Three generations of selection for growth rate in fall-spawning rainbow trout
A selection program to improve the growth rate of rainbow trout (Salmo gairdneri) was begun in 1967 with random samples of fall‐spawning fish from the Manchester and Wytheville National Fish Hatcheries. All matings were made as one male to one female, and selection was based on family mean fish weight of the progeny at 147 days post‐fertilization. The selected families in each year class were mated in all combinations at 2 years of age (age II) and 3 years of age (age III). Selection was applied only to the age II progeny. A control group was established in 1971 from the base population and carried through the remaining portion of the study. All families were reared under standardized rearing conditions. During the three cycles of selection studied, mean 147‐day weight of progeny from age II parents increased from 2.02 g in the first generation to 5.28 g in the fourth generation, a gain of 161%. Of the 3.26‐g total gain during the study, net genetic gain in weight from selection was 0.98 g or 30.1%. The rest of the gain (69.9%) resulted from improved rearing conditions. Gains in percentage hatch and percentage fry survival to 147 days were not significant. Differences in the performance of progeny from the age III parents, as compared to that of progeny of age II parents from 1970 to 1974 showed an advantage for the age III parents in all three traits; 6.4% for percentage hatch, 25.0% for percent fry survival, and 22.3% for 147‐day weight.