Puzzled and Upset.

Can someone please advise i have large-ish pond in our garden it
contains pond plants is about 5-1/2ft deep in the deepest part and
stepped at one end.I am no expert in pond keeping/fish but have managed
to maintain a healthy environment for our fish. Recently we have had the
regular attentions of a heron (poor thing looks emaciated)the pond is
thoroughly/ properly netted!, but fish have disappeared. Now we have
found the grandchildren's largest goldfish and our pet koi carp have
been attacked, plus two dead on the lawn with just a few large bite
marks out of them around the head and stomach. Other fish in the pond
also in a maimed condition,sadly they had to be put out of their
agony.What can be causing this?! it cant be cats?! what ever it is has
had to have got under the netting at night we believe..Please advise




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Twixt