Effects of intense inbreeding on growth traits In Tilapia (Oreochromis mossambicus)

Tilapia species plays an important food security role in developing countries, but is often farmed by farmers with very little technical knowledge about breeding. When animals are kept in groups and depend on natural mating, inbreeding is inevitable, as inducing spawning by artificial means is impossible. Large amounts of inbreeding in these farming communities may reduce production efficiency. In as much as inbreeding is unavoidable, understanding its effects may aid in the decision making processes of breeding programmes, especially in developing countries. The extent of inbreeding in small-scale production systems farming with tilapia should be estimated to create awareness and alleviate the effects of inbreeding. Therefore, the objective of study was to evaluate tilapia (Oreochromis mossambicus) productivity under the maximum possible inbreeding conditions. Full-sib matings were conducted on O. mossambicus for three generations at Welgevallen experimental farm at the University of Stellenbosch. A total of 25 males and 25 females were used as the parental stock, where a 14-day spawning period was followed by three generations of full-sib mating. Measurements of body weight (BW), standard length (SL) and specific growth rate (SGR) were recorded for each of the sixteen randomly sampled fish per replicate, at two weeks growth intervals for the period of 90 days at each generation. Regression analysis was used to determine the rate of phenotypic depression per unit increase in F and analysis of variance used to establish the difference between the means. The results show that BW, SL and SGR decreased with increase in inbreeding at each generation. Average inbreeding depression for BW ranged from 8.35 to 46.57%, while the average inbreeding depression per 10% increase of inbreeding ranged from -12.42 to -18.62%. For SL, the mean inbreeding depression was 18.15 and 17.95% at G₂ and G₃, respectively. Inbreeding depression coefficients for SL were -7.2 % at F = 0.250 and -4.79% at F = 0.375 per 10% increase of inbreeding. The inbreeding depression for SGR was 21.76 and 20.34% at F = 0.250 and F = 0.375, respectively. Per ten percent increase in inbreeding, inbreeding depression coefficient was -8.70% at F = 0.250 and -5.43% at F = 0.375 for SGR. These results show the extent of inbreeding that can be expected under extreme conditions where inbreeding is not controlled, especially where technical knowledge is lacking. They indicate that inbreeding reduces the performance of O. mossambicus; hence such decrease in performance may be addressed by minimizing inbreeding, which can be achieved by avoiding the mating of full-sibs. Therefore, if inbreeding is not checked in the small-scale production systems, overtime production will decrease.