The Biology and Culture of Tilapias

ponds, as used in Brazil, to allow the easy separation of hybrid fingerlings
from their parents. The latter incorporated a set of screens between the
ponds to sort out different sizes. Dr. Coche reports the separation of finger-
lings by size to be an effective means of increasing the percentage of males in
mixed sex culture in the Ivory Coast. Dr. Guerrero notes that Pruginin and
Shell used a mechanical grader for this purpose.
The design of efficient structures for screening and sorting fish can be
a complicated process. Solid gratings may be preferred to soft mesh as the
geometry of flow can be quite important when they are operated in flowing
water. Much more work of this sort seems to have been done in connection
with the design of fish passes than by aquaculturists. The behavior of the
species being sorted is also important. Tilapia fishermen tend to agree that
adult fish, at least, are extremely wary of gill nets, but it is probably not
known whether this is a question of retreat from a "new" object in the
environment, or from some more specific stimulus.
In the control of reproduction of tilapias, it may be useful to make
a distinction between the requirements for hybrid seed production systems,
where almost complete control over wild spawning is needed, and the
requirements of growouts where less control may be needed. Perhaps more
work should be done on combining the various methods of inhibiting natural
spawning and/or of increasing the malelfemale ratio of progeny. We should
also determine whether the faster growth of males is genetically controlled.