The Biology and Culture of Tilapias

and eastern parts in 1960, and was abundant by 1964; it later became
the dominant species in the former S. variabilis areas. Meanwhile S. nilo-
ticus and S. leucostictus, both rarely seen in commercial catches before
1962, became much more abundant from 1964 onwards, S. leucostictus
becoming the dominant species in lagoons around the lake and near the
papyrus and in the shallows of muddy bays, S. niloticus on the former
S. esculentus grounds, where they were caught both in surface and bottom
set gillnets. S. niloticus from offshore areas grew much larger than the
indigenous tilapias (personal observation c.1970). The T. zillii lived in
shallow marginal waters all round the lake, fry and juveniles (to 5 cm TL)
inhabiting sandy shelving rock shores in sheltered places, situations favored
as S. varia bilis nurseries.
Welcomme considered possible competition between juveniles, and also
between adults for feeding and breeding places. There appeared to be little
competition for food between adults: T. zillii taking higher plant material,
S. esculentus planktonic diatoms, while S. niloticus and S. variabilis had
more flexible diets, feeding either on the bottom or on epiphytic or plank-
tonic diatoms depending on the habitat. S. leucostictus seemed to feed
exclusively on bottom material (Welcomme 1966, 196713). Spatial segre-
gation of feeding individuals helped to reduce any competition, S. leucos-
tictus being confined to shallow margins of the lake near, and often behind
the papyrus fringe (in water 30 cm to 5 m deep), a habitat shared with
young S. esculentus (of 13 to 18 cm TL), while T. zillii and S. variabilis
tended to spread into harder-bottomed habitats and more exposed waters.
The larger S. esculentus remained in mud-bottomed bays 5 to 10 m deep.
S. niloticus was found in most of the habitats. All the introduced species
grew to a large size in the lake and were in good condition (high weight for
length).
Competition for breeding grounds was more apparent. The possible
lack of suitable areas for spawning, and of nursery grounds for the very
young, appeared to be a factor limiting population expansion. This view
was supported by the dramatic increase in S. esculentus catches in 1964165
following the unprecedented increase in lake level (1.4 m above the previous
highest recorded level) in 1961-62. This flooded lagoons behind the papyrus
fringe, places then used as S. esculentus nurseries (Welcomme 1970). Clean,
firmer substrates were needed on which to spawn by the guarder species
T. zillii, and by the very small S. variabilis, and the ousting of S. varia-
bilk by T. zillii appeared to be due to competition between them for such
places for breeding and nursery grounds. S. leucostictus spawned in shallow
(30 cm deep) water in mud-bottomed areas at the edges of lagoons; here
the males in breeding colors guarded their nests at sites not frequented by
other species. S. esculentus evidently spawned deeper, where it could find
firm enough bottom in the sheltered gulfs, but the juveniles lived in the
lagoons at the lake edge. S. niloticus appeared more catholic in its habits and
could be found in association with any of the other species; its preference
for hard substrates was reflected in some hybridization with S. variabilis, but
later it thrived in S. esculentus areas.
Welcomme's (1964) study of juveniles on nursery beaches showed that
juvenile tilapia were abundant on gently sloping 'gradient' beaches, Haplo-