I finally figured out that it is a mink (or some kind of weasel), and not the heron, that has wiped out my fish for the last 2 winters (except for the black ones). I put out a trap, but only succeeded in trapping my cat. I also read that mink avoid traps.

So I had an idea to invert black laundry baskets in the pond, with weights to hold them down. The idea would be that the fish could swim in through the holes but the mink could not. I also read that mink hunt by sight (hence all the living black fish), so I was going to try dieing the water black in the winter as well.

Has anyone tried either of these solutions, or others, with any success? Thanks!