December. Their nesting sites are ecologically distinct. T. rendalli nests along
the lake margins in water 60 to 100 cm deep, sometimes in colonies, the
parents depositing the alevins in pits. The S. mortimeri nests appear to be in
water 4 m deep. The juveniles move from the nurseries into deeper water at
night, but in Lake Kariba tilapia keep near weed cover till 17 to 19 cm TL
(though in dams which lack Hydrocynus, tilapia schools of all sizes mingle at
varying depths). The largest tilapia in Hydrocynus stomachs were 18 cm TL.
In Kariba Hydrocynus seldom enter water shallower than three times their
own length; small Hydrocynus taken in the primary nursery areas were
still feeding; on invertebrates. Other piscivores here are the catfishes Clarias
gariepinus and Eutropius depressirostris; Morrnyrus deliciosus also forage
in the deep nurseries. The combination of active predation (mainly by
Hydrocynus) and fluctuating water levels has enhanced the importance of
submerged plants in providing cover for tilapias in Lake Kariba.
The Lower Zarnbezi is the type habitat for S. rnossambicus, now a circum-
tropical pond fish. Bell-Cross (1976) described this species as an omni-
vorous feeder, taking plankton, algae, aquatic and terrestrial insects, shrimps
and vegetable matter. A polygamous mouthbrooder, several broods are
produced during the summer (starting about September but varying with
water temperature). Some move up tributaries to spawn. The most thorough
ecological studies of this species in natural waters are those made in Lake
Sibaya (Kwazulu, South Africa).
S. mossambicus lives in Lake Sibaya with T. rendalli swierstrae and T.
sparrmanii (see Bruton 1979). S. mossambicus nests on terraces in open,
shallower water than that used by the other two species, whose nests are
among macrophytes. Male S. mossambicus move in to establish territories in
September, when the water temperature first exceeds 20°C. There may be
up to 13 nests per 1,000 m2. There is intense intraspecific conflict between
males prior to excavating nests. Small males (10 to 13 cm SL) are repulsed
and returned to deeper water; they may establish nests if the lake level rises,
but not at low lake level. After a brief active courtship the female leaves
the nest and mouthbroods eggs and fry for 14 to 22 days in the macrophyte
beds. The fry when about 9 mm SL are released in very shallow water (10 cm
deep) along barren sandy shores or in marginal pools. They occupy pro-
gressively deeper water by day as they grow larger, but all move into deeper,
warmer water at night, where they lie motionless. The juveniles, which
return to the shallows when the temperatures first exceed those of the
main lake in the morning, form small schools (up to 20 fishes) on terraces by
day for feeding; the composition and size af the feeding groups change
constantly.
Scale studies have shown that males grow faster than females after the
first year, as discussed below. This Lake Sibaya S. mossambicus population
is breeding precociously, then growth deceleration occurs and the final
size is smaller than in other natural systems in southern Africa, for reasons
discussed by Bowen (this volume).