Genotypic selection - Genetic selection and hybridization in Aquaculture

Genotypic selection

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 selection:

R_f  = i_f σ _f h²_f

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 clipping

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.