UK Predator problems- Not A Heron?

Please help my Dad

I'm posting this on here because something is attacking his pond and eating his fish!!!

I went to see the evidence the other day. Bear in mind that he already has a net which completely covers ALL of the pond and has a heron decoy at the side and despite living in the countryside, has not had any problems with herons since he moved there about 6 years ago!

To date he has found at least six of his larger fish at the rear of a nearby shed with their heads eaten and their bodies left. Considering that the pond is over 5ft deep in the centre and there is no access for the herons to feed through the netting, could something be getting into the pond, catching the fish and then dragging them to the nearby shed to nibble at the heads and then get bored, leaving the rest? But what???? Has anyone got any ideas? Please help before my dad loses all of his fish, which he's had since I was a boy!

All suggestions welcome.

Many thanks

I'm sorry to hear about your sneaky predator. Here are a few ideas that may help that I posted in another article... http://www.gardenpondforum.com/showt...ight=predators

Hope if helps.