Dissertation (PhD (Agric))--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: 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). / AFRIKAANSE OPSOMMING: ‘n Teelprogram gerig op die verbetering van groeitempo in reënboogforel is in 1988 ingestel onder toesig van die Departement Genetika aan die Universiteit van Stellenbosch, in sameweking met die plaaslike forelprodusenteverenigings. Die eerste fase van die teelprogram behels die versameling, evalasie en seleksie van die beste beskikbare genetiese materiaal vanuit, 13 verskillende genetiese groepe (nege plaaslike en vier van oorsee) om twee basispopulasies te ontwikkel in elk van die gelyke en ongelyke jaargange. Die doel daarvan was om ’n basispopulasie met hoë genetiese meriete en variasie te ontwikkel met die aanvang van die teelprogram gerig op genetiese verbetering, deur middel van seleksie. Statisties betekenisvolle en ekonomies belangrike genetiese verskille in massa- en lengtetoename is aangetref, tussen die onderskeie genetiese groepe. Die daaropvolgende twee generasies binne die teelprogram behels die uitvoering van ’n reeks enkel- en dubbelkruisings ten einde ’n verdere toename in genetiese variasie in die basispopulasies te bewerkstellig, sowel as om die voorkoms van heterose en algemene, sowel as spesifieke kombinerings-vermoë tussen die kruisings te bepaal. Betekenisvolle vlakke van heterose (6.7% tot 9.6%) sowel as algemene kombineringsvermoë, is aangetref ten opsigte van massa- en lengtetoename in opeenvolgende groeifases. Daar kon geen aanduiding van betekenisvolle, spesifieke kombineringsvermoë gevind word nie. Die kruising van geselekteerde nageslag vanuit die oorspronklike genetiese groepe, gevolg deur ‘n multi-fase seleksiemetode vir groeitempo binne nageslaggroepe, het bygedra tot die ontwikkeling van ‘n tweede en derde generasie broeipopulasie wat beskik oor hoër vlakke van genetiese variasie en verbeterde individuele meriete ten opsigte van groeitempo. Die benutting van nie-additatiewe genetiese variasie binne die basispopulasies deur middel van kruisteling en heterose tydens die vroee stadium van die teelprogram is uitgestel ten gunste van die benutting van additatiewe genetiese variasie deur middel van ‘n multi-fase seleksiemetode, wat berus het op die toepassing van hoë vlakke van seleksie-intensteit binne en tussen familiegroepe. Die beraming van genetiese parameters tydens die vierde generasie het die voorkoms van hoe vlakke van additatiewe variasie binne die onderskeie jaargroepe bevestig. Hoë oorerflikhede van 0.40 tot 0.53 is beraam vir ligaamsmassa en -lengte op die ouderdom van 150 dae. Genetiese korrelasies tussen die kenmerke was ook hoog met waardes van 0.74 tot 0.82. Die saamgestelde gerealiseerde seleksierespons van 50% vir liggaamslengte vir die “EVEN”-jaargroep na afloop van ses generasies van seleksie (8.3% per generasie) en die 33% van die “ODD”-jaargroep na afloop van vyf generasies van seleksie (6.6% per generasie) het die doeltreffendheid van die multi-fase seleksiemetode bevestig ten opsigte van die benutting van die additatiewe variasie vir groeitempo binne die onderskeie basispopulasies/jaargroepe. Die teelprogram duur steeds voort en sal die 7de generasie in 2004 bereik. Die program voorsien nagenoeg 50-60% van die kommersiele materiaal vanuit direkte voorsiening van teelmaterial, eiers en vingerlinge asook die indirekte voorsiening via kommersiële teelstasies. Die teelprogram was die eerste van sy soort met betrekking tot akwakultuurspesies in Suider Afrika en het bygedra tot die implimentering van programme van genetiese verbetering in drie inheemse spesies, naamlik die tilapia (Oreochromis mossambicus), die baber (Clarias gariepinus) en die perlemoen (Haliotis midae).