The revival of otter numbers in Norfolk's rivers is seen by many as a boost to the natural environment.

But the comeback is causing a growing problem for fish farmers and fans whose ponds and lakes are being raided by the predator.

Eleven expensive fish, including six ghost koi, were dragged from a pond off Cromer Road, Trimingham, and left to die under bushes in the garden between last Friday and Sunday.

Owner Jennifer Hugman, 71, who first got the pond life 11 years ago, believes her fish were killed by an otter after calling nearby Gimingham Lakes.

The pond was covered with a net to protect the fish from herons but it proved no defence against the otter.

Mrs Hugman said: 'It was upsetting. Everybody would walk past and look at my pond and say what nice fish I had. I won't get any more fish in case the predator comes back. I would say to people be aware.'

Daniel Gotts, Gimingham Lakes owner, agreed with the otter theory, saying: 'Attacks on fish are a growing problem. Since otters were reintroduced in the early 1980s it has been a free-for-all.

'People are trying to increase numbers of otters and they are lovely things to look at but they wreak havoc, which no-one has looked into.'

'There is not a great deal people can do except put up fencing.'

He added otter attacks on fish were costing commercial fisheries across Britain millions of pounds each year.

Jonah Tosney, senior project manager at Norfolk Rivers Trust, said the Trimingham fish attack could have been done by an otter or mink.

Dr Tosney said: 'Otters are quite capable predators and a pond full of fish is a dream come true for them. There are a lot of positives to them but unfortunately they do eat fish.'

But he said the trust did not see otters, which were on the brink of extinction across Britain in the 1980s, as a problem.

'They are a natural part of the ecosystem. We are delighted to see them. They keep nature in balance. We feel otters are good for rivers, not bad for rivers,' Dr Tosney added.

He said the Norfolk otter population had balanced out and could not imagine much room for expansion.