The first harvest of hybrid fingerlings is 1.0 to 1.5 months after stocking
the broodstock and is continued at approximately 3 week intervals. The
hybrid fingerlings, 1 to 2 g, are harvested with a seine in the feeding areas
where the fingerlings concentrate. No mention is made of the length of time
the tilapia adults are left in the spawning pond before removal to prevent
backcrossing. The small hybrid fingerlings are transferred to nursery ponds
and stocked at 30,00O/ha. The nursery ponds also serve as growout ponds
for carp (Cyprinus carpio) adults and fingerlings.
The hybrids are grown until they reach a sexable size of 50 to 100 g.
In most cases the percentage of males is not high enough to permit direct
stocking into growout ponds so the hybrid fingerlings have to be sexed
to eliminate the females. Hybrid males must be overwintered for stocking
the following summer in growout ponds. Tal and Ziv (1978) state that the
production cost of 50 g hybrid fingerlings in Israel is very expensive: as
much as 35% of the market price of the tilapia.
Pruginin (1967) describes a system used in Uganda to produce 100%
male hybrids by crossing S. niloticus x S. hornorum. The need to regularly
drain hybrid spawning ponds was a handicap in many areas of Uganda. Also,
the handling of small fingerlings during transfer to nursery ponds caused 25
to 30% mortality. To reduce the need for regular draining and transfer of
small hybrids to nursery ponds, a pen spawning system was tested. In a 0.2
ha earthen pond, pens measuring 8.8 x 6.0 m were placed in shallow water.
The pens were made of welded steel fencing with a 2.5 to 3.5 cm mesh. Each
pen was stocked with 8 female S. niloticus and 6 male S. hornorum. The
small hybrid fry were able to pass through the mesh into the spawning pond
where they would grow to 30 g before removal. The larger hybrid fingerlings
were unable to reenter the spawning pens at maturity so the chances of
backcrossing were reduced.
Lovshin and Da Silva (1975) divided a 350-m2 spawning pond in the
shallow end with a fence to form a 100-m2 enclosure to facilitate the removal
of broodstock after the spawning period. The mesh of the fence permitted
the hybrid fingerlings to pass out of the spawning area containing the parents.
This allowed easy removal of the adults when the spawning pond was lowered.
This system worked fine in most cases, but, for each 3.5 month fingerling
production period (consisting of 2.5 months of spawning and 1 month of
additional growing time) 1 month of spawning time was lost while the fry
were growing.
Lovshin and Da Silva (1975) describe a spawning-nursery pond designed
to continually produce all-male tilapia hybrids and eliminate the need to
handle hybrid fry. Figure 3 shows a modified design that is being built
in Brazilian government hatcheries to produce tilapia hybrids. Mature
adults are stocked in the 400-m2 upper spawning pond. When the spawning
period has terminated, the dam boards dividing the spawning and nursery
units are removed and the water from the spawning pond drains by gravity
into the nursery pond carrying hybrid fingerlings and fry. The broodstock
are retained in the spawning pond by a screen. Large fingerlings descending
from the spawning to nursery pond can be collected with net graders and
stocked immediately. Small fingerlings and fry needing more growth before
handling pass into the nursery pond. The spawning pond can then be pre-
sean pound
(Sean Pound)
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