GENOTYPE BY ENVIRONMENT INTERACTION FOR HARVEST WEIGHT, GROWTH RATE AND SHAPE BETWEEN MONOSEX AND MIXED SEX NILE TILAPIA Oreochromis niloticus
In Kenya, Nile tilapia (Oreochromis niloticus) is mostly grown in ponds. To avoid excessive reproduction and stunted growth, fingerlings are treated with methyl-testosterone to make all-male populations (monosex). For a national breeding program that aims to provide genetically improved broodstock to hatcheries that supply monosex fry to smallholder pond farmers, it is important to assess the genetic correlation (rg) for traits between the mixed sex breeding candidates from the breeding nucleus and monosex production fish. The purpose of the study was to estimate genetic parameters for harvest weight (HW), daily growth coefficient (DGC) and body shape and investigate genotype by environment interaction (GxE) for these traits between mixed sex and monosex populations.
Forty-eight sires and 76 dams from the F2 generation of a local O. niloticus strain, kept at Sagana Aquaculture Research Station, Kenya were used to produce 76 full-sib families. Mixed sex fry (3 days old) from each full sib family were divided into two groups of 50 individuals each. One group (monosex) was fed a diet treated with methyl-testosterone to induce sex reversal while the other group (mixed sex) was reared on a control diet. After hapa rearing, tagging and weighing, fish were randomly divided and stocked in six earthen ponds, three for mixed sex and three for monosex fish. After 5 months of growth, 2105 fish fish were harvested, photographed and weighed. Results indicated that monosex groups had a higher percentage (>94%) of males compared to mixed sex groups. Generally survival till harvest across the treatments groups ranged between 55.9 - 89.5%, with mixed sex recording lower survival rates. Mean HW and DGC were higher in monosex groups (120.6g and 1.52) than in mixed sex groups (100.2g and 1.43). Genetic parameters and genetic correlations for HW, DGC and shape traits for mixed sex and monosex groups are given in table 1.
Genetic correlations for HW between monosex and mixed sex was 0.74 ± 0.14. The corresponding rg for DGC and shape were; 0.59 ± 0.10, and -0.19 ± 0.11, respectively. Based on these results, it is concluded that GxE between the mixed sex nucleus and monosex production fish is important, and that a breeding program for Nile tilapia needs to include production performance from monosex siblings.