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.