The key with tilapia is the much higher survival rate of the yoy. Both tilapia and BG can (not will but can) spawn quickly and often, BG can spawn at 4/5 mths and every 28 days while tilapia also 4 mths and IIRC every 21 days. This assumes proper temp , water quality and food resources. BG usually have higher total yearly offspring numbers but much lower yoy survival rate.