SESSION I: BIOLOGYChairman's OverviewAt the start of this first session of the Conference it is very important to
stress the aims of the meeting, as set out in Roger Pullin's letter of invitation,
namely, "the urgent need to bring tilapia biologists and culturists together to
exchange views and information and to define approaches and priorities for
'future work". We need to keep this firmly in mind and aim to produce the
most constructive guidelines for future research.
The two groups of people mentioned, the tilapia biologists who have
been working mainly under field conditions or in laboratories (or both)
and those with practical experience of aquaculture, have much to give
one another of very different experiences that can help to solve one another's
problems. This is a two-way process. As a field worker myself, I know only
too well the complexity of working conditions in natural waters in the
tropics, where so many species interact and where, for example, it is often so
difficult to determine fish growth rates. We have to look to those who grow
tilapias in ponds to solve many problems posed in the field, in particular,
those concerning the switch from growth to reproduction (nanism, dwarfing)
which is such a special feature of tilapia biology. I regard this as possibly the
key problem for our attention. If we could really understand the mechanism
which controls the switch from growth to reproduction, this would be one
of the biggest steps towards improving tilapia culture.
This session is concerned mainly with field studies, and should provide a
good background to our subsequent consideration of the physiology of
tilapias from both field and laboratory work. For those who grow tilapia in
ponds, we must stress the vast scale of the theatre for field studies. The
equatorial Lake Victoria in East Africa is over 6.5 million ha and the Kafue
floodplain, another site for much tilapia research, around 121,000 ha.
Sampling such huge areas presents special problems, and errors arise in
extrapolation of results. Catch statistics are of very variable reliability
compared with emptying a pond and weighing all the fish. Furthermore,
what is loosely referred to as 'fish production' from these vast areas is really
the fish catch and only a very small percentage of the total biological pro-
duction (unlike ponds from which most of the total production may be
channelled into spawners).
In this session we have four papers.
Dr. Ethelwynn Trewavas first introduces the taxonomy and speciation
of tilapias, and says why it is so important to know which species we are
dealing with when making studies of any kind. We all owe a great debt of
gratitude to Dr. Trewavas who has toiled away for very many years putting