chromis off more steeply shelving 'non-gradient' beaches. The small tilapias
could withstand temperatures up to 38"C, lethal to the Haplochromis.
Tilapias moved on and off these gradient beaches in accordance with the
diurnal temperature fluctuations, the shallows cooling rapidly at night and
the fish then moving into deeper water. An approximately linear relationship
was found between the size of fish and depth of water. This enables small
tilapia to feed without disturbance from larger individuals. Also they are
then out of reach of the piscivorous catfishes, Clarias mossambicus and
Bagrus docmac, often caught just off the tilapia beaches. The nursery beaches,
though sharing high temperatures, differed in their dissolved oxygen tensions
(DO), for which the different species had characteristic, though to some
extent overlapping, preferences. S. leucostictus young were confined to areas
of low DO (0.6 to 2.0 ppm), S. esculentus and S. niloticus lived in slightly
better aerated water (1.0 to 3.0 ppm and 2.0 to 4.0 ppm respectively), while
T. zillii and S. variabilis needed well-aerated slightly exposed areas. Com-
petition for nursery areas appeared greatest between T. zillii and S. variabilis.
The introduced Lutes niloticus fed mainly on cichlids in Lake Victoria
and included some tilapia in its diet (Gee 1968,1969).
Zambezi system tilapias have been much studied as part of the pre- and
post-impoundment surveys of hydroelectric schemes at Kariba and Cabora
Bassa on the Middle Zambezi and on the Kafue tributary (see Balon and
Coche 1974; Bowmaker et al. 1978; JFRO Annual Reports, and for informa-
tion on the Barotse floodplain on the Upper Zambezi, Duerre 1969). Tilapias
flourish better in the more lake-like 'reservoir rivers', which retain water
and a good growth of aquatic vegetation throughout the year, than in the
'sandbank rivers', which dry out into pools devoid of plant cover in the
dry season. Throughout the Zambezi system the presence of large piscivores,
especially Hydrocynus vittatus (Hepsetus odoe in the Kafue), appear to
control the ecology of the tilapias. The Upper Zambezi and Kafue have two
indigenous species of Sarotherodon, S. macrochir and S. andersonii, and
two Tilapia, T. rendalli (which greatly resembles T. zillii in the northern
soudanian floodplains) and the smaller T. sparrmanii. In the Middle Zarnbezi
S. mortimeri (which greatly resembles S. mossambicus in the Lower Zambezi)
is the dominant species, though S. macrochir is also present; S. andersonii
had not been recorded from the Middle Zambezi until 1971, after Lake
Kariba had been established for twelve years. This example stresses that
tilapias may move about more than realized, but strays do not get established
unless they find suitable ecological conditions, in this case due to a man-made
change.
Summarizing information on Upper Zambezi tilapias, Bell-Cross (1974,
1976) described S. macrochir there (the subspecies with a 'volcano' shaped
nest) as being well distributed and abundant throughout the Upper Zambezi
and its larger tributaries, growing to 1.8 kg. It feeds mainly on periphyton
and detritus, though other algae are taken in summer months when they are
abundant. Breeding starts early in the summer, probably triggered by increas-