Abstract:

Despite its introduction in the 1950s, Nile tilapia (Qreochromis niloticus) farming in
Uganda is still mostly at subsistence level. Three private fish farms (A, B, C) stocked
with mixed sex tilapia were studied between January 2009 and July 2009. Management
practices differed in the three farms: Fertilization was done before stocking in Farm C but
lacked in farms A and B. Rice bran and cabbage leaves were administered as
supplemental feeds in Farm A; and maize bran and cabbage leaves in Farm B. The
quantities of maize brain and cabbage leaves in Farm B were twice and thrice that of rice
bran and cabbage leaves in Farm A respectively. Fish in Farm C depended on natural
foods only. The farms were subjected to partial harvesting prior to this study. The major
objectives were: to identify and quantify major natural foods within the fish ponds,
determine diet, and life history traits of O. niloticus. Four phytoplankton taxa:
Bacillariophyta, Chlorophyta, Cyanophyta and Euglenophyta were identified.
Zooplankton comprised of mainly Rotifers, Copepods and Cladocerans. The dominant
macro-invertebrates taxa were Gastropoda, Diptera, and Oligocheate. Farm B supported
the highest densities of natural foods: 52 ± 8 cells.mF1; 1048 ± 277 organisms.I'1; and
2147 ± 598 organisms.m’2; while Farm A had the least 5 ± 1 cells.mF1; 289 ± 53
organisms.!’1 and 520 ±111 organisms.m'2 for phytoplankton, zooplankton and macroinvertebrates
respectively. Kruskal-Wallis test detected significant differences in
numerical densities of foods between farms. Chief dietary items were higher plant
material, detritus, and algae. Fish from Farm B ingested more zooplankton and macroinvertebrates
than that from the other farms. Supplemental feeds constituted only 3% and
<1% of the fish diet from farms B and A respectively. Sex ratio (Chi-test) did not differ
significantly from a 1:1 ratio. Size at first maturity was below 20g (except for males from
Farm B) and the modal size class was <20g. Males grew bigger than females, and
ANCOVA test for logarithmic relationships showed significant differences both within
and between farms. Fish from Farm B were heavier and grew isometrically; fish from
farms A and C displayed a negative allometric growth. Excess cabbage leaves provided
primarily as feed probably supplied manure that enhanced higher production of natural
foods to provide a better diet which perhaps contributed to improved performance of Nile
tilapia in Farm B. Farmers are encouraged to stimulate natural food production, provide
supplemental feeds, farm all-male Nile tilapia, and observe management practices.
Research to establish age at maturation in farmed Nile tilapia is recommended.