develop towards the end of the culture period. This problem can, however,
be solved much more effectively by including a predator in the polyculture,
such as the Nile perch (Lutes niloticus) in Africa (Pruginin 1965, 1967;
Meschkat 1967), the mud fish (Channa striata) in the Philippines (E.M.
Cruz, pen. comm.) and Thailand (Chimits 1957), Cichlasorna rnanaguense
in Central and South America (Dunseth and Bayne 1978); and the sea
bass (Dicentrarchus labrax or D. punctatus) in Israel (Chervinski 1974,
1975). The last two species are marine fish which can adapt to freshwater.
The predators are in most cases stocked at about 10% of the tilapia popula-
tion. The use of predators in tilapia ponds, despite its promise, has not
received wide application and has been practiced mainly on an experimental
scale, partly because of the difficulty in obtaining predator fry. This is true
for both the Nile perch and the sea bass. Their inability to breed spon-
taneously in the pond can be considered an advantage but a source of fry
must be found for commercial application.
In the subtropics the winter is usually too cold for growth of tilapia
and the growing season is restricted to the summer. Balarin and Hatton
(1979) quote Bishai (1965) who gives the range of 17.2 to 19.6"C, below
which the growth rate of most tilapias decreases. For spawning to take place,
temperatures must also be higher than this. The lowest temperature for
spawning is 20 to 23°C (Huet 1970; Uchida and King 1962). This restricts
the length of the season for culture of young-of-the-year even more. In
Israel spawning starts only in May, and fry of approximately 1 g for stocking
growout ponds are not available before June. This limits the season for
rearing young-of-the-year from June to October so that only one cycle can
be carried out. The fry are transferred from the spawning pond into the
growout ponds when they are over 1 g. The culture period is sufficient to
bring them to a market size of 200 to 300 g.
Since tilapias grow well above lB°C, the early part of the summer (March
to June) can also be utilized for culture but the only fry then available are
those hatched the previous year. If a mixed sex population is cultured they
will spawn in the ponds when the temperature reaches 20 to 21°C in May.
Since this occurs shortly before harvesting, the fry thus produced do not
reach a size which can cause much harm in the pond and the stocked tilapia
reach market size unaffected. Halevy (1979) reported that unsexed fry
hatched in the previous year could be cultured during the first cycle and
young-of-the-year during the second. A typical example of one growing
season during 1977 is given in Table 2. In this case it can be seen that the
cool spring weather is an advantage and high yields of market-sized tilapia
can be obtained.
It is obvious that fish size, the time of stocking and especially the time of
harvesting may be crucial to the success of culturing the previous year's fry
in spring. The later the stocking and the smaller the fry, the smaller the fish
at harvest time and the later the harvesting, and the more troublesome is the
"wild" spawning. This is a considerable drawback in large farms where, due
to market demand, the stocking and harvesting times are usually phased so
that some ponds are stocked in late spring and harvested in mid-summer.
These cannot, therefore, be used for early spring stocking and growout.
sean pound
(Sean Pound)
#1