An initial twenty species in the reservoir fish fauna included marine species
which survived but did not reproduce, rice paddy and riverine fish from the
drainage basin, and Chinese carps stocked in 1967-71. The S. mossambicus
were escaped pond fish which established themselves and soon became the
dominant species in gillnet catches. Chiefly a detritus-feeder, the tilapia here
had the most diverse food spectrum of any species studied, thus proving to
be a good reservoir fish since almost all items were exploited, and when
certain items were scarce a change to others was possible. Tilapia also helped
to control algae and chironomids, and to remove nutrients and decaying
organic detritus from the mud, thus indirectly helping to reduce algal growth.
Earlier studies on S. mossambicus in Indonesia had provided much basic
information about this species in Asian waters (Vaas and Hofstede 1952).
In Lake Moyua in Nicaragua (Central America) the stocked S. mossam-
bicus matures between 12 and 14 cm (90 to 100 g) when 5 to 6 months old.
This lake lacks piscivorous fishes, but numerous water birds feed on the
tilapia fingerlings.Where several tilapia species share a water body, nesting areas are usually
distinct in place (depth or type of bottom) or time. In substrate-spawners
the elaborate courtship behavior probably helps to prevent any hybridization.
Courtship behavior is generally much briefer in the mouthbrooders. Cases of
hybridization, or suspected hybridization in natural waters, due to introduc-
tions include those of:
S. niloticus x S. spilurus niger in Lake Bunyoni (both introduced)l(Lowe-
McConell1958)
S. niloticus x S. esculentus in Lake Nkugute (both introduced) (Lowe-
McConnell1958)
S. niloticus (introduced) x S. variabilis (indigenous) in Lake Victoria
(Welcomme 1966)
T. zillii x T. rendalli (both introduced) in Lake Victoria (Welcomme 1966)
S. amphimelas (indigenous) x S. esculentus (introduced) in Lake Kitangiri
(Trewavas and Fryer 1965)
S. s. niger x S. leucostictus (both introduced) in Lake Naivasha (Elder
et al. 1971, who considered how these hybrids fitted into the ecological
zonation in the lake). These hybrids, abundant in 1961, following
introductions in 1956, had disappeared by 1974 (Siddiqui 197713).
Tilapia Growth Rates in Natural WatersThe most reliable growth rate data come from waters with marked seasonal
changes which affect the biology of the fish, leaving marks on scales, bones,
otoliths, etc., and where seasonal spawning allows length-frequency mode
progressions to be traced in the young fish. Examples are given in Figure 4,
based on data from nilotic tilapias by Blache et al. (1964), Ben-Tuvia (1959),
Daget (1956), Jensen (1957), Lauzanne (1978), Payne and Collinson (in