ConclusionsTilapias constitute a diversified and plastic group of great interest to
ecologists. But until now most studies have been qualitative and limited to
one particular aspect of their biology. Future ecological research should
above all consist of quantitative overall studies of populations (cf. Bruton
and Allanson 1974 and Bruton and Boltt 1975 for S. mossumbicus) and
studies of population dynamics: analyses of the causes of temporal fluc-
tuations of the populations; demographic comparisons of several species
in the same water body; demographic comparisons on the same species in
contrasting milieu; evolution of populations in water bodies subject to
artificial modifications (cf. Dudley 1979), and interactions between indi-
genous and introduced populations (cf. Noble et al. 1976).
Another important objective is to show the precise geographical distri-
bution of each species and the introductions and transfers with their effects
in Africa and in the other regions (i.e., build up a data bank); this would
provide a reference point for the easier evaluation of the evolution of future
populations.
It is equally necessary to develop quantitative research on the environ-
ment in order to understand better the ecological factors which influence
the distribution of different species and their absolute abundance, as well as
their preferred habitats and microhabitats. This type of study should lead to
preparation of an inventory of populations (ecophenotypes) adapted for
particular conditions (temperature, salinity, etc.) and interesting to exploit
for culture. But parallel to this research on the environment, it appears
indispensable to undertake, in a more systematic manner, experiments on
environmental tolerance limits and preferences. For this it is necessary to
first define a standard methodology in order to eliminate the extreme vari-
ability of the results which appear in the literature. A systematic study of
preferences and tolerance limits in regard to temperature (McCauley 1980)
is especially important.
Finally, concerning introductions, the errors made and the past failures
(notably with S. mossumbicus) by stocking without sufficient knowledge
of the biology of species and ecosystems, encourage very great prudence
concerning introductions of new species (S. aureus, S. hornorum, S. niloticus)
in all parts of the world. It is good to report that many countries, without
doubt because they are subject to the unpleasant consequences of ancient
and uncontrolled introductions; are starting to take control measures on the
spreading of tilapias (e.g., Colombia, U.S.A. and Papua New Guinea). The
burden of decision in this matter ought to rest on an evaluation of the risks
of perturbation of the native biocenoses and ecosystems. However, often in
such ecological studies it is difficult to generalize one way or the other
(total prohibition or total liberty). Numerous examples show that the same
species introduced into different waters often reacts in an unpredictable
and unequal manner depending on the milieu and the action of physico-
chemical factors and biocenotic characteristics. Only intensive research on
the particular case will permit the understanding of the mechanisms involved
and the implications for the management or conservation of the aquatic
milieu.