Of the several families only those showing good phenotypic value are selected.
When between-family selection is used relatively few families are saved. This retroactively lowers Ne of the previous generation to a very small number.
If select brood fish are not marked and are mated at random, then because the select brood fish came from relatively few families, relatives will mate, and the inbreeding that will be produced can become quite large.
The easiest way to prevent inbreeding during between-family selection is to mark the select brood fish and to prevent consanguineous matings. When the fish cannot be marked, the only way to prevent inbreeding or to minimize it during between-family selection is to treat each of the select families as if it were a cohort and use rotational mating to produce successive generations.
After the first generation of selection, between-family selection will choose the best families from each “cohort”.
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