Salinity tolerance in superior genotypes of tilapia, Oreochromis niloticus, Oreochromis mossambicus and their hybrids

Tilapias are important candidate species for brackish water aquaculture and are increasingly cultured in coastal ponds including in polyculture with shrimp, thus creating a demand for tilapia genotypes well suited to elevated salinities. The Nile tilapia Oreochromis niloticus dominates freshwater tilapia culture and has been the subject of recent efforts in genetic improvement but is one of the least salt tolerant tilapia. Oreochromis mossambicus, are more tolerant of high salinity but tend to have lower growth rates. This study compared the culture performance of two strains of genetically improved O. niloticus (GIFT and Fishgen-selected), and two stocks of O. mossambicus (Boesmans and Kasinthula-both newly introduced to Asia directly from southeast Africa) and their hybrids. These comparisons were made over a range of fixed salinities (0, 7.5, 15, 22.5 and 30 ppt) with fish grown for 75 days in cages within concrete tanks with three replicate cages of each genotype at each salinity. Attempts to produce reciprocal hybrids failed due to very low spawning frequency in O. niloticus x O. mossambicus crosses, thus all hybrids were only of O. mossambicus maternal origin. Results for the trials were consistent across replicates and a number of interesting trends were apparent. Overall, growth was higher for all genotypes at elevated salinities with O. niloticus relatively faster growing at low salinity and O. mossambicus at the higher salinities. The hybrids revealed high average positive heterosis for weight gain (1.24) and biomass gain (1.33) but less for FCR (1.08). Regression analyses of relative weight and biomass gain showed highly significant correlations with salinity. The hybrid was superior to O. mossambicus at all salinities and to O. niloticus at salinities above 10 ppt. Survival of O. niloticus was lower than the other genotypes at 22.5 and 30 ppt due to susceptibility to disease and possibly also stress. The implications for the development of tilapia for brackish water aquaculture are discussed.