Effect of larger fish and size grading on growth and biomass gain in nile tilapia (Oreochromis niloticus) in cages

In this study the effects of larger fish and size grading on growth of Nile tilapia were investigated. Fish close to mean population size (10.24±3.09 g) were raised either in presence (20% of the population), or absence of larger fish (39.95±2.12 g) for 60 days. At the end of the study growth was calculated as weight gain, feed conversion ratio, specific growth rate (SGR), and total net weight gain.According to final results; graded group had a larger final average weight (62.35 ± 10.41 g) than that bulk fish of the mixed (large + small) group of smaller fish (49.18±6.35 g) (P<0.05). Daily weight gain of the graded group (0.869±0.035 g.day-1) was higher than the mixed group (0.654±0.023 g.day-1) (P<0.05). Mean specific growth rate was higher in the absence of larger fish (3.00±0.141) indicating a negative effect of larger fish on growth of smaller fish (2.70±0.042) (P>0.05). Feed conversion ratio of graded group (1.29±0.052) was lower than the other group (2.17±0.039) (P<0.05). Total net weight gain of the groups were 5.211±0.208 kg.m -3 and 3.926±0.139 kg.m-3 respectively (P<0.05).