The use of alternative breeding schemes to enhance genetic improvement in rainbow trout: II. Two-stage selection

The potential of selection in stages to increase genetic gain and profit in rainbow trout breeding was assessed. The theory of Cochran was utilized to predict genetic gains under alternative breeding schemes mimicking the actual Finnish breeding programme. The first stage of selection was based on the within-family deviations of body weight after the first growing season, and the second stage was based on a genetic index that included two body weight measurements at the fresh-water nucleus/central station, and body weight of sea ranched fish at harvest (the breeding objective). A fixed number of 200 family tanks were assumed in all schemes but initial family size and proportions of individuals selected at the two-stages were varied. The deterministic predictions showed that the two-stage selection schemes resulted in 7% and 14% higher genetic gains in the breeding objective compared to the one-stage schemes with equal number of fish reared from tagging to spawning. This result however depended on the population size. The results showed further that by switching from one-stage schemes to two-stage schemes, population size of a breeding programme can be considerably reduced without a decline in the rate of genetic gain. A linear profit function that included the most evident costs (tagging, rearing and management expenses) and returns from the programme showed that the maximum profit was achieved when selection was relatively strict in the first stage of selection. Due to high fertility and biological flexibility of fish, two-stage selection schemes can be easily applied; and this study confirms their benefits in reducing management costs and improving genetic gain in fish breeding