Principles of Genetics and Breeding

5.1.1 Hybridization in fishes

Hybridization is a conventional technique commonly used in fish breeding. Hybrids are defined as the products of crossing distinct species. A considerable number of hybrids have been produced between different fish species. The occurrence of natural hybridization in fishes is known since long. It is more common in freshwater fishes than in marine.

• The purpose of hybridization is to bring out or combine useful traits by crossing fish with different phenotypes and genotypes.
• The F₁ hybrids will have new phenotypes and genotypes and usually exhibit heterosis.
• Hybrids with obvious heterosis in the first generation can be used directly in commercial fish production, and also in fish breeding.

• Hybridization can be used in one of several ways to improve productivity. This can be incorporated into a selection programme as a final cross to produce animals for grow-out. In this case, the fishes are selected in two lines that have been shown to produce good hybrids; after the selection programmes, the selected lines are hybridized.
• Interspecific hybridization was successfully obtained in many fish and shellfish genera and/or families (carps, catfishes, cichlids, moronids, salmonids, sparids, sunfishes, oysters, crayfish) as a means of improving production traits (e.g., growth rate, survival, disease resistance) as well as to manipulate sex ratios.
• Sometimes an interspecific hybrid does not exhibit heterosis for any trait, but is still quite important for aquaculture as it expresses a good combination of beneficial traits from both parent species.
• Most of the hybrids evaluated did not prove beneficial for aquaculture. However, a few valuable hybrids are commercially exploited. Hybridization in fishes has the following application in aquaculture.