I suspect that they will stay around as long as the catching is easy. I have 2 ponds, the smaller one being close to a feeder stream known to have otters. Last year a pair "found" my small pond (3/4 acre) and ate everything bigger than 6 inches in a few days. Fortunately they didn't go upstream to my big pond, I have enough trouble there with Herons and Cormorants. Aggression is probably a good idea, at least to make them feel unwelcome.