There is another serious drawback to mixed sex spring culture which
is economic in nature. The cost of overwintering tilapia fingerlings is high
and the rate of growth of unsexed fingerlings in spring is lower than that of
all-male fingerlings. This is because males grow faster and because large fish
grow better than small ones in culture ponds. At much the same cost,
therefore, one can raise overwintered all-male fingerlings which have been
nursed to a larger average weight. This is the reason why most tilapia culture
in Israel uses all-male populations.
men reeng young-of-the-year in warmer tropical climates, it should
be remembered-that 'their age is a most important factor affecting sexual
maturity. Stunted fry cannot beIcuitured because they will breed early
h the p-onds, It is important therefore to use recently hatched fry. It is also
important to completely drain the rearingpond between cycles and eliminate
all the remaining fish, if necessary by poisoning. The intrusion of fish from
outside the pond should be prevented by screening the water inlets.
All-Male CultureA monosex male population can be obtained in three ways: manual
sexing; crossing two species of Sarotherodon to produce all-male or a high
percentage (90% and over) of male hybrids and sex-reversal at an early age
by incorporating hormones in the feed.
While much experimental work is being carried out on hybridization
and sex-reversal and these methods appear promising for the production
of all-male populations, it is only very recently that commercial use has been
made of monosex hybrids and no commercial application has yet been made
of sex-reversal. The main method used today to achieve an all-male population
is still manual sexing. This is a relatively simple procedure. In many tilapia
species the sexes can be distinguished by the genital papilla which has one
orifice in the male as compared to two orifices in the female. The female
often also has a smaller genital papilla.
It is important to sex the fish carefully. The less errors in the sexing, the
less "wild" spawning occurs. The earlier the sexing is done the better since
the females are then discarded. Early sexing thus saves space which can be
used for rearing of the males and minimizes feed wastage on unwanted
females. There is, however, a certain minimum size of fish for sexing with an
acceptable degree of confidence. In field conditions the optimum size for
sexing in most tilapia species is 50 to 70 g. This means that the fry have to
be nursed to at least this size before growout. The hybrid crosses that
produce high percentages of males are advantageous even if they do not
produce 100% all-male population. Using such hybrids, fewer females are
discarded, space is saved and sexing can be done on large fingerlings with a
greater degree of confidence.
Here again, climate is an important consideration. Where a cold win-
ter exists, nursing is done in the summer and growout is usually postponed
to the following year. The final weight of the nursed fingerlings will depend
to a large extent on the length of the nursing period. Those which hatched
early and were stocked in the nursing ponds early in the season (end of