If you or your family owns the land, I would start by calling a game warden and tell them that someone has been poaching fish from your pond. Get him to come out and see what's going on, and ask him. Make up your mind (or your parents mind) to prosecute them if caught. If they aren't prosecuted, then there's no penalty for them fishing there, and they will continue to come and fish. Make sure the game cam is hidden pretty well - they might steal it if they think they are caught on it.
