The movements of baby eels are being monitored by infra-red cameras to help conserve stocks.
Elver numbers are believed to have fallen by 95% since the 1970s
The elvers are being captured - but only on videotape - as they migrate along the River Parrett in Somerset.
The technology has been set up at the Oath Lock at the river's tidal limit to discover the number and habits of the species and aid conservation.
Elvers are a delicacy and are vulnerable to poaching, where a big profit can be made from the young eels.
Pete Sibley. Project Manager at the Environment Agency, said: "It helps to monitor the life stage we're interested in - these little elvers coming up this part of the river from the tidal area up into the fresh water. "
European eel stocks have been in major decline since the 1970s. The number of young eels reaching the UK's shores is thought to have fallen by 95%.
Weirs, locks and pumping stations have all caused eels numbers to dwindle.
Elvers, which are about 7cm (2.8in) in length, spend at least a year drifting across the Atlantic in the Gulf Stream.
They then swim up rivers to find a suitable place to live and grow, and when they mature, they return to sea and swim across the Atlantic to spawn south-west of Bermuda.