overland from Israel to Scotland in hit lorries suffered no mortality
despite exposure to cold temperatures.
Several participants expressed the view that private sector hatcheries
afforded the best hope for a rapid and efficient increase in tilapia seed
production. However, the economic viability of tilapia seed production
has been tested in very few countries, such as Israel and Taiwan. Government
hatcheries or government subsidies to private sector producers may be
needed in the future.
Culture Techniques and SystemsThe discussion concentrated on methods of recruitment control other
than monosex culture of sex-reversed or hybrid progeny, which it was felt
were adequately discussed elsewhere. Attention was drawn to the high
success (84%) of male fry production by size selection alone in the Ivory
Coast. There was also a brief discussion on the destruction of urinogenital
papillae. This has been accomplished at Auburn University, Alabama, by hot
wire cautery, but the papillae regenerate and the fish spawn normally.
The main method discussed was the use of predators to remove unwanted
recruits. It was recognized that although tilapia recruits have different
feeding preferences to their parents, they do compete for food and oxygen
in culture ponds.
Dr. Pullin commented that Channa striata was being studied at Central
Luzon State University in the Philippines to control tilapia recruitment in an
S. niloticus (85%):Cyprinus carpio (15%) polyculture system. Very low
numbers of this predator (20 to 30 stocked as fingerlings) could obliterate
tilapia recruitment in 0.1 ha ponds containing 850 or 1,700 S. niloticus
stmked at average weight 3 g and grown for 90 days, during which first
maturity is reached and spawning occurs. If the culture period is extended to
180 days, however, the predators are unable to cope with the increased level
of recruitment and up to 200 to 300 kg of tilapia recruits/pond can be
present at harvest. Therefore, the size and fecundity of the breeding tilapias
and the length of the culture period must be considered, not just numbers of
fish. Channa striata is a very useful controlling predator and has a high value
as a by-catch. It is used in commercial culture of tilapias in Taiwan-largely
in monosex culture where it mops up any recruits resulting from inaccurate
sexing or poor quality hybrid progeny.
Dr. Coche pointed out that smaller predators than Channa should also be
considered. Although of negligible value as a by-catch, they have the advan-
tage of always selecting the smallest recruits as prey and there is, therefore, no.
danger of them eating the cultured crop as well as unwanted recruits. He also
reported work using Clarias lazera in the Cameroons. This species ceased to
be an effective controlling predator when the water became very turbid and
it was also useless in systems where high protein supplemental food was
given to the cultured tilapias: it preferred to eat the supplemental food.