Fish in the base population (G0) were ranked according to their individual breeding values for growth (recorded as body weight at harvest).
Due to the limited number of full-sib families produced in the G0 generation, however, it was decided to conduct a within-family selection of an
equal number of males and females from all available families at harvest.
In later generations (G1–G3), all harvested fish were ranked according to
a selection index including individual breeding values for growth and
family breeding values for survival traits (i.e., pond survival and, in the
G3 generation, also cold-water tolerance). All breeding values were
BLUP values (i.e., based on all available information) and family breeding values were estimated as the average of the parents' breeding values.
Prior to calculating the selection index values, the distributions of breeding values of growth and survival traits were all standardized to a mean
of 100 and a standard deviation of 10. The relative weighting of growth
was gradually decreased in the selection index with increasing number
of generations to allow more selection pressure on survival traits. A total
of 500–600 breeding candidates (i.e., 130–150 males and 370–450
females) were pre-selected in each generation (G0–G3) to produce
new generations of full-sib families. The number of selected individuals
from the same full-sib family was restricted (max. 3–5 males and 5–15
females per family in different generations) to reduce inbreeding. The
final selection of breeders to produce full-sib families (Table 1)
depended, however, on the sexual maturation and readiness to spawn
of the pre-selected female breeders.