Growth of livestock and some fish species (footnote 14) has been readily improved by selective breeding. However, growth rate is a complex trait to measure, because the growth rates of fish change during their development from juveniles to adults. For the development of GIFT, harvest weight at 90 or 120 days was chosen as the trait to be improved. This was a reasonable choice as an indicator of overall growth rate, given that historical harvest sizes for farmed tilapia in Asia had ranged from about 100 grams (g) to about 350 g, from cropping cycles of up to 9 months.23 After 90 or 120 days, any farmed tilapia would be expected to have a harvest weight indicative of its growth rate during its main phase of growth. These were also convenient periods for experimentation: long enough to show growth differences among tilapia strains and short enough for multiple trials and progress in selection.