Genetic improvement of growth rate in rainbow trout (Oncorhynchus mykiss)

A breeding programme aimed at the genetic improvement of growth rate of rainbow trout was initiated in 1988 by the Department of Genetics, University of Stellenbosch, in collaboration with the local trout producer's organisations. The first phase of the breeding programme included the collection, evaluation and selection of the best available genetic material from 13 different genetic groups (nine local and four overseas) to make up two separate base populations as odd and even year-groups. This was done to establishment a base population with high genetic merit and variation at the onset of the breeding programme. Statistically significant and commercially valuable genetic differences in terms of weight and length gain were detected between the various hatchery groups. The next two generations of the breeding program included a series of single and double crosses in order to increase the levels of genetic variation in the base populations, and to investigate possible heterosis and specific and general combining ability among the crosses. Significant levels of heterosis (6.7% to 9.6%) and general combining ability was found for weight and length gain during consecutive growth stages. No evidence was found for specific combining ability among the crosses. The crossing of selected offspring from the original genetic groups followed by the application of intensive multi-stage selection for growth rate within progeny groups has led to the establishment of second and third generation parental populations with higher levels of genetic variation and improved individual genetic merit with regard to growth rate. The exploitation of non-additive genetic variation within the base populations through crossbreeding and heterosis during the early stages of the selection programme was delayed in favour of the utilization of additive genetic variance through a procedure of multi-stage selection that incorporated high intensities of selection within and between family groups. The estimation of genetic parameters during the fourth generation on the basis of a hierarchical half-sib family structure confirmed the presence of high levels of additive genetic variation within the respective populations/year-groups. High heritability values in the range of 0.40 to 0.53 were recorded for body weight and length at 150 days. Genetic correlations between the traits were also high, in the range of 0.74 to 0.82. The cumulative realized response of 50% in body length for the EVEN year-group after six generations of selection (8.3% per generation), and the 33% for the ODD year-group after five generations of selection (6.6% per generation) confirms the efficiency of the multi-stage selection procedure to exploit the available additive genetic variation for growth rate within the respective populations. The programme is still ongoing, entering its 7th generation in 2004 and is supplying about 50-60% of commercial material through direct supplies of broodstock, ova and fingerlings and indirect supplies via multiplier stations (commercial hatcheries). The programme was the first of its kind in relation to aquaculture species in the Southern African region, and has since initiated the introduction of programmes of genetic improvement in three other indigenous species, namely tilapia (Oreochromis mossambicus), African catfish (Clarias gariepinus) and abalone (Haliotis midae).