The Farmers' Union of Wales today urged people living in the countryside to be vigilant for any suspicious activity after a farmer reported a case of sheep rustling on the Black Mountain near Brynamman, Carmarthenshire.
Hugh Davies, 67, and his 61-year-old wife Mary noticed 150 sheep were missing from their 1,000-head flock when they collected them from the mountain at the end of October.
They reported the theft to the police a fortnight ago and a reward of £5,000 has been offered for any information leading to the recovery of the sheep or conviction of the thieves.
"The problem is that the sheep are not replaceable if they have not been bred on the mountain. We are now keen to highlight this problem that is causing havoc among sheep farmers," said Mr Davies.
"To get the sheep off the mountain you need very good dogs and I suspect that someone who knows what they are doing is watching me closely.
"A few years ago, when we were enrolled in a scheme to cut scrapie, all our sheep were registered and we didn't lose any at all during those years. It makes you think.
"We usually lose about 30-40 sheep a year and some of our neighbours have also lost some stock this year but nothing to this extent has ever happened to us before," added Mr Davies.
Dyfed Powys Police confirmed they are investigating the theft of 150 sheep from the Black Mountain between Upper Brynamman and Gwynfe between the end of May and October 25.
The actual time of the theft is difficult to pinpoint due to the terrain and the amount of time between gatherings
"Prevention of rural crime such as this can be tackled but it needs the farmers, police and the wider rural communities to work together to combat it," said Sgt Matthew Howells, who represents the police on rural issues.
"The current prices sheep are fetching, although good news for the farming industry, does then lend itself as being a target for criminals intent on making money from their criminality.
"If we can work with the farming community in setting up Farm Watch schemes as recently showcased in North Ceredigion then we will hopefully be on the right track to reduce rural crime.
"I would be very interested in helping the community affected here to set up a Farm Watch scheme along with the local Neighbourhood Policing Team."
FUW president Emyr Jones said sheep rustling is an extremely disturbing development and a huge problem for Welsh farmers. A survey carried out by the union in 2004 revealed thousands of sheep were being stolen from farms across Wales over the course of a year.
The problem is much more widespread than thought, with the majority of stolen animals believed to end up in unlicensed abattoirs and slaughtered illegally.
"The FUW is urging everyone to be vigilant for any suspicious activity in the countryside and to use the latest camera phone technology in a bid to help the police catch the thieves," said Mr Jones.
The FUW's 2004 survey revealed a large number of thefts over the year. In Llangynidr, near Abergavenny, more than 1,000 sheep were stolen. The Brecon to Builth Wells area saw 487 sheep reported stolen, while seven cattle disappeared in the Sennybridge area.
In Monmouthshire, 570 sheep were reported stolen to the police from six farms and farmers in Llandinam, near Newtown, reported the theft of 150 ewes. Two farms at Ponterwyd, near Aberystwyth, suffered the theft of 115 sheep 23 went missing from a farm in Llandysul.
Some 270 sheep were reported stolen from a farm in Llanllwni, Carmarthenshire. In many cases the rustlers were taking 10-20 sheep out of a flock of more than 100 so that the thefts were not immediately noticed. The figures were based on the number of sheep reported stolen to the police - the true figure could be much higher.