A series of dog attacks on Dartmoor has left 13 sheep dead over a period of ten days.
Farmers said they cannot explain why there has been such a sudden series of attacks on the National Park.
Park rangers are now appealing to owners to keep their pets under careful control.
The owners of animals which are not kept on a tight leash can be fined or the animals killed.
National Farmers' Union (NFU) spokesman Ian Johnson said the extent of the problem is often underestimated.
He said: "It's a huge worry, not just in terms of finance, though there are tough trading times at the moment and there is the cost of these, but also in animal welfare terms.
"If it's not killed, the sheep can be very badly injured. And, at lambing time, it can cause them to abort, which is a terrible problem."
The penalty for not keeping control of a dog can be a fine of £400. But, for an owner, there can be a greater cost.
If their pet does start worrying sheep, a farmer is entitled to shoot it.
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