Frogs and toads have had their legs eaten
At first it was thought a mystery virus was responsible for the discoveries at Westermill Farm, near Exford, in the centre of the national park.
But all the dead amphibians had the same baffling characteristic - mutilated rear legs.
The discovery has been taken as a sign of a growing population of otters on Exmoor - setting for the famous Tarka the Otter stories.
The virus attacks the frogs' legs.
But scientists are satisfied otters are to blame, even though mass killings are rare.
They say there is a simple explanation for their victims' legs being mutilated.
Toxic lunch
Joe Harper, a conservation officer with the Environment Agency, said: "They are probably the tastiest bits.
"Toads particularly are not very palatable.
"They have fairly toxic skin so the animals will remove the legs and skin them, and take the meat within."
Farmer Oliver Edwards said: "I was very concerned because it was wiping out our frogs.
"We like to see small frogs and all the other things that go with conservation areas.
"To wipe out the frog population in the food chain is not very good.
"I'm glad that the otters are here and long may it last, but I'm not so happy that they are eating my frogs."
Checks have confirmed that amphibians are still surviving around Mr Edwards' pond - at least for the time being.