Volunteers help ZSL to save the European eel

To celebrate London Rivers Week, Joe Pecorelli from our Europe Conservation Programme blogs about monitoring the critically endangered European eel.

Eel monitoring Thames
Eel monitoring on the Thames

The European eel (Anguilla anguilla) is classified as ‘Critically Endangered’ by the IUCN Red List of Threatened Species. ZSL has been working to conserve these iconic London inhabitants as part of our Tidal Thames Conservation Project. 

Over the last 5 years, April, May and June have been busy months out and about on rivers restarting ZSL’s citizen science eel monitoring project across London.

It’s my absolute pleasure at this time to catch up with volunteers and project partners who have helped us over a number of years and, at the training events we run at each site, meet the new cohort of volunteers. This year the number of volunteers joining the project has reached 110.

From April to October, the duration of the upstream freshwater migration, these volunteers will count, measure and release eels from 11 monitoring sites and then upload the data to the ZSL database. By doing so they contribute to vital information on trends in the numbers of young eels (also known as elvers) joining the adult population (which is called recruitment) which, given we have been doing this for 11 years, is now becoming a very meaningful dataset of both regional and national significance. In addition the data collection also provides the evidence on which we prioritize eel conservation work in the region. 

The traps we use for monitoring are positioned at structures in rivers such as weirs or sluice gates. Once recorded, eels are released upstream of these structures so that they can carry on their migration. Weirs and sluices have the effect of reducing the amount of habitat available for eels by preventing or hindering upstream migration. This impacts the size of populations in freshwater that will metamorphose into silver eel, the last stage their life cycle, and then start the epic journey back to the Sargasso Sea to breed so this loss of habitat is believed to be one of a number of pressures on eel populations contributing to their decline.

When we analyse the monitoring data across multiple sites lower than anticipated catches often highlight barriers that are restricting migration. Eel passage over barriers can be aided with the addition of eel passes, simple structures that provide a channel of flowing water over a substrate for eels to crawl through. We have had several very gratifying incidents, as part of the project, where monitoring has yielded a zero catch over a year, or in the case of the River Crane three years.

Zero catches have prompted the building of an eel pass downstream – once passes go in we subsequently start to catch elver upstream of them and see migration into rivers that, our monitoring data suggests were formerly difficult for eels to access.

Each of our citizen science sites are run in partnership with a local organisation that coordinates trap checking and works with us to deliver the training that all new volunteers must attend to be part of the project.

Traps are checked as a minimum twice per week and at the end of the season everyone involved is invited to ZSL London Zoo to hear feedback on the data gathered across the region and the conservation work undertaken in response.

ZSL thames eel survey
Young eels, also known as elvers

As a volunteer you can give as much time as you can spare to the project, we are grateful for all help received. But, a word of warning - monitoring eels can become addictive! I have seen volunteers become drawn in to the curious world of the eel, trying to work out the patterns that emerge from the catches we get.

Every year we see the first elvers of the season appear at sites on the eastern end of the Thames catchment, as you would expect as these sites are nearer the eels oceanic origins. Catches generally then increase in the warmer summer months and, at sites within or near the tidal limit, bigger tides bring bigger catches. So there are some rules that govern the eel’s migrating behaviour but the truth is there is still a great deal that perplexes about the eel and still much to learn.

One thing we do know for certain though is that recruitment has crashed since the 1980’s to the extent that the European eel is now listed as Critically Endangered by the IUCN so the contribution of our volunteers to this work is enormously important and we look forward to welcoming more to the project in the future.

volunteer on our eel conservation programme 

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