There are several reasons why fishes suddenly disappear in a certain place. Perhaps, trouts have found a better home; and there are several factors why they do this. Perhaps, it has something to do Wirth their habitat. There might be changes that happened along the process and they need to find a better place; same as the place and environment where they grow up because it's really important for them. Trouts are non-adaptive kind of fish, and they are always willing to find the kind of place in accordance with their desires.