The carp are large, so 20lb plus ?
If they are that size it is an Otter, a heron may stab the odd big carp by mistake, but in general eats the smaller stuff, same goes for Cormorants.
A mink can deal with a fish of maybe 5 or 6 pounds, past that size and happening regularly it's an Otter every time.
Not much you can do about it, except Otter proof fences all thew way round, not a cheap proposition, not sure if help funding the fence is still available or not.
Neil.