Project status

With the vital help of citizen scientists and partner organisations, ZSL has been working for the survival of Critically Endangered European eels.

  • 1107 volunteers
    and 22 partner organisations have been involved in this project to date.
  • Access to 139 hectares
    of additional habitat has been improved in the Thames River Basin over the duration of this project.
  • ZSL’s work for the European eel in the Thames

    European eels (Anguilla anguilla) once thrived in London’s rivers but the number of young joining the adult populations has dropped dramatically since the 1980s and the species is now classified as ‘Critically Endangered’ by the IUCN Red List of Threatened Species. ZSL’s work on European eels started in 2005 and now ranges from informing international conservation policy and leading UK research on eel behaviour to regional-scale best practice conservation delivery in the Thames Catchment. Here, through research, river improvements, public engagement and advising policy, we work in partnership with the Environment Agency and a network of other organisations to address the key conservation issues facing eels.

    Reconnecting broken migratory pathways

    A principal conservation action for eels in freshwater is to restore migratory pathways by removing barriers such as weirs. If removal is not possible, the impact of barriers can be partially mitigated by installing eel passes that allow eels to move upstream over them. With over 2000 barriers to migration, the Thames rivers are highly fragmented. One of ZSL’s key goals in the region is to reconnect migration routes. So far, eel passes commissioned by ZSL have made 138.95 hectares of additional habitat accessible.

    migratory pathways for eels

    Assessment of Adult European Eel Stocks in the Thames River Basin

    ZSL has monitored the recruitment of juvenile eels into the Thames River Basin since 2005. The data collected to date supports the findings that the annual recruitment of the European eel across its range has reduced by 90-95% since the 1980s. It is likely that this reduction in recruitment will have had an impact on the numbers of adult eel in the Thames River Basin. In 2019, ZSL repeated surveys of adult eels undertaken in 1987 and 2011 to assess the geographic spread of eel populations in the Thames and identify the effect of the range of measures that have been introduced to improve eel stocks.

    Thames Eel Management Plan 

    ZSL is a key partner in delivering the Environment Agency’s Thames Eel Management Plan. The management plan outlines key targets and measures to improve the population and conservation of the European eel. Given the scale of the challenges faced by eels, working together is essential. – if you or your organisation would like to join us in this work please get in touch.


    ZSL are working with partners in the Thames Rivers Trust Thames Catchment Community Eels Project to co-develop a standardised methodology for citizen science river walkover surveys with the aim of mapping barriers and assessing their impact on upstream eel migration. The barrier assessment method will be based on the EBAT scoring system and data captured using the River Obstacles App, which ZSL worked in partnership to help relaunch in 2021. This is an exciting development which will help fill data gaps on where barriers are in our rivers nationally and help river Catchment Partnerships make decisions about how to prioritise conservation action for eel and other fish species by restoring source to sea habitat connectivity.

    fieldwork eel conservation
    close up for eel Thames estuary

    The importance of marshes to European eels

    close up of eel out of water with grey background
    European eel underwater

    How can you help?

    From March to June each year, ZSL and project partners recruit citizen scientists at sites across the Thames region to help check eel traps and measure and release any eels present. This gives us data on the number of young joining the adult population and can highlight the impact of barriers, one of the principal threats to eels in freshwater.

    Eel Barrier Assessment Tool

    In 2018, ZSL produced a field guide for assessing the 'passability' of man-made river structures by European eels entitled the Eel Barrier Assessment Tool (EBAT). This step-by-step guide has been designed for use by NGOs, consultants and regulators in the UK and other European eel-range countries. The field guide includes an overview of eel behaviour in freshwater and a series of questions about the structure and its environment to produce an overall assessment score. This is a coarse assessment that can help influence and prioritise future conservation action for the Critically Endangered European eel. You can download the EBAT and the EBAT Form below: 

    Download the Eel Barrier Assessment Tool

    Download the Eel Barrier Assessment Tool Form 

    Project information

    Key species

    • European eel (Anguilla anguilla)

    People Involved

    • Joe Pecorelli and Thea Cox work on the European eel conservation project.

    Partners and Sponsors

    • Canal and Rivers Trust;
    • Environment Agency;
    • Ham United;
    • Kingston University;
    • Medway Valley Countryside Partnership;
    • National Trust;
    • Thames Anglers Conservancy;
    • Thames Rivers Trust Thames Catchment Community Eels Project;
    • Thames Water;
    • Surrey Wildlife Trust;
    • Wandle Heritage.

    Kindly funded by

    • The City Bridge Trust;
    • Disney Conservation Fund;
    • Panton Trust.
    • European Maritime Fisheries Fund