It has traditionally been the preserve of bird-watchers gazing out over the waters of one of the Broads' most pristine habitats.

But over the past four years visitor numbers at the RSPB's Strumpshaw Fen reserve, near Norwich, have more than doubled as an exciting secret has gradually spread.

Regulars at the reserve now know that as well as being a site of national repute for spotting bitterns and other rare birds, including barn owls and marsh harriers, it is also an amazing place to encounter otters.

Reserve warden Tim Strudwick said: 'After nearly becoming extinct nationally in the 1970s, otters started coming back to Strumpshaw in the late 1990s.

'However, until about three years ago they were rarely seen, very secretive and mostly active at night.

'But then suddenly a female otter began to be seen with her cub on a regular basis.

'Since then the whole population has learned to be more active during the daytime.

'They will swim past, look up and go on about their business completely unconcerned about people watching them close by.'

Mr Strudwick said otters were now being seen and photographed from the reserve's hides on a daily basis.

He said: 'This time last year seven were seen together; a female breeds here and visitors regularly see her at the moment with her cub. She had another cub which we found dead –we think of natural causes.'

Manifold signs of the otters include trampled paths down to the water and the occasional half-eaten pike left on a pathway.

'In the cold spell last December we would see them playing on the ice at the edge of the river,' he said.

It is thought the RSPB's mid-Yare reserves, including Strumpshaw, are home to three territorial males and four or five breeding females, as well as a number of younger males.

Mr Strudwick described the return of the otter as 'one of the greatest conservation success stories'.

He said the work of the former Otter Trust at Earsham, near Bungay, in breeding and releasing the mammals had played a part.

However, it was the improvement to water quality in rivers that had really driven the recovery.

'The River Yare was filthy in the 1970s but there has been a big improvement in the treatment of sewage,' he said.

'Agricultural pesticides getting into fish and poisoning otters was also an issue in the 1950s and 1960s.

'Otters disappeared completely in many parts of England but in Norfolk they just about managed to hang on.'

Mr Strudwick said the work they did in maintaining the ditches and waterways and ensuring a good fish population made Strumpshaw the perfect haven for otters.

And the improvement in water quality and general habitat was attracting an increasing variety of rare birds – ospreys were now frequent visitors and cranes were also seen on the reserve.

stephen.pullinger@archant.co.uk