Yes you should assume they have moved on..... then assume they will be back. I have otters show up every October and sometimes they are just passing through, but sometimes they stay. The DNR will tell you they will eat the slow moving fish like Bullheads and Carp and not bother to chase game fish, don't believe it. What happens is they eat what they like best and work there way down. They do love Crawfish, and they do eat Bullheads and Carp, but no fish is safe. They tend to eat the biggest fish first and work their way down. I once saw one eat a painted turtle shell and all.

They will stay where the fishing is easy. Adding structure to the pond will give the fish a chance and make it harder for the Otters. They will leave if it's not easy fishing. I watched a pair of Otters eat 48 bullheads in 45 minutes. They are extremely good at catching fish and I have seen them come up with more than one fish in their mouth at a time!

My ponds are part of a large wetland near a river so I know I can't eliminate them. I have built large boxes with chain link fence all the way around. I cut slits to allow fish in but not wide enough for the Otters to get in. I place these out on the ice where there is no natural structure and let them sink when the ice melts. This works and frustrates Otters, but you can't fish around them either. I have watched Otters circle the cages for along time then give up and leave. If you have a bowl shaped pond with no structure it's possible they will clean out every fish.

If they are spending time at your pond you will find black tar like scat in piles in spots on shore. They mark their territory like dogs and fire hydrants. They are not hard to trap. The ones that know what traps are will leave if they know traps are there and that's what you want anyway. They will not clean you out overnight but if there is sign there try to get rid of them by any means.

The other issue is that stocked fish are not wise to predators and are easy targets. It's not the end of the world but it sure feels that way when you see them eating your fish. I see some every year, if they stay I deal with them, and I always still have fish in the ponds.