Principles of Genetics and Breeding

5.1.1.3 Producing monosex fishes

Hybrid sunfishes(Centrachidae) and several combinations of Tilapia hybrids produced monosex populations. The major problem in increasing the production of market size tilapias is their uncontrolled reproduction resulting from their early sexual maturity. Uncontrolled reproduction generally leads to overcrowding, reduction in growth, and a high proportion of unmarketable fish. The most simple, effective, and therefore widely used method to prevent uncontrolled reproduction is monosex culture. Hybridization is one of the most promising techniques to produce monosex tilapia cultures. Some methods developed from the numerous hybridization experiments resulted in a 100% male hybrid population. The production of monosex population in different species of Tilapia was due to their sex determination mechanism.

• Hickling (1960) obtained a 100% fertile male population when he crossed T. hornorum males (ZZ) with T. mossambica females (XX).
• Pruginin (1969) obtained similar results by crossing T. nilotica and T. hornorum.
• Of the many interspecific and intraspecific tilapia crosses which have been attempted at least three have produced 100% male offspring.
• These are: T. macrochir x T. nilotica (a difficult cross to produce). T. mossambica (zanzibar stock) x T. nilotica (lake Albert stock). T. hornorum x T. mossambica.
• Better than 98% males were also obtained from the intraspecific mating. T. mossambica (African stock) x T. mossambica (Malaysian stock).
• Hybridization between nile tilapia and blue tilapia (O. aureus) result in the production of predominantly male offspring.
• Attempts to produce monosex stocks by tilapia have generally concentrated on obtaining males since, unlike females, males continue grow during breeding periods.
• However, males have one disadvantage; whether or not there are females present, they optimistically construct spawning nests. No crosses which consistently produce high percentages of females have been reported.
• In many interspecific crosses, however, significantly skewed sex ratios are observed. Gross morphology of hybrids may be intermediate when expressed as a general hybrid index, but a sizable number of morphological characters strongly resemble only one parent.
• All male progeny are obtained from hybridization among Australian freshwater crayfishes and the technology can be implemented by crayfish culturists who currently use manual sexing to establish all-male population.