Lake Chilwa, lying southeast of Lake Malawi, has a subspecies of S.
shiranus, S. s. chilwae as a predominant member of its very small fish fauna
(which includes T. sparrmanii, one species of Barbus, one of Haplochromis
and two of Clarias). Mass mortalities of the fish occurred when this lake
virtually dried out in 1965-68 (see Furse et al. 1979). Tilapia populations
started to decline ahead of the main drying out-possibly due to sandy
shallows used as spawning grounds being reduced in area (which seems to
happen about every six years or so in this lake). Populations of dwarfed
tilapia survived in the permanent streams of the basin and pools in the
marshes; these fish were able to repopulate the lake when it refilled. But
aided by stocking, it still took three years before tilapia numbers were high
again. This subspecies is smaller in adult size than S. shiranus in Lake Malawi,
matures earlier, produces smaller and more numerous eggs, and has two main
spawning periods in an extended breeding season. It also matures more
quickly in freshwater when rains come again. In Lake Rukwa in Tanzania,
Ricardo (1939) had found S. rukwaensis to have dwarf populations in river
pools near the lake after it dried out. These appeared adapted to repopulate
the lake after it refilled.
Endemic lacustrine tilapias have also evolved in small crater lakes in west
Africa. Lake Bosumtwi (Ghana), whose fish communities have been studied
very comprehensively by Whyte (1975), has three tilapias, Sarotherodon
galilaeus multifasciatus (an endemic subspecies), Tilapia discolor (endemic
and very like T. zillii), and T. busumana. The adult S. g. multifasciatus live in
midwater feeding on phytoplankton, here mainly blue-green algae and
desmids. Unlike other tilapias (and S. galilaeus in Lake Chad) this species is
said to feed at night, between 6:00 P.M. and 5:00 A.M. undergoing a diurnal
vertical migration to do so; sinking to 30 m by day and rising to surface
waters at dusk to feed. Adults of both Tilapia species are bottom dwellers. T.
busumana feeds mainly on aufwuchs on stony and sandy substrates by day
and moves into deeper water (3 to 7 m) by night. T. discolor feeds mainly on
periphyton on reeds (Typha) and water plants, and on aufwuchs, but feeds
mostly by night, moving into deeper water (5 to 10 m) by day. The juveniles
of all three species are surface dwellers round the lake edge, using different
foods from their adults; they all share the allochthonous insects on the
surface, eaten together with phytoplankton in juvenile S. g. multifasciatus
and T. discolor, while T. busumana juveniles take aufwuchs instead of
phytoplankton. There is thus separation in feeding habits between species,
between juveniles and their adults, and also in vertical use of space, juveniles
living in surface waters, S. g. multifasciatus in midwater, both Tilapia species
at the bottom, one feeding by day the other by night.