Longitudinal genetic analysis of Nile tilapia (Oreochromis niloticus L.) body weight using a random regression model

Genetic parameters for body weight at ages over approximately 120 days are scarce in Nile tilapia. In this study, genetic parameters for body weight in Nile tilapia were estimated for ages ranging from 100 to 326 days. To this end, five repeated observations of body weight were collected on 2483 pedigreed fish. The fish originated from a population descending from four different genetic backgrounds. Genetic parameters were estimated using a random regression model with covariance functions. The heritability of body weight was fairly constant around 0.2, which offers good prospects for selection on body weight. Genetic correlations were estimated between all ages (100-326 days). At higher ages, genetic correlations were clearly more stable: for example genetic correlations were over 0.9 between the age of 100 days and all ages up to 115 days of age; at higher ages genetic correlations were over 0.9 between the age of 223 days and all ages up to 326 days of age. The estimated genetic parameters showed that early selection results in higher selection response than direct selection, when the target trait is body weight at the age of 326 days. This is due to somewhat higher heritabilities at lower age and a shortened generation interval. Furthermore, evidence was found for genetic differences in growth patterns of fish of different strains. This means that the possibility to change the shape of body weight curves by selection exists and that the choice of strains should depend on the target market weight of the production chain. From the raw data we concluded that differences in body weight between male and female fish were significant already at early ages (100 days). Results from a bivariate genetic analysis, where body weight of male and female fish are treated as separate traits, suggest that body weight in male and female fish is most likely controlled by the same genes. Prospects to decrease the difference between mature male and female body weight by selection are therefore unfavorable in Nile tilapia.