Pipes are being put in rivers across eastern England to help eels swim upstream to favourable habitats and return to the sea for spawning.
Structures put in rivers allow young eels to swim upstream
Obstacles such as weirs and dams have contributed to a 95% decline in numbers of eels in the region since the 1980s.
Eels are caught for the table at Ely in Cambridgeshire but are also a staple food for bittern and otter in Essex and Suffolk, the Environment Agency said.
The Environment Agency believes falling eel numbers could damage river ecology.
Nigel Tomlinson from the agency said: "Rivers in the East play an important role in eel life-cycles. Without them there would be a serious impact on river ecology."
The agency is installing simple structures such as pipes to allow eels get past weirs and other obstructions to swim upstream and to return easily to the sea for spawning.
Special traps in the rivers also check how many eels are migrating.
Mr Tomlinson said: "They are also an important source of food for humans as well as species such as the bittern and otter."
Most of the eels seen in the East are females and these are especially important for conservation of the species.
Mr Tomlinson, fisheries principal officer at the Environment Agency, said: "We are at a point where eels could become extinct, which is why we're doing this work now to stop that happening.
"Fishermen must be licensed, but at present there is no limit on how many eels they may take so laws are being reviewed to bring more protection."