Pure genetic strains that are known to give 100% male hybrid offspring
should be carefully isolated from other tilapia species and hybrids so that
contamination does not occur. This point cannot be overstressed, as a major
drawback to producing tilapia hybrids is the failure to preserve pure genetic
lines over long periods of time. If possible, pure strains should be held in
tanks or small ponds isolated from the rest of the hatchery ponds and
supplied with water free of natural tilapia populations. Well water would be
the ideal water source but surface water can be used if care is taken. In
Pentecoste, Brazil, pure stocks of S. niloticus and S. hornorum have been
held for 7 years in 36-m2 concrete tanks with earthen bottoms supplied with
irrigation water. Water entering the tanks passes through several screen
filters and a gravel filter before entering the tanks. The tanks-are covered
with nets to eliminate the entrance of predatory birds and animals. Thus, if
at any time contamination of broodstocks is observed, the broodstock
production ponds can be drained and restocked with pure strains taken from
the concrete tanks.
Pure-line S. niloticus and S. hornorum fingerlings or adults are stocked
into individual earthen spawning ponds to produce the number of brood-
stock needed to obtain commercial numbers of hybrid fingerlings. When
pure S. niloticus and S. hornorum fingerlings reach 20 to 30 g and are still
immature, they are sexed and the female S. niloticus and male S. hornorum
are stocked into broodstock preparation ponds isolated from the opposite
sex. If pond facilities are limited, fish can be stocked into cages to mature.
Immature female S. niloticus and male S. hornorum should reach sexual
maturity in 2 to 3 months: 60 to 100 g when stocked at 2 to 3/m2 and fed
5% of their body weight daily. These fish should not be overstocked and
should be well cared for so that numerous, healthy gametes are produced.
Mature male S. hornorum and female S. niloticus with swollen genital
papillae are placed in the hybridization pond at the ratio of one male to one
female, stocking one female17 m2 of pond surface area. The water depth of
the spawning ponds should be less than 1 m. The broodstock are fed agri-
cultural by-products (bran and oilseed cake) at 5% of their body weight daily
during the hybridization period. After 2.5 months, the spawning ponds are
drained and the hybrid fingerlings transferred to a nursery pond where they
are stocked at up to 10/m2 and fed agricultural by-products to attain further
growth. Lovshin and Da Silva (1975) recommended that spawning ponds be
drained after 3 months to avoid backcrossing with the female S. niloticus,
but now a 2.5 month spawning period is recommended as mature hybrids
have been found before 3 months. When transferring hybrid fingerlings to
nursery ponds, a number (25 to 50) of large, sexable fingerlings should be
checked to determine whether all-male hybrids have been produced.
Upon draining a hybrid spawning pond female hybrids are sometimes
found or an extremely large number of small fry, 1-2 cm in length, that are
too small to sex at harvest but contain a high percentage of females when
sexed at a later date. The presence of female hybrids is almost always the
result of backcrossing between a mature all-male hybrid and female S.
niloticus: human error. After a 2.5 month spawning period, the pond has to
be completely dried to eliminate any small hybrids that may remain. If the
spawning pond cannot be dried, the pond should be carefully poisoned.
sean pound
(Sean Pound)
#1