Thanks Cecil, the spawn was my first guess but this is my first year with fish in the tank and I'm a little paranoid. I've used two scarecrows situated 180 degrees around the pond, which has kept the blue herons away for 3-4 weeks. I don't think too many of my 10-inch bass survived the blue attack. The green herons are another story and I believe wound a few fish. We have bass with dorsal fins torn off, punture wounds etc. The greens are not impressed by the scarecrows (or me for that matter) and will only retreat to nearby trees and squawk at me (even with loud explosions coming from my direction). I don't expect them to bother me forever.