Not in my opinion, because, except for the weight of originally stocked tilapia, all "new" Tilapia weight came from nutrients consumed in the pond...no net gain or loss of matter. It's just that the nutrient is in a changed form.
If you stocked 20 pounds in the half acre pond described in this thread, that 20# is the only additional load. All growth and reproduction is supported by currently present nutrient. If feeding fish, that would not be the case as the weight of feed would be a new nutrient load.
I would actually take that one step further. I think you need to account for the amount of nutrients that the fish expends in the form of energy. What they consume doesn't all go into the growth of the fish. I would think that swimming around for four or five months (depending on climate) would actually "burn" off nutrients. That energy has to come from somewhere.
