Principles of Genetics and Breeding

12.1.5 Inbreeding studies in fish

There have been relatively few inbreeding studies of fish.

• Some work has been done in rainbow trout, Atlantic salmon, brook trout, brown trout, common carp, T. mossambica, channel catfish, zebra danio.
• With few exceptions, the studies showed similar trends: Inbreeding depressed production phenotypes such as growth, viability and survival, and increased the number of abnormalities.
• Kincaid (1977) estimated that the critical level of inbreeding in rainbow trout is about 18%; below 18%, inbreeding produced few problems, but above 18%, productivity was depressed significantly.
• One generation of brother-sister mating in rainbow trout resulted in increased fry deformity (37.6%), decreased FCE (15.6%) and fry survival (19%).
• In the common carp one inbreeding generation after the crossing of sibs retards the growth of fish by 10% to 20%, this is accompanied by decreased viability and a marked increase in the number of malformations.
• Complex polygenic heterozygous system involving many traits related to productivity, including the growth rate and viability, developed in the course of natural selection of fishes. These systems are destroyed by inbreeding.
• The harmful consequence of inbreeding during fish reproduction have been noted by many authors. Inbreeding depression has been noted in rainbow trout for hatchability, survival in different phases and growth rate in freshwater as well as sea cages. But it should be pointed out that in many aquarium fishes inbreeding does not appear to be accompanied by any marked depression. Severe and thorough selection of the best individuals by aquarists overrides the harm done by inbreeding.
• Fishes are more prone to inbreeding in hatchery environment for their high fecundity. For example, the species of Indian major carps can produce about 0.25 million eggs per female per breeding season. The hatchery environment ensures high rate of survival in the absence of competition for food and challenges from predator and parasite. So,logically speaking brood stocks in a carp hatchery can be obtained from a single pair of parents. If the brood stocks are not exchanged or replaced, the hatchery manager will breed the close relatives. This has happened in some Indian major carp hatcheries in Southern India,where the brood stocks remained genetically close. The inbreeding rate in these hatcheries were found to be between 2 to 17% (Eknath and Doyle, 1990).
• Studies that have been conducted with other species of fish showed that inbreeding usually decreased means for production phenotypes, although the studies have produced mixed results; for example, inbreeding reduced the return rate of Atlantic salmon, Salmo salar; reduced growth and viability of Mozambique tilapia, Oreochromis mossambicus; increased growth of channelcatfish, Ictalurus punctatus, in one study but reduced it in another.
• The only unambiguous result was that inbreeding produced some abnormal offspring. It has been suggested that the breeding programmes used by most fish farmers will produce inbreeding of 3-5% per generation.
• If this occurs, inbreeding depression could begin to affect productivity and profits after only 3 to 5 generations.