Thesis (MSc)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: The genetic status of H. midae broodstock in the South African aquaculture industry reflects that of random samples originating from undomesticated wild populations. The nature of growth in abalone is very slow, taking between three and four years to reach a marketable size of between 60 to 100 grams. It is therefore of paramount importance to improve this trait in order to ensure global competitiveness and economic viability within the industry. Improving this negative characteristic through conventional selection methods is a long-term venture and alternative means that would yield instantaneous results had to be considered. Crossbreeding was identified as an alternative, short-term strategy to improve growth rate. A crossbreeding experiment was performed between two populations of the abalone, Haliotis midae, from the East (E) and West (W) Coast of South Africa. This was done to investigate the occurrence of heterosis for growth among the crossbred genotypes (East x West, West x East). Fifteen males and females from both the East and the West Coast populations were mated in a complete dialelle crossbreeding experiment to produce four progeny groups (WW, EE, EW and WE). Progeny groups were evaluated for weight (bW) and length gain (bL) over a specific growth period of 9 months. The results provided no evidence of significant differences in weight (P = 0.085) or length gain (P = 0.244) between the four progeny groups, giving no indication of significant heterosis for weight and length gain among the crossbred progenies of these East and West Coast populations. It is recommended that further efforts to obtain improved growth rate in the abalone, Haliotis midae, through crossbreeding only be considered in light of clear evidence of substantial genetic differentiation between such populations. / AFRIKAANSE OPSOMMING: Die huidige status van perlemoen, soos dit voorkom in akwakultuur bedrywighede in Suid Afrika, weerspieël dié van ‘n ewekansige monster vanuit wilde, natuurlike populasies. Perlemoen is inherent ‘n stadig groeiende organisme wat tussen drie en vier jaar neem om tot ‘n bemarkbare grote van 60 tot 100 gram te groei. Dit is dus uiters noodsaaklik om hierdie eienskap te verbeter ten einde die bedryf ekonomies lewensvatbaar en mededingend op wêreld markte te maak. Konvensionele seleksie as ‘n metode om hierdie negatiewe eienskap te verbeter is ‘n langtermyn onderneming wat die identifisering van ‘n korttermyn metode, wat ondmiddellike resultate lewer, noodsaak. Kruisteelt is geïdentifiseer as geskikte korttermyn oplossing aangesien dit onmiddellike resultate lewer. ‘n Kruisteel eksperiment is uitgevoer tussen twee populasies van die perlemoen, Haliotis midae, van die Ooskus (E = East) en die Weskus (W = West) van Suid Afrika. Dit is gedoen om die omvang van heterose vir groeitempo in die gekruisde nageslag (East x West, West x East) te bepaal. Fyftien mannetjies en wyfies van beide die Oos- en Weskus populasies is met mekaar gepaar in ‘n volledige dialleel kruising om vier nageslag groepe (WW, EE, EW en WE) te vorm. Die nageslag is geëvalueer ten opsigte van massa (bW) en lengte (bL) toename oor ‘n spesifieke groei tydperk van 9 maande. Die eksperimentele resultate dui daarop dat die vier nageslag groepe nie betekenisvol van mekaar verskil het ten opsigte van massa (P = 0.085) en lengte (P = 0.244) toename nie en dat daar dus geen aanduiding van heterose vir massa en lengte toename in die nageslag van kruisings tussen die Ooskus en Weskus populasies bestaan nie. Daar word aanbeveel dat kruisteling as ‘n metode van genetiese verbetering van groeitempo in Haliotis midae slegs oorweeg word in die lig van nuwe molekulêre bewyse van genoegsame genetiese differensiasie tussen sulke populasies.