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
G. Degraaf et al., 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, Aquaculture, 146(1-2), 1996, pp. 85-100
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%).