MCM have a look here: – Stoat Info
It may be that if the fish was already dead, a Stoat would then eat it. Herons would eat those sized carp in one, no left overs, no chunks, nothing. This time of year they would also have eaten half of the pond if not more.
As Herons are wading birds, the way to protect your pond is to have deep edges (over 2 feet) with some thing like a rope edge fencing close to the edge.
This gives enough of a deterent as it’s sparse and unatural, ponds with wide, open borders that the Heron can access easily are normally the main targets.
If all else fails take the fish to your aqautic centre, they will let you know stright away if their any good.