May-beginning of June) can reach a final weight of 100 g and over by autumn.
However, those hatched later and stocked in the nursing ponds in July-August
will only reach aweight of 40 to 60 g. The minimum weight for overwintering
fingerlings is about 20 g. With smaller fingerlings the survival rate during
winter is very low. The stocking density preceding overwintering is adjusted
according to these expected final weights. When fingerlings are expected to
reach a final weight of 100 g, the stocking density is about 50,00O/ha. In
order to better utilize the natural productivity of the pond, fry are often
stocked at a density of 100,00O/ha for the first part of the nursing period
until they become 50 g fingerlings. They are then thinned out to 50,00O/ha.
Fry of later spawning, which can reach a final weight of 50 g before over-
wintering are stocked at a density of 100,00O/ha.
One of the most important considerations in determining the desirable
final weight of the nursed fingerlings in regions with cold winters is the
available overwintering capacity. If special facilities and large investments are
required for retaining a higher than ambient water temperature in the
overwintering ponds (e.g., covering of the pond and/or warming the water),
then this usually means a restricted overwintering pond area and, therefore, a
limitation of the standing crop of fish that can be held over the winter. A
certain number of fingerlings are required for stocking the culture ponds in
spring and the maximum average weight of these can be obtained by dividing
the standing crop which can be overwintered by the required number of
fingerlings. Smaller fingerlings can be obtained by increasing the fry stocking
density in nursing ponds relative to the desired final fingerling weight. These
fingerlings can be nursed again in spring if necessary.
Nursing can also be done in polyculture ponds while rearing other fish
(sometimes including large male tilapia) to market weight. In such cases the
density is sometimes lower than for nursing ponds. Table 3 gives an example
of such nursing on an Israeli farm where tilapia were introduced in late
summer (August 9).
The culture of an all-male population removes the restrictions on final age
and thus on weight at harvest associated with mixed sex culture. The fish can
therefore be cultured to a much larger final weight, usually 400 to 600 g,
even though this takes longer. Growout can be done either in polyculture or
in monoculture systems. The advantages of polyculture have already been
discussed above. The stocking density used in polyculture is not much
different to that for young-of-the-year culture, but since males grow better
and the growing period is longer, they usually attain a much larger final
weight.
Here again, the density depends on the levels of inputs (fertilization,
manuring and feeding). Pruginin (1967) stocked all-male hybrids (S. niloticus
x S. hornorurn) of 50 g average weight at densities of 1,000 to 1,5001ha. The
daily weight gain was 1.5 to 3.0 g/fish, and the average weight at harvest was
200 to 450 g after a culture period of only 100 to 150 days. No information
was given on fertilization or feeding rates. She11 (1968) conducted an exper-
iment to study the stocking of male S. niloticus using supplementary feeding
(pelleted Auburn No. 2 fish feed, rich in protein). The growth rate was
independent of fish density up to 5,000 fish/ha at 2.1 to 2.3 g/fish/d. Rearing
at this density gave the highest yield of 1.6 t/ha/163 d. The feed conversion
sean pound
(Sean Pound)
#1