ing pure genetic strains that give a high percentage of male hybrids takes
great care and dedication on the part of hatchery workers. Large earthen
ponds are not ideal units for holding pure genetic lines. Small, controllable
earthen ponds, fiberglass, or concrete tanks are preferred.
A small number of pure broodstock couples can produce enough finger-
lings to initiate even a large earthen pond tilapia hybrid fingerling production
operation. If or when the stocks become contaminated, they can be elimi-
nated and pure genetic lines reintroduced. Facilities for producing hybrids
should be laid out so that ponds containing pure tilapia broodstock are
isolated from each other as well as from hybrid spawning ponds. Hybrid
nursery ponds should also be isolated from hybrid spawning ponds. In
essence, at least three groups of fish are involved in any hybrid hatchery
operation: the two parental stocks and the progeny. Care should be taken to
eliminate any contact between groups except in the spawning ponds.
It is my opinion that the primary obstacle to producing commercial
numbers of all-male tilapia hybrids is the low number of progeny produced
per spawning. In Brazil, an average of 2,763 all-male hybrids were produced
in 2.5 to 3.0 months when 50 female S. niloticus were stocked with 50 male
S. hornorurn in a 350-m2 pond. It seems reasonable to assume that the
average number of eggs produced per female was about 400. Thus, if survival
of eggs to fingerlings was loo%, about 7 female S. niloticus spawned i.e. 14%
of the females stocked. Even if the survival of eggs to fingerlings was not
loo%, it seems unlikely that more than 20% of the females spawned. This,
moreover, assumes that a female spawned only once per spawning period. If
some of the females were able to spawn more than once then the percentage
of spawning females would have been even less.
It appears at first glance that the number of hybrid fingerlings produced is
small because the females are dying during some stage of development:
hence the all-male survival. However, Hickling (1960) and Lee (1979) proved
this to be untrue; females that spawn produce normal numbers of hybrid
offspring. Lovshin (unpublished data) demonstrates that there is a relation
between the number of females stocked in a 350-m2 spawning pond and the
number of hybrid fingerlings produced per female (Table 4). These data were
collected over a 6-year period and do not consider the size of the female S.
niloticus stocked, the variation in ratio of S. hornorurn to S. niloticus
stocked, or length of the spawning period (which varied from 7 1 to 11 1 days).
The fewer the female S. niloticus per unit area of spawning pond the greater
the number of hybrid fingerlings produced, calculated on a per female
basis. Thus, it appears that the percentage of females that actually spawn
increases with a decrease in the stocking density of females. It is not known
why this happens. Increasing the stocking density of females increased the
number of hybrids produced up to a value of 1 917 m2, but further increases
up to 1914.7 m2 and 1913.5 m2 failed to give hybrid increases. There
appears to be some behavioral or chemical factor present that reduces the
compatibility of female S. niloticus and male S. hornorurn so that at high
densities of female and/or male broodstock the percentage of females that
will spawn is greatly reduced. The role of the male S. hornorurn and the
impact that surplus males or S. niloticus females may have on male territorial
and sexual behavior must be considered. The densities of males given in
sean pound
(Sean Pound)
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