Fisheries officiers are using military-grade sonar to fight illegal eel fishing.

Commercial catches are strictly-regulated because of falling numbers of the once-common eel.

But traps and fyke nets are often placed in waterways in the Fens - particularly during the coarse fishing close season, when the banks are deserted.

EA officers say new sonar equipment has allowed them to see, with amazing clarity, where they've never been able to before - under the water.

'This makes checking for nets, especially illegal equipment, much easier and makes us more efficient and in some cases 100pc effective,' the agency said.

'A recent audit of a river near King's Lynn resulted in a record illegal eel net seizure, 16 nets in total capable of significantly impacting an emigrating eel population – hundreds of eels were released.'

Eels are not the only fish which can become caught in illegal fyke nets, while otters have also been found drowned in them.

'Using boat mounted sonar equipment we can travel up to 10km per day, checking every inch of channel for illegal and submerged equipment,' the EA said.

'This is our greatest weapon in halting the illegal exploitation of eels and gives us a chance to completely rid waters of illegal instruments. We are using this technology to regulate other areas of illegal fishing including poaching, netting and trapping.

'You can see from the images how easy it is for us to spot illegal equipment, here both ends of a fykenet can be clearly seen stretching across the river channel. We use boats and specially trained enforcement staff to check traps and remove any illegal ones we find.'

Scientists say eel stocks are in decline, recruitment is at an all-time low, and exploitation of the stock is currently unsustainable.

The EA regulates the fishing industry, which is carried out by only a handful of licensed fishermen. It also protects the eel's habitat.

The mysterious species comes to our shores as a tiny elver, lives in rivers and drains until it is ready to breed, and then returns to its birthplace in the Sargasso Sea, off the Gulf of Mexico, where all Europe's eels spawn.