have been migrating to the Colne for thousands of years and it is this inherent instinct that directs them here. 

How long is a mature eel?
60-100cm. Bigger in some exceptional cases. 

Find out More

If you have a question about European Eels or want to know more about our project, please contact us at This email address is being protected from spambots. You need JavaScript enabled to view it.

Updates via the Colne CAN and Colne Valley Regional Park websites will be provided as the project proceeds over 2021. 

Design by LTD Design Consultants and build by Garganey Consulting. From an original concept by the Herts and Middlesex Wildlife Trust.