My understanding is straight sides on the pond help reduce predators from wading in to grab your fish. But I watched a heron standing on the edge and dive into my pond, grab a big goldfish and take off with it. It surprised me since I thought herons waded in to get fish. My pond has straight sides but isn't deep along the edges. So there are varying factors. I don't have raccoons that I know of but herons come by during the fall and spring migration so I put a net over the pond in the fall and leave it until mid to late spring. That has worked keeping the herons from snacking on my fish.
