Yup been there. Don't give up on your fish pond, here are some lessons I learned. Otters are opportunist and a pond like fish bowl will get cleaned out. First thing to do is add structure to the pond that your fish can hide in or escape too. It needs to be strong so the Otters can't rip it apart. Keep in mind an Otter can get through a 3" hole. I built boxes out of pallets with the slots vertical for fish to fit through but not wide enough for an Otter. I also made 55 gallon drum structures. Cut slots for fish to go through but too narrow for the Otter. Some cement or rocks inside to hold it in place. Do several in a group if you can. A pile of logs and brush can be a benefit also. A frustrated Otter goes where the fishing is easier. The worst case is multiple Otters as one will go in the brush an others will catch what comes out. I built several chain link boxes that look like a dog kennel that have slots big enough for fish, added structure inside them and sunk them in deeper water. My ponds are too large for a fence but I have heard of people putting a fence around their ponds. Otters don't jump or climb a fence so a short fence buried on the bottom works. Another idea is a cattle electric fence wire near the ground but you need to keep it mowed so weeds don't short it out. You can trap Otters or use other methods of permanently removing your Otters but more will replace them. Make your pond suck for the Otters and they will move on.