ratio (FCR) was 2.31.
In Israel the stocking densities for all-male tilapias in polyculture systems
are usually 3,000 to 5,00O/ha, when the fish are fed supplementary feed (25%
protein pellets). Table 4 presents results obtained from a well-managed
polyculture pond in Israel which included sexed all-male tilapia hybrids (S.
niloticus x S. aureus). The tilapia stocked in spring were nursed the previous
year and then overwintered. Those stocked in the second cycle in August
were young-of-the-year, which were nursed to 48 g and then sexed. The total
annual yield of tilapia was 1.36 t/ha: over 10% of the total pond yield.
The most economic stocking density is not necessarily that which results
in the highest average growth rate, but rather that which results in the highest
yield per unit area. Up to a certain density, an increase in stocking density
does not depress growth rates proportionally and the yield per unit area
increases. This effect is quite pronounced with tilapia. At densities of 3,000
to 4,00O/ha, when fed protein-rich pellets, each fish can gain 3 to 5 g/day.
The daily weight gain drops when the stocking density is increased, but at
20,00O/ha it is still 1.5 to 2 g/day, which results in a high yield. The stand-
ing crop of such densely stocked ponds can reach 25 t/ha. This is only
possible, however, when no restrictions are imposed on the length of the
culture period and the fish can be given longer period to reach market weight:
as in all-male culture.
Lovshin et al. (1977) cultured all-male S. niloticus and all-male hybrids of
S. niloticus x S. homorum at densities of 10,00O/ha. The tilapia were stocked
at 60 to 63 g and harvested at an average weight of about 240 g. The yields
after a 180day culture period were 2.8 and 3.2 t/ha for the S. niloticus and
hybrids, respectively.
Sanchez (1974, cited by Lovshin and Da Silva 1975) stocked male S.
aureus at 40,00O/ha with an average weight of 113 g. After 60 days he
harvested the fish at an average weight of 143 g. Though the yield was high
(2.1 t /ha), the growth was very low (0.5 g/fish/day). This may have been
due to the feed which contained 30% coffee pulp.
Sarig and Marek (1974) stocked male S. aureus at even higher densities:
up to 60,00O/ha. The fish were fed a pelleted diet containing 25% protein
and in spite of the high density gained 2.39 g/day. The mean feed conversion
ratio (FCR) was somewhat high (3.4) but the total yield during the 67day
culture period was 15.6 t/ha.
When the stocking rate is so high, the existing natural food resources of
the pond are divided between a great number of fish, and the role of natural
food and productivity in the overall nutrition of the fish decreases. The
advantages of polyculture are then very limited and the extra work involved
/in sorting the different species of fish at harvest becomes a burden. Mono-
culture is therefore more rational at high stocking densities. This can, how-
ever, be modified by environmental factors. In Israel filamentous algae
develop early in the growing season and become a nuisance when the fish
are harvested. In order to control this a number of common carp are intro-
duced into the ponds (stocked at up to 20% of the tilapia density). By
burrowing in the mud, the common carp prevent the development of the
filamentous algae. The monoculture thus becomes a duoculture, with the
tilapia as the dominant member.
sean pound
(Sean Pound)
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