Individual or mass selection can only be used on traits that can be recorded on live animals and is not very efficient for traits with low heritability. In such cases, other types of selection have to be resorted to. The two types of genotype selection that have applications in aquaculture are family selection and progeny testing.
Family selection and sib-selection
Family selection is of special interest in selection for characteristics
of low heritability, such as survival, meat quality and age at maturation. Use
of full and half sib families in a selection programme has the advantage that
the genera-tion interval will not be increased, compared to individual
selection. However, a disadvantage is that usually each family has to be reared
in separate tanks, as it is generally difficult to mark newly hatched larvae or
fry. This will introduce environmental and tank effects on characteristics,
such as body weight, between families. Because of this, Falconer (1981)
recommends a combination of individual and family selection.
In family selection, several families are grown under identical
conditions to determine the ones to be maintained for breeding. To obtain
separate progeny (family), either one male/female pair or a small group of
spawners can be used. The response equation is essentially the same as in mass
Rf = if σ f h2f
The intensity of selection appears to be lower than in mass selection,
as it is not possible to grow such a large number of families. Similarly, a
reduction can be observed in the standard deviation, as this denotes the
variation in the family and not individual variation. However, the heritability
is much higher.
If the individuals have to be sacrificed for examination, the brothers
and sisters of the individuals from the best families can be maintained for
breeding. This is known as sib-selection. Kirpichnikov (1971), in his
description of the methods of family selection, underlines the importance of
carrying out crossings, egg incubation, larval rearing and grow-out of families
separately, under as identical timings and conditions as possible. The main
disadvantage of family selection is the practical difficulty in simultaneously
growing many families under identical conditions. Marking of individuals will
reduce some of the problems, as communal growing will then become possible. Fin
and cold or hot branding have been used in many large-scale selection
programmes. Molluscs can be marked more easily on their shells, whereas in
crustaceans moulting habits make marking difficult.