The Biology and Culture of Tilapias

ing water temperature and/or light periodicity; there appears to be a correla-
tion between onset of rains and breeding activity, in the river at least one
brood of young is produced before the floods, and one or more during the
highwater season (in ponds four broods may be produced between September
and February). The female mouthbroods the young. Once juveniles leave the
floodplain, where they grow fast, they are restricted to lagoons and back-
waters until they reach a size large enough to coexist with piscivorous
Hydrocynus.
The Mweru/Luapula system has a subspecies of S. macrochir which has a
characteristic star-shaped nest; this undertakes seasonal migrations from the
deeper north end of Lake Mweru to the shallower southern end and up the
Luapula River during the period July to October (it breeds September to
March), with a reverse movement northwards in February-May; it appears to
need shallow water in which to spawn, and cover for the young. Juvenile S.
macrochir have a higher tolerance of low DO than adults, and remain on the
spawning grounds up to a length of 18 to 20 cm. Marshall (197913) gives egg
numbers ranging from 1,000 to 1,500 in 16 to 27 cm SL female S. macrochir.
S. andersonii, a larger-growing species (males up to 3.2 kg) occurs widely
except in small streams and is found in all types of habitat, but it is a shy
fish which likes to hide in deep pools. Though it takes the same food as S.
macrochir, it appears to be less specialized and will also eat aquatic and
terrestrial insects and small Crustacea. The female mouthbroods, and seems
to produce only one brood a year, between November and January. The eggs
are large (374 to 593 eggs in females of 17 to 25 cm TL; Mortirner 1960).
T. rendalli, widely distributed and abundant throughout the Upper
Zambezi, moves up tributary rivers and onto the floodplain, but spawns
once in the dry season before leaving the river as well as on the flood-
plain; juveniles in tributaries continue to move upstream. This species
prefers water with dense growths of aquatic plants. In addition to macro-
phytes and detritus, it eats aquatic and terrestrial insects. It grows to 1.4 kg.
A multiple spawner, it breeds during the rains between October and February;
the nest is a series of small holes made in shallow water where both parents
guard eggs and young.
The smaller T. sparrmanii (growing to 0.25 kg in rivercthe largest, up to
25 cm in Lake Young) is the commonest cichlid *m the Upper Zambezi
and distributed throughout the system; it prefers quiet water with god
weed cover, in riverine, swamp and lake habitats. It is a fairly omnivorous
species, feeding primarily on periphyton, aquatic plants, detritus and algae,
but Crustacea, insects (terrestrial and aquatic) and even worms and fish may
be eaten. A multiple spawner, batches of eggs are produced between October
and February, both parents guarding the eggs and young. In Lake Young
Ricardo-Bertram (1943), who described the spawning behavior there in
shallow water, considered that suitable breeding grounds with sandy bottom
were very limited, and there was great intraspecific competition for nest
sites, the nests almost touching one another.
Predators on these Upper Zambezi cichlids include piscivorous cichlids,
Serranochromis spp. (though these feed mainly on cyprinids and cyprino-
donts). The characoid Hydrocynus vittatus takes many cichlids and its