Optimization of Nile tilapia breeding schemes for monosex culture conditions in smallholder production systems

The aim of this study was to optimize the design of a nucleus Nile tilapia breeding program for harvest weight, growth rate and survival in the presence of genotype by environment interaction with monosex (MS) culture of all-male fish. A nucleus breeding program was deterministically simulated using pseudo-BLUP selection index theory, implemented in SelAction. First, we investigated the rates of genetic gain for growth rate, expressed as TGCms using different selection indexes with varying degrees of genotype by environment interaction (G x E), expressed as the genetic correlation (r(g)) between the nucleus and a monosex production environment. Selection strategies were (1) mass selection in nucleus only, (2) Best Linear Unbiased Prediction (BLUP) with full/half sib information from the nucleus fish only, and (3) BLUP with information from full and half sibs in the nucleus and in the monosex production environment. In the second step, we extended the breeding goal to include harvest weight (HWms) and survival (Sms) to derive desired weights and genetic gains for these breeding goal traits. Finally, we maximized genetic gains in Sms while restricting the loss in TGC to 5%. Results: the presence of G x E lowered accuracy of selection (r(IH)) which led to a loss in genetic gain for TGCms. At r(g) of 0.7 and less, incorporating half sib information from monosex sibs into the selection index resulted in higher genetic gains of TGCms. Using 8 offspring in the nucleus as selection candidates and 32 offspring in monosex production environment, resulted in highest accuracy with lowest rates of inbreeding. This index had the highest relative genetic gains in all r(g) tested, implying that this index is least sensitive to G x E. Phenotypic variance ratio had no effects on r(IH) but had effects on predicted genetic response. Maximizing gains for HWms and Sms caused a large reduction of genetic gain in TGCms. However, allowing a 5% loss of genetic gain in both TGCms and HWms resulted in a 33% increase in genetic improvement of Sms. A breeding goal that maximizes survival while restricting loss in TGC to 5% had the highest response in US$. Statement of relevance: This is the first study that presents optimization of Nile tilapia breeding schemes for monosex culture conditions in smallholder production systems of the tropics.