Individual heterozygosity levels and relative growth performance in Oreochromis niloticus (L.) cultured under Fijian conditions

Tilapia (Oreochromis spp.) (Pisces: Cichlidae) are important freshwater fishes that are widely cultured worldwide and have been introduced to the Fiji islands as an aquaculture resource. An O. niloticus (L.) 'Chitralada' stock cultured under Fijian conditions was studied to assess the potential of using individual multilocus heterozygosity as an indicator of relative growth performance. Allozyme analysis was undertaken in each of three generations (n = 50/generation) of 'Chitralada' grow-out. Subsequent correlation and regression analysis of 25 allozyme loci with individual body weights demonstrated no significant association between individual heterozygosity and growth rate in any of the generations. Correlation analysis between heterozygosity based on nine microsatellite loci screened in a subset of 'Chitralada' individuals (n = 43, generation 2) and morphological measurements, demonstrated a significant relationship between heterozygosity at locus UNH146. Homozygous individuals at this locus were characterized by larger lengths and weights than heterozygotes. No significant correlations with either length or weight were observed at any of the other eight microsatellite loci. Selecting broodstock therefore based solely on individual allozyme or microsatellite heterozygosity levels is unlikely to increase relative growth performance in the Fijian cultured O. niloticus 'Chitralada' stock.