stocked. Immature fish were used and allowed to mature in the aquaria to
form spawning families, which greatly reduced male aggression on the
females. Spawning either took place naturally in the aquaria or the eggs were
stripped from ripe females and fertilized with milt taken from the male. The
eggs were incubated artificially and the young fry were raised in aquaria,
concrete tanks, cages or ponds. Maintaining the water temperature between
25 and 29°C and daily artificial illumination of 12 hours permitted year-
round spawning.
However, some hybrid crosses are very difficult to perform in aquaria
because of male aggression, resulting in the death of the female. Lee (1979)
eliminated female mortality in 100-1 aquaria, stocked with 1 male and 3
females, by surgically removing the premaxilla of the male fish. The male
fish continued his aggressive behavior but he was unable to damage the
female without his premaxilla. This technique increased the number of
successful aquarium spawnings. Lee also increased the frequency of spawning
in female S. niloticus and S. aureus without decreasing egg production by
removing the eggs from the mouths of the females and incubating them
artificially. Spawning intervals of females from which eggs where removed
were every 13 to 18 days as opposed to every 30 to 60 days for females that
mouthbrooded eggs and young.
Lee produced tilapia hybrids by stocking 3 males and 9 females in 3-m
diameter plastic pools of 3,500-1 with or without a bottom substrate. Hulata
et al. (1980) produced a number of tilapia hybrid crosses in 700-1 plastic
tanks stocked with 1 male and 5 to 10 females using the system described by
Rothbard and Pruginin (1975). The S. niloticus x S. hornorurn cross could
not be successfully completed in these plastic tanks. However, when 3 males
and 10 females were stocked in 4 m2 concrete tanks, all-male offspring were
obtained. I have also been unable to produce the S. niloticus x S. hornorurn
hybrid cross in 80-1 aquaria or 1000-1 cement-asbestos tanks. However, Lee
(1979) produced the S. niloticus x S. hornorurn hybrid in 100-1 aquaria
stocking one male and three females.
The use of artificial systems to produce tilapia hybrids permits control
of the environment allowing year-round spawning in temperate climates.
Taking the eggs from an incubating female increases the spawning frequency
and thus, the number of hybrids that can be produced in a given period of
time. Survival of eggs to fry is higher because of controlled conditions
and better care. Finally, genetic control is much greater because of the
reduced chance of contaminating pure genetic lines. However, some hybrid
crosses are difficult to carry out in small, confined environments. There
is no conclusive proof that an aquaria or tank-based hatchery will be able to
produce commercial numbers of tilapia hybrids economically.
Tilapia hybrids have traditionally been produced in earthen ponds where
the environment cannot be controlled and genetic lines are more difficult
to maintain. Lovshin et al. (1977) and Lovshin and Da Silva (1975) describe