include data on growth and production of tilapias in natural waters.
From the title one might expect discussion of the role of tilapias in
relation to their competitors and predators, and the partitioning of resources
where more than one tilapia species is present in a community. There has,
however, been little direct study of competition with other species, or of
differential predation, though more is known about partitioning of resources.
While reviewing the main sources of information on ecology and behavior
of tilapias in natural and semi-natural fish communities (for the species listed
in Table I), the aim here was to consider:
- The partitioning of resources of (i) food, and (ii) space and time
breeding (now dealt with by Philippart and Ruwet). - Some of the factors which appear to control tilapia numbers in fish
communities, with special reference to the size and age at which dif-
ferent populations switch from growth to reproduction. - Growth rates in different water bodies.
- Production and yield from various natural waters.
Tilapias are preadapted by their breeding and feeding habits to thrive
in still water and most of the important tilapia fisheries are in lakes. In rivers,
tilapia fisheries are usually important only where there are extensive flood-
plains, and the fishery is then a seasonal one, geared to the annual flood
cycle. In the last two decades the creation of large man-made lakes behind
hydroelectric dams on African rivers has stimulated pre- and post-impound-
ment surveys, which have taught us much about the ecology and behavior of
the riverine fish communities and how they adapt to lacustrine conditions.
As we have already seen in the previous papers, riverine tilapias tend to be
widely distributed, for example, those of the 'soudanian' region and in the
Zambezi system. The main river systems of these regions cross savanna
floodplains lying at comparable distances north and south of the equator,
where seasonal floods follow the overhead sun, so that they flood in their
summer season after temperatures and day lengths have started to increase.
The aquatic life is geared to the welldefined seasonal cycles, the high-
water season being the main feeding and growing time for the fishes, including
the tilapias, when growth is fast and fat stores are laid down to last through
times when food is scarce in the low water season. In these vast river systems
floods do not necessarily coincide with local rains, as they may result from
rain far upstream arriving downriver after local rains are over. Tilapias are
less dependent than are the ostariophysan fishes living with them on the start
of the rains, or flood, to initiate spawning. These riverine tilapias generally
spawn at the end of the dry season, producing at least one batch of young
before the water level rises, and continuing to produce batches in succession
through part of the highwater season. The growth checks in the dry season
show on the tilapia scales as annual rings, so more is known about ages and
growth rates of floodplain tilapias than about those dwelling in large lakes,
where more climatically stable conditions throughout the year lead to
continued growth, making scale ring analyses very difficult. As the flood-
plains dry out, the tilapias along with the other fish retreat to the main
rivers, or are trapped in pools on the floodplain until released by rising
waters the next year. Predation from piscivorous fishes, the numerous
aquatic birds and other animals, including man, is most intense as the waters