The Biology and Culture of Tilapias

feed on phytoplankton. S. lidole, the most openwater-living, may move
considerable distances to do so (80+ km), the others may turn to bottom
feeding if phytoplankton is scarce. These tilapias have evolved relatively
short annual breeding seasons at staggered times. Season and place of spawn-
ing, reinforced by differences in colors of the breeding males, keep the
species distinct. S. saka, which has a black breeding male, spawns from
August to November, the hot weather before the rains in relatively shal-
low water (4 m deep) near macrophyte beds, off weedy or sandy shores;
brooding females move into weed or reed beds. S. squamipinnis, the non-
breeding fish of which are scarcely distinguishable from S. saka, spawns in
the rainy season, December to February, in deeper water (about 16 m), gen-
erally off more sandy, open shores; brooding females also retire into the weed
and reed beds; the breeding male is sky blue with a white head which is clearly
visible to the ripe females which swim above the nesting arenas (see Berns et
al. 1978). S. lidole, like S. saka, also has a black breeding male and breeds
before the rains, but in much deeper water or off open sandy, clean weedless
beaches.
Fecundity appears to be much reduced in these Malawi Sarotherodon,
by late maturation (not spawning till three years old), the production of
but one brood a year (rarely two?), and of relatively few but large ova
(370 to 549/fish in S. squamipinnis), the young being brooded to a larger
size than in other tilapias (to 30 mm TL in S. squamipinnis, 52 mm in S.
lidole). In Lake Malawi, S. saka and S. squamipinnis mature when about
25 cm TL, S. lidole at 28 cm TL, with no size difference between the sexes;
in aquaria they can mature at a smaller size (10 cm SL-for details of aqua-
rium behavior see Berns et al. 1978).
S. shiranus, of different stock, lives in a more inshore zone of Lake
Malawi and has a more extended breeding season, but mainly December-
January. T. rendalli appears to be confined to vegetated lagoons round the
lake.
Malawi tilapias show adaptations to inhabit increasingly openwater
conditions, from the inshoredwelling S. shiranus to the most offshore
living S. lidole, viz: (a) the food contains a higher proportion of phytoplank-
ton rather than bottom diatoms and bottom debris; (b) the tendency to
collect in small, close schools increases; (c) the amount of movement increases,
S. shiranus populations being very localized and S. lidole far-ranging fish; (d)
the young are brooded to an increasingly large size; (e) the rate of growth in
length appears to increase, but growth in weight for length decreases, giving a
more streamlined form in openwater species. Each species spawns in the
clearer water end of its range, but broods young in the richer, greener water.
The brooding females carry the eggs and young inshore, from spawning to
brooding or nursery grounds. It is possible to tell from the characteristics of
the water, such as clarity, color, and whether the shore is of mud, silt, sand
or rock, shore vegetation, etc., which species are likely to predominate off
a particular beach, though mixed catches are often taken. S. shiranus pre-
dominates in muddy swampy areas, S. saka off sandbanks near weedbeds
close to swampy (Papyrus) and reedy (Phragrnites) shores, S. squamipinnis
off more open beaches with Phragmites, and S. lidole off open, clean,
clearwater beaches or where the bottom shelves steeply, and out into open-
water.