Some species, such as Tilapia zillii or Sarotherodon mossambicus, breed
when they are young and small and cannot reach market-size before matura-
tion. Monosex populations have been obtained, mainly by manual sexing,
but this can only be done reliably when the fish have reached a size of 20 to
50 g, so they must first be nursed to at least this size. In many cases these
species will breed at an even smaller size. Pruginin and Arad (1977) report
that in Malawi, S. mossambicus bred and growth stopped due to stunting
when the fish reached 30 g. As a consequence the yield after 100 to 150 days
culture did not exceed 300 kglha. It may be advisable, therefore, to choose
species which spawn at an older age, such as S. niloticus, S. aureus or others.
Pruginin (1965) found that while S, hornorum in Uganda reached not more
than 150 g in about one year, S. niloticus usually reached 250 g in 5 to 6
months by the time they reached sexual maturation.
Culture techniques, such as pre-nursing, choice of size at stocking, stock-
ing density, fertilization, feeding, etc., are affected to a large extent by
the choice of management approach: mixed sex or all male. However,
other factors such as climate and the type of culture system (monoculture
or polyculture) have also to be considered.Young-of-the-Year CultureFor unsexed young-of-the-year populations, the period before they reach
sexual maturity is short: about 3 to 6 months. Thus the management method
is based on having 2 or 3 cycleslyr, with complete drainage between cycles.
In order to ensure rapid growth to the desired market weight during this
short period, the stocking density is usually relatively low and depends on
the inputs used, e.g., fertilization, manuring and feeding.
Van der Lingen (1959a) has found that with increasing levels of manage-
ment, the maximum standing crop (carrying capacity) which can be sustained
in the pond increases. He gives the following carrying capacities for S.
mossambicus: natural feeding (no fertilization), 840 kg/ha; high level fertil-
ization, 2,466 kg/ha and high level supplementary feeding with fertilization,
'6,165 kg/ha. Van der Lingen (1959a) further states that over the same
culture period and under the same conditions the yield per unit area at any
level of nutrition is dependent upon the initial stocking biomass per unit area
and not on the number of fish per unit area. He therefore recommends
stocking by weight according to the expected carrying capacity. However,
this statement is based on only a few experimental observations. Since the
relative growth rate (i.e., relative to unit body weight) of small fish is higher
than that of larger fish (Ghosh 1974; Hepher 1978), it is usually found in
practice that for a given biomass of fish, the higher the density the higher the
yield, provided enough food is available. Moreover, the practice of stocking
by weight may lead to stunting of the population if the average weight of the
stocked fry is low, since the number of fry per unit of stocked weight will be
large and they will reach the carrying capacity while still small. Van der
Lingen (1959a, 195913) stocked fingerlings of 21 to 56 g average weight. His
recommended stocking weights and densities relative to the carrying capa-
cities for different inputs are given in Table 1. The number of fish stocked