Hundreds of eels were given a health check by the Environment Agency fisheries staff during a two-week operation assessing the population in the River Wandle.
About eight members of the Environment Agency’s sampling and monitoring team took part in the survey at 12 sites along the river’s south London stretch earlier this month.
Electro-fishing techniques were used to temporarily stun the 671 European eels, making it possible to catch and measure them.
Information was also taken from other fish caught in the survey, including a sample of their scales, which reveals their age and how well they grow in the River Wandle.
Tanya Houston, an Environment Agency officer, said: “This was an extremely useful exercise as it is important to see how the eel population is faring, while getting extra information on the other fish in the river.
“It will tell us a lot about how eels are using the River Wandle in their life cycle.”
Eel populations are in decline across the whole of 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 same stretch is due to be examined in two years, which will reveal if numbers have gone up or down.
All eels were returned to the river once the operation finished.
The work is partly funded by income from rod licence sales and part of a larger monitoring effort for fish and eels in the Thames Region.
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