If stocked true Mozzambique, they are VERY well camouflaged! I think you mistakenly recall Grass Carp as seeking flowing water and swimming downstream. While many fish will get caught in outflow current and go over the spill way, all your other fish, but the GC, will swim UP stream when forced.
Tilapia spawn every 3-6 weeks depending on many conditions. You rarely see fry in FA bottom growth. The fry will eat it from inside and as the T grow, other fingerlings hover and pick off the T as the FA disappears.
