Near-extinct otters are returning to Lincolnshire's waterways, according to a survey by the Lincolnshire Wildlife Trust and the Environment Agency.

The sleek, amphibious mammals were almost wiped out in England the mid to latter part of the 20th century but, after a conservation effort, they are making a comeback.

Phil Smith, an Environment Agency biodiversity officer, said: "An otter is a very elusive creature – very nocturnal – and it's not very often you see one.

"While I've not seen one in Lincolnshire, I've looked for and seen the signs they leave behind, so they are definitely here."

The Wildlife Trust's Fran Southgate said otter populations have probably been in decline since the industrial revolution, so any reappearance is a good thing.

She said: "The intensive nature of post-Second World War farming and development accelerated the decline, destroying habitat, putting more pressure on water resources and, from the 1950s, releasing organo-chlorines into our rivers and waterways.

"These chemicals, particularly dieldrin and aldrin, built up in the tissues of top predators, causing blindness, immune system collapse and breeding failure.

"By the mid 1970s much of England had lost almost all its otters, with counties such as Lincolnshire taking the brunt of the force due to the predominance of arable land and widespread use of organo-chlorines.

"Fortunately the situation was recognised in time and the otter became a flagship species – an indicator of the general health of our wetlands."