RECRUITMENT CONTROL OF NILE-TILAPIA, OREOCHROMIS-NILOTICUS, BY THE AFRICAN-CATFISH, CLARIAS-GARIEPINUS (BURCHELL 1822), AND THE AFRICAN-SNAKEHEAD, OPHIOCEPHALUS-OBSCURIS .1. A BIOLOGICAL ANALYSIS

The recruitment of Oreochromis niloticus (stocked at 20 000-22 000 ha( -1)) was completely controlled by large Clarias gariepinus (6.8-130 g) and large Ophiocephalus obscuris (75-206 g) at stocking densities of 8300 catfish ha(-1) or 725 snakeheads ha(-1), The difference in predat ion efficiency between the two species is related to their feeding str ategies: omnivorous vs piscivorous, The elimination of the Nile tilapi a fingerlings caused the growth rate of male and female Nile tilapia t o increase significantly (P less than or equal to 0.05) with both pred ator types. However, the net yield decreased in the case of predation by snakeheads (P less than or equal to 0.05) but did not change in the case of predation by catfish. Analysis of the results indicated that food shortage is a limiting factor. This limitation can be removed by increasing the feed supply directly with additional feed or indirectly by the elimination of the Nile tilapia fingerlings. The results led t o the conclusion that stunting of somatic growth in Nile tilapia is ma inly related to the husbandry techniques applied. Small catfish (weigh t less than or equal to 3.65 g) and small snakeheads (weight less than or equal to 2 g) were not capable of controlling recruitment complete ly; at harvest 3.7% and 8.9% of tilapia fingerlings (of total harveste d biomass) persisted for, respectively, catfish and snakeheads at the highest stocking densities. Incomplete recruitment control by small, s tocked predators is mainly related to the later onset of piscivorous f eeding and the low survival rates (24-34%).