Response to bi-directional selection for frequency of early maturing females in Nile tilapia ( Oreochromis niloticus)

Abstract

Twenty tagged individuals were randomly sampled from each of 42 full-sib families within 21 randomly chosen half-sib families of Nile tilapia ( Oreochromis niloticus) among the families representing the third generation of selection for improved growth performance in the GIFT project, and communally stocked in an earthen pond. The occurrence of sexual maturation in the females was recorded four weeks after the first swim-up fry were observed in the pond. Broodstock was selected from full-sib families with a high (>75%) or a low (<20%) frequency of mature females. After about 4 weeks of single pair stocking in breeding hapas, 16 pairs of breeders from families with a mean frequency of 83% early maturing females and nine pairs of breeders, all from families with 0% early maturing females, had produced fry. The fry from each pair were reared separately until a size of 3–5 g, when totally 3179 fingerlings equally representing the 25 progeny families were tagged and communally stocked in three replicated ponds for testing. Recording of sex, body weight and occurrence of sexual maturation in the females was carried out 2, 3 or 4 weeks, respectively, after observing the first swim-up fry in the three ponds. The fish were restocked and reared until recording at harvest. The age corrected least square means across ponds and across families within selection groups for frequency of sexually mature females at intermediate recording was 57.0% and 33.6% in progeny of breeders from full-sib families with a high or a low frequency of early maturing females, respectively. The response to selection, measured as the difference between the two progeny groups, was highly significant ( P=0.0002). The timing of the recording was appropriate in all three ponds. A significant correlated response in body weight of males at harvest ( P=0.027) and a nearly significant correlated response in body weight of females at intermediate recording ( P=0.052) was also observed. The age corrected least square means of body weight were higher in the progeny of breeders from full-sib families with a high frequency of early maturity in the females, suggesting an unfortunate genetic association between the two traits in Nile tilapia used in aquaculture. It is proposed to carry out combined selection for body weight and frequency of early maturing females in applied breeding programs for farmed Nile tilapia.