The Biology and Culture of Tilapias

ing and suggests that they may be more readily attacked by disease and
parasites.
Manures and fertilizers seem to be more effectively utilized in pond
culture systems. In tropical countries tilapia culture is particularly attractive
, when it is combined with pig or duck farming. Drs. Hepher and Pruginin
point out in their paper that organic manures, by directly stimulating hetero-
trophic food-chains, are better than inorganic fertilizers for the microphagous
tilapias.
Only pond and cage culture systems have been reviewed. It is also valuable
to refer to two other systems. One is the very intensive culture of tilapias in
tanks and in raceways, systems which are particularly appropriate in utiliza-
tion of waste thermal waters. The other is extensive culture in small reservoirs,
natural lakes and ponds. The highly intensive culture systems pose special
problems of providing complete diets as well as removal of wastes from the
system. Management of extensive systems, on the other hand, should be of
interest in relation to the extent to which cage or pen culture can or should
be combined with harvest of the more free-living stocks. This is an important
question in, for example, the improvement of the fisheries of the 10,000 or
so small reservoirs of Sri Lanka. In small reservoirs, there are some interest-
ing possibilities for the control of reproduction of tilapias through water
level control: for example, by exposing nests at critical periods.
Where cage culture has prospered, local administrators have often been
unprepared for the resulting legal and environmental problems. Dr. Coche
cites several cases in the Philippines where this has happened and I have
been told of similar sorts of problems in Polish trout culture. It is, however,
not only administrators who have been caught unprepared. There is very
little information available on the effects of cage culture in lakes, ponds or
bays upon which to base predictions of permissible levels of occupancy. At
high levels of occupancy, reduced production in each cage unit becomes
obvious to the operators. As in capture fisheries, however, the overutiliza-
tion of water bodies for cage culture is not likely to be reduced to optimal
levels voluntarily. Unfortunately, we know little about predicting permis-
sible levels of occupancy before production begins to drop, nor, to my
knowledge, has there been any attempt to define effluent standards for cage
culture, analogous to those which have been adopted in some places for
pond culture.
Speaking of environmental matters, Dr. Coche also notes that cage cult-
urists and others should take care that inappropriate species are excluded
from waters to be used for cage culture. Competition for food between S.
aureus and wild S. mossambicus in cages in Puerto Rico has been cited as an
example of this problem. There, the wild mossambicus were also thought to
be a source of parasites and disease for the cultured aureus.
There is a problem not mentioned explicitly in any of the papers, but
which will emerge as tilapia culture becomes more intensive and more highly
selected strains are used for culture. This is the increased movement of both
broodstock and seedstock across national boundaries and the attendant risks
of transfer .of parasites and disease, and of contamination of stocks with
genes of related species. In most countries, there is as yet very little control
on either the species, or more importantly, the health of the fish transported.