Comparative assessment of the production performances of different strains of nile tilapia and the evaluation of genotype × environment interaction

Aquaculture is acknowledged worldwide as a major contributor to food security.Nile tilapia (Oreochromis niloticus) is the second most produced freshwater fish, and an important source of affordable animal protein in Africa.Tilapia production in Africa is characterised by traditional small-scale subsistence farming systems and more recently the introduction of large-scale intensive farming systems.The efficiency of tilapia production can be improved by farming with genetically improved strains that are developed for particular types of production systems.Genetically improved tilapia strains have been developed under controlled conditions, without proper assessment of their suitability for culture in both intensive and extensive systems, where conditions likely may differ from which the strains were developed in.It is important to determine the potential effect of genotype X environment interaction (GxE) on the performance of these improved genotypes in production systems in Africa.To date the potential GxE effect on the production performance of improved Nile tilapia genotypes in different production systems has not been investigated, and validated in Africa.This study therefore assessed the performance of six unrelated strains of Nile tilapia to determine the possible influence of GxE on the growth rate, proximate carcass composition and haematological parameters.Unrelated strains were randomly allocated to a high input systems (represented by a high energy (HE) diet in the form of a commercial feed), and a low input system (represented by a natural low energy (LE) diet in the form of duckweed (Lemna minor) during two trials conducted during summer (Experiment 1) and winter (Experiment 2), respectively.The strains (displayed significant differences in relation to a wide range of growth parameters for both treatments (diets) and seasons, with the GIFT strain having the best overall performance.The growth rate of strains fed the HE diet was significantly higher than strains fed the LE diet, for all strains during both seasons.There was no significant difference in yield between strains and treatments for Experiment 1.There was significant difference observed in Experiment 2, where the Red strain was characterized by lower yields.The condition factor for both diets was within acceptable ranges, and no difference in survival was observed in relation to strains and diets.The HE diet resulted in a significantly higher carcass fat content, whereas the LE diet result a higher carcass ash and moisture (90 -95%) content.Diet did not influence carcass protein content.Red blood cell (RBC), haemoglobin (HB) and hematocrit (HCT) values did not differ between diets, while significant differences were observed for mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), mean corpuscular haemoglobin concentration (MCHC), white blood cell (WBC), neutrophils (NEU), eosinophils (EOS), and basophils (BASO).The study reports on the first phenotypic biserial correlations for Nile tilapia for growth parameters, with all bi-serial correlations reported all growth traits for Experiment 1 being above the threshold of 0.8, indicating an absence of GxE interaction.The bi-serial correlations calculated for Experiment 2 similarly indicated a level of non-significant GxE for most strains, with Strain S6, where a weak degree of GxE was observed (0.729-0.777).In some cases, re-ranking was observed, however, this re-ranking was found to be not significant.This study confirmed significant differences between the strains in relation to various growth parameters, although there was no clear indication of GxE effects associated with the LE and HE diets.The superior strains may therefore be recommended for application in relation to both treatments applied in this study.GIFT strain (S4) perform significantly better than other strains in the wider coverage of this experiment.The findings of this study indicated that it is important to determine the GxE influence on the performance of improved genotypes, especially when they are propagated for use in low input, small-scale production systems, which may represent conditions that differ from those under which the genotypes have been developed in.A greater awareness also needs to be created amongst subsistence and commercial tilapia farmers in Africa about the possible effect of GxE on the production performance of wild and genetically improved genotypes under different environmental conditions.