Just to add to what Sherrie wrote, you can use the large gray bricks with holes in the middle to support the wire and the wire will hold the weight of the critters and the fish can swim through them. This is how the local nursery showed it in their pond.
My Brother-in-law had the same problem with his pond. What he did was measure the pond area and he sunk chicken wire just below the water line (you can't see it at all) and the cats and other critters can't get any of the fish out. You can still feed your fish and do everything else..I hope this helps.
Try either netting over the top of the pond ( the most economical approach), or a motion activated sprinkler (more costly than netting, yet more effective-- available at most home and garden stores).
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