Hundreds of eels have been given an electric shock as part of a project to monitor fish populations in the river Wandle.

The Environment Agency’s (EA) sampling and monitoring team used the electro-fishing techniques to temporarily stun 671 European eels - making them possible to catch and measure.

Information was also taken from other fish, including a sample of their scales, which revealed their age and how well they grow in the Wandle.

Species caught, including chub, barbel, roach, gudgeon, dace and perch, highlighted the diversity of fish species in the river, an EH spokeswoman said.

Tanya Houston, EA environment officer, said: “This was an extremely useful exercise to have carried out as it is important to see how the eel population is faring, whilst getting extra information on the other fish in the river. It will tell us a lot about how the eels are using the River Wandle in their life cycle.”

Eel populations are in decline across Europe and although the survey found a lot of eels living in the Wandle, it probably represents a fraction of the population that would have lived there before the Industrial Revolution.

The Wandle was fished by Lord Admiral Nelson, but deteriorated due to effluent discharged into it from the large number of mills built on its banks.

London - which was once a major eel market - and the Thames in particular, has been associated with eels for thousands of years.

The same stretch of river is due to be examined in two years, which will reveal if numbers have gone up or down.

The eels and fish were returned to the river once the operation finished.