To date, commercial strains of Nile tilapia (Oreochromis niloticus, L.) are not available for aquaculture in saline water environments. The overarching objective of my PhD thesis was to develop a genetically improved strain of Nile tilapia not only having fast growth but also with good adaptation to a range of production systems. Specific aims of this study/thesis were: i) To assess genetic progress achieved from a selection program for high growth in moderately saline water (15 – 20 ppt); ii) Examine whether selection in a challenging environment produces Nile tilapia genotypes that can thrive in a range of production systems; iii) Estimate genetic variation for novel traits such as early survival of Nile tilapia fry during the early phase of rearing in brackish water environment (5 – 10 ppt); iv) Evaluate growth, physiological parameters, flesh and eating qualities of three genetically improved tilapia strains reared in fresh and saline waters; and v) Identify genes and gene expression associated with growth and salinity tolerance in the selected line cultured in both moderate saline- and fresh-water. Genetic evaluation of the selection program involved 36,145 animals with individual body traits records collected over seven generations from 2007 to 2014. They were offspring of 564 sires and 754 dams. Restricted Maximum Likelihood (REML) method applied to a multi-trait mixed model analysis showed a significant improvement in growth performance with an average genetic gain of 7 – 8% per generation (one generation was completed per year). The selection program for high growth, however, did not have detrimental effects on survival rate during grow-out. The estimate of heritability for the selection criterion (i.e. body weight) was moderate, suggesting that the population will continue showing response to future selection.
Submitted in the fulfilment of the requirements of the degree of Doctor of Philosophy, University of the Sunshine Coast, 2017.