Le contrôle du sexe chez les tilapias : bilan et perspectives de 10 années de recherche à la Station aquacole de Tihange

Due to their rapid growth, natural breeding in ponds and resistance, the tilapias belonging to the Oreochormis sp. offer high potentialities for extensive and intensive culture. Sexual growth dimorphism in favor of males oriented most applied research activities towards the production of monosex male populations. Since the early 80's, the Laboratory of Fish Demography and Aquaculture applied or adapted for this purpose some conventional techniques (manual sorting and hormonal sex reversal). Adding 17 a-metyltestosterone to the fry's feed during 20-40 days gives 89-100 % male offspring in tank (20-60 mg/kg feed) and 83-97 % male offspring in breeding ponds (60-150 mg/kg feed). It was experimentally demonstrated that the natural hormone 11 ß-hydroxy androstenedione gives similar results - at equivalent doses- on sex reversal as with the artificial hormone. However the high cost of the natural hormone prevents its use in large scale sex reversal programmes. We thus developed new techniques based on hybridization : i.e. crossing male O. aureus with female O. niloticus gives 92,5 % male offspring but the overall fecundity is lower than with female O.aureus (- 34 %). Since O. aureus exhibits a male homozigocy (male with ZZ genotype), an elegant alternative is provided by the use of feed added ethynylestradiol to produce a theoretical 100% female phenotype offspring grouping true females and transsexual fish (pseudofemale with ZZ genotype). Crossing a pseudofemale with a ZZ male gives a theoretical 100 % ZZ genotype male offspring that have never faced artificial steroids. However it appears that sex determination in 0. aureus relies on a polygenic process and the male ratio obtained from this technique ranges between 86 and 91 %. The development of these news techniques that minimise the use of hormones - and subsequent risks for consumers - could be further enhanced by the mastering of environmental influences on phenotypic sex determination (e.g. thermal effects), in order to produce "clean" (hormone free) 100% male, fast growing tilapias.