Six hundred Nile tilapia Oreochromius niloticus monosex fingerlings (mean 0.98g ± standard error 0.02) were stocked (50 fingerlings / m2) into 6 different hapas to constitute two experimental groups, control group (C) and base group (B). After 4 weeks, the larger 20 fish in each hapa in the base group were designated to establish the experimental group (I1). After another 4 weeks, the 15 larger fish in each hapa in the base group were also designated to form another experimental group (I2). All formed experimental groups were reared for another 8 weeks. The coefficient of variation (C.V. %) showed a notable increase at the end of the experiment for all experimental groups including (C) from 22.1 to 39.2%; (B) from 21.5 to 26.2%; (I1) from 18.4 to 27.3%; and similarly (I2) from 15.6 to 25.5%. Final coefficient of variation (C.V. %) in the control group (C) 39.2% was significantly higher (p ˂0 .001) than those of all another experimental groups. Size distributions for all experimental groups also typically skewed positively towards the larger fish with time. The coefficient of skewness increased from -0.3, 0.21, 0.7, and 0.56 for C, B, I1, and I2, respectively to 1.84, 0.7, 1.15, and 1.22 at the end of the experiment. Specific growth rates % of the base group (B) were significantly (p˂0.001) higher than those of all other experimental groups after larger fish designation indicating an enhancement in the growth of the remaining individuals. The final body weight of I1and I2 (15.91 & 16.39 g, respectively) were significantly (p˂0.001) higher than those of C and B groups (12.88 & 12.89 g, respectively). Growth hormone receptor (GHR1) mRNA level was highly up regulated (5 folds) and Insulin growth factor-I (IGF-I) mRNA level was lowly down regulated (0.76) in the liver of secondary isolated lager fish (I2) than those of the subordinate individuals remaining in the base group (B). The results of the current study led to suggest that repeated isolation of the larger fish from the rearing unit may help on reducing the final size variation and increasing the probability of fast growth for more individuals. The level of GHR1 mRNA was highly depending on the relative position in the hierarchal structure within the same rearing unit. |
