I am no expert but it makes me wonder what happened to the fish you stocked. With a lake that big it is hard to believe they are all gone. Putting in more fish without knowing is a big gamble. Is there a predator species you may not know is in there, is there a creek or channel or over flow that allowed them to escape? You haven't seen any floaters? I'm assuming you stocked a mix of known males and females. Like I said I'm no pro just some thoughts.