The efficiency of family-based selection relies on the fact that the
environmental deviations of the individuals tend to cancel each
other out in the family mean value. This selection method has advantages
over other types of selection especially when heritability for a
trait of interest is low and when environmental deviations constitute
a large part of the phenotypic variance. However, because fish are
too small to be individually tagged at (or soon after) spawning they
usually remain in their full-sib group until they are large enough
to be tagged (5–10 g). This management results in a common (to
full-sibs) environmental effect that contributes to the between family
variance. From a managerial viewpoint it is desirable to reduce the
common environmental component to a minimum by standardization
of the environment for all families (Gjedrem, 2005). Thus, individuals
from all families should be tagged as early as possible before communal
rearing together in the same tank, pond or cage. However, familybased
selection based on physical tagging has to accept a minimum
period of separate rearing before tagging, which usually introduces a
common environmental effect that varies in magnitude according to
circumstances