Principles of Genetics and Breeding

13.1.8. Individual or mass selection

Individual or mass selection is the simplest, oldest and most often effective method of selection, where the best individuals are selected from a population on the basis of their own 'phenotypic value' compared to the population means, e.g., the largest individuals, the survivors of a disease, or according to their colour in ornamental fish. Generally the top 10 to 15% individual are selected.

• Fish that meet or exceed the cut-off value are saved, and the others are culled.
• Family relationships are totally ignored, an individual is selected or culled based on its own merit.
• Individual selection should be the basis for collecting young brood fish candidate.
• In the individual selection, it is necessary to know about the origin of population, performance of the given strain, typical external characteristics of species. After selection, the young brood fishes are kept under optimal conditions before choosing the most appropriate female and male individuals for hatchery operation.
• The base population should be genetically diverse for the trait(s) of interest. At least 50 pairs of fish should be selected in each generation and contribute to the next generation to minimize inbreeding during selection-in small selected populations, (inbreeding can counter the effects of selection).
• Selection is a cumulative process. The genetic gains may be 5-10% per generation, but this adds up over time. Fore.g., a mean genetic gain of 7.5% per generation would result in a 43.5% cumulative improvement compounded over 5 generations.

Most tilapia farmers tend to conduct their own selection either directly or indirectly by culling smaller fish in the breeding stock and continually propagating only large individuals.

Several farmers adopt mass selection since breeding space in most farms is limited. Improved traits in the selected stocks are gauged in subsequent generations through performance comparison against control lines.