niloticus predominates, and in the southern deltas of the inflowing rivers
where T. zillii lives over varied types of bottom (Blache et al. 1964). The daily
food consumption of S. galilaeus in Lake Chad, studied by Lauzanne (1978a,
1978b), depends on the water temperature; the conversion of the detritus
eaten is very low (only 3%). In the new man-made lakes of Volta (Ghana),
Kainji on the Niger, and NasserINubia on the Nile, S. galilaeus has become
the dominant tilapia, though in the Volta lake S. niloticus also became fairly
abundant on the eastern side of the lake, and T. zillii in certain western arms.
In Lake Kainji the buildup of tilapia populations was delayed for some years,
perhaps because the large annual drawdown (10m) affected potential spawning
grounds; furthermore Citharinus became very abundant when Kainji first
filled, the lake filling coinciding with, or stimulating, a good Citharinus
spawning year, and conditions for the survival of the young were good as
predator populations had not yet built up. In Lakes Turkana (Rudolf) and
Albert, which both have nilotic faunas, S. niloticus is the larger-growing and
more abundant tilapia, feeding mainly on phytoplankton. Though here again
S. galilaeus is found off sand banks; T. zillii is rarely seen in the main lake.
Lake Albert also has another inshoredwelling Sarotherodon, S. leucostictus.
In Lakes GeorgeIEdward, which drain into Albert, S. niloticus and S. leucos-
tictus are abundant (the latter in the inshore zone), but S. galilaeus and T.
zillii are absent from the fauna. Both Albert and Turkana have populations
of dwarf S. niloticus, living in lagoons along the Albert lake shore, and in
crater lakes on Central Island in Lake Turkana.
S. niloticus was also stocked into many lakes and dams in Uganda. In
some of these the fish grew very large while the populations were small, but
then multiplied rapidly and became dwarfed; in other lakes they apparently
never grew large, which may have reflected the paucity of suitable food.
Data on S. niloticus from many of these lakes and dams, summarized by
Lowe (McConnell) (1958), brought out the following points:
The distribution of S. niloticus within a water body varied with the
ecological conditions. Phytoplankton, epiphytic diatoms or bottom
algae were used as food according to whatever was available.
The size to which the fish grew, and at which they matured, varied
greatly; fish from large lakes matured at, and grew to, a larger size
than those from lagoons or ponds. (The largest up to 64 cm TL
came from Lake Rudolf, Worthington and Ricardo 1936). (Figure 1)
In the lakes the males and females did not differ significantly in
maturation size or final size, but in small bodies of water where
the fish were dwarfed, males were larger than females, and also much
more numerous than the females.
Fish in poor condition (low weight for length) matured at a much
smaller size than fish in good condition (Figure 2). The growth rates
and ages of large and small fish at maturation were not known, but in
some newly stocked waters S. niloticus grew to 350 g (equivalent to
about 26 cm TL) in one year, whereas in ponds they grew to 17 cm
TL (98 g) and started to breed when 7 to 8 months old.
In equatorial waters S. niloticus in breeding condition may be found
at any time of year, but peak spawning coincides with the two rainy
seasons (a finding later confirmed by Gwahaba 1973). At higher
sean pound
(Sean Pound)
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