Maybe your rat prefers your eelgrass to apples! I have usually caught them the next morning when baiting with apples. Musk lure should get him near the trap if not in. The apple slices should bring him the rest of the way. If this doesn`t work and you can`t put a 110 conibear in a hole, setting up with a .22 would be my next step if the area around your pond is safe to shoot around.