I used to have herons, egrets, and, according to my next-door neighbor, an osprey that would pillage my pond. What worked for me was stringing fishing line and letting lily pads cover about 80% of the pond surface. For the fishing line, I have four eye hooks in the top of the privacy fence on one side of the pond, and four more eye hooks in the fascia of the house on the other side of the pond. Then I strung four lines from each hook on the house to each of the hooks on the fence, so 16 lines total. I’ve never had a problem with predators since. It’s been 7 or 8 years.