species to be hybridized mean similar spawninghehavior resulting in increased
spawns and hybrid fingerlings?
Fish culture researchers have traditionally tried to answer these questions
with practical research that often solved the problem without explaining
why the result was obtained. Seven years of such work with the S. niloticus
x S. homorum hybrid has not provided the required increase in hybrid
fingerling numbers on which to base large commercial cultures. I believe that
the answer, if, one exists, is to be found in basic research by knowledgeable
behavioralists and physiologists aimed at understanding the barriers to tilapia
hybridization and then discovering methods to overcome them.
Further investigation is needed to determine the best installations for
producing tilapia hybrids. What are the comparative advantages of producing
hybrids in small units, where a high degree of control is possible, over a
welldesigned pond hatchery? Which system is more economical and tech-
nically feasible?
Other areas still in need of further research are the optimum stocking
densities and sex ratios of broodstock, the length of time for which they
can be used before replacement, and the influence of broodstock nutrition
on fecundity and fingerling survival.
Other tilapia species should be hybridized to discover new crosses that will
produce 100% male offspring. The discovery of an all-male herbivorous
hybrid would be very valuable.
Little comparative data is available on the growth and production of
hybrids. Studies are needed to determine which hybrid crosses respond
best to feeding and which to fertilizing so that intelligent choices of hyW
can be made.
The vast majority of cultured tilapia hybrid crosses give less than 100%
male progeny. These fish are often marketed at a small size (Taiwan) or the
males are hand-sexed so that larger fish can be raised (Israel). The commercial
culture of all-male hybrids presents problems that have still to be researched.
The consistent production of 100% all-male hybrid fingerlings in sufficient
numbers to stock a large growout operation has not been worked out.
The level of technology needed to raise all-male hybrids is beyond the
reach of most farmers in tropical developing countries and great care should
be exercised in introducing all-male hybrid culture into countries with n@
fish cultw tradition. Culture of pure species appears to be the logical first
step in introducing tilapia culture to such countries. However, in countries
with a tradition of fish culture, where trained biologists are available and
money .exists to build hybrid fingerling hatcheries, culture of all-male tilapia
hybrids may hold great potential if researchers are able to improve fingerling
production techniques.
AcknowledgmentsI would like to thank the National Department of Works Against the
Droughts (DNOCS) and the Centro de Pesquisas Ictiologicas for their support
and patience over the 6.5 years I spent in Fortaleza, Brazil, performing fish