Survival. In tilapia farming, as in most farming, the most important performance trait is survival. A farmer needs to be able to harvest as many as possible of the fish he or she bought as seed. However, survival to harvest depends on a multitude of genetic and environmental factors and on interactions among these factors. Throughout the history of tropical aquaculture, many farmers have overstocked their ponds and cages as a countermeasure against unpredictable, and sometimes high, fish mortalities. This overstocking itself has often contributed to such mortalities and has resulted in wasted expenditure on seed and feed and in lost revenues because of reduced harvests. Researching the genetic determinants of fish survival on-farm is difficult. Fortunately, most tilapia are hardy fish, and they usually have high survival in diverse farm environments within their natural tolerance ranges for temperature and salinity. Up to the 1990s, tilapia farming remained remarkably free from serious disease problems.21 These factors make it difficult to attempt to improve tilapia survival by selective breeding. However, it was also essential for the developers of GIFT to determine whether selective breeding can have adverse consequences for survival. GIFT developers monitored the survival of different tilapia strains under well-defined conditions, on-station and on-farm, rather than attempting to select for survival.