An Eye on Eels

Joe Pecorelli, Zoological Society of London

All is not well with the European eel (Anguilla anguilla). Research over the last thirty years indicates that the number of eels arriving each year (recruitment) in some rivers in Europe, from the Sargasso Sea, is believed to have declined by up to 95%. In 2008, in recognition of this worrying decline, the International Union for Conservation of Nature listed the European eel as critically endangered. In turn, this has led to the development of statutory regulations in the UK to protect eel populations.

The reasons for the decline are not fully understood, but it is mooted to be a combination of habitat loss, barriers to migration, presence of a swim bladder parasite, over-fishing, and climate change affecting the oceanic currents that carry the passive leptocephali (larval eels).

Since spring 2005, the Zoological Society of London (ZSL) has been monitoring the upstream migration of elvers using simple traps. The data collected so far feeds into the Environment Agency’s Eel Management Plan for the Thames River Basin. Since 2011, thanks to the Esmeé Fairbairn Foundation, in collaboration with the Environment Agency, ZSL have expanded the monitoring programme by enlisting the support of volunteer eel monitors and by working in partnership with a number of local organisations. At the end of the second year of monitoring we now have thirteen monitoring sites and work in partnership with; Kingston University, the Wandle Trust, Medway Valley Countryside Partnership, North West Kent Countryside Partnership, the Thames Rivers Restoration Trust, London Wildlife Trust with Friends of the River Crane, Thames21, The Wildfowl and Wetland Trust, and the Thames Anglers Conservancy.

The most important element of the programme is that all involved are working safely. To that end, all our volunteers are obliged to attend a training session before they join the programme. Training principally consists of a health and safety briefing but also includes information on eel biology and instruction on how to collect the data and upload it to the ZSL website. To date over 160 volunteers have gone through our training. All monitoring equipment is provided by ZSL.

It is essential that we keep gathering information on the movement of eels through the Thames catchment and do what we can to help turn around this worrying trend. One of the problems for eels in the Thames catchment is river barriers such as weirs. These prevent or hinder upstream migration and reduce the amount of habitat available to eels. The Environment Agency has identified 2,393 barriers within the Thames catchment. The eel monitoring programme adds much needed additional capacity to tackle this problem. Knowing which rivers are being used by the eels and which barriers are most hindering their migration can guide the Environment Agency and others in prioritising and planning for more eel passes over these barriers. The more habitat the eels have access to, the greater the population that can be supported.

We are already planning next year’s sites and we need your help to expand this important scheme. If you would like to get involved and help the magnificent European eel we would be very interested to hear from you: marineandfreshwater@zsl.org.

For more information go to the ZSL website.