Internal and external factors controlling reproduction in the African catfish, Clarias gariepinus

The African catfish, Clarias gariepinus, is a highly appreciated species for aquaculture, because of its favourable food conversion, its resistance to diseases, its relatively low requirements for water quality, the possibility for high stocking density and the excellent meat quality. For those reasons even in the Netherlands, there is a modest, but Europe's largest and still expanding, African catfish farming activity. Although this species grows and matures in captivity, there is no spontaneous reproduction. We could demonstrate that the failure to reproduce resides in the brain-pituiary-gonad axis. Hormones required for oogenesis and spermatogenesis are being produced in sufficient quantities. However, final oocyte maturation, ovulation, spermiation and spawning behaviour do not occur, due to the lack of a gonadotropin surge. In nature, the prespawning gonadotropin surge is induced by environmental factors such as the water level in the spawning area. Under farming conditions the environmental cues are hard to identify and/or to mimic. In combination with unavoidable stress this causes a blockade of the release of gonadotropin releasing hormone (GnRH). Consequently, gonadotropin surge release fails to occur, which is enforced by an effective hypothalamic dopaminergic inhibition. The gonadrotropin surge induces the conversion of 17 alpha OH-progesterone into 17 alpha hydroxy-20 beta-dihydroprogesterone, the final maturation inducing substance. Based on these data, several protocols for artificial propagation could be developed. They include either a treatment with a GnRH analogue in combination with a dopamine receptor antagonist, a treatment with homologous gonadotropin or HCG, or a treatment with 17 alpha OH-progesterone. Since a number of years we have used the African catfish as a model for fundamental research on fish reproductive endocrinology. Till now one gonadotropic hormone (GTH) could be demonstrated. Its amino acid composition and sequence was analysed and appeared to be homologous with known forms of the maturational GTH (GTH-II). Specific radioimmuno assays for the complete hormone and its alpha- and beta-subunit respectively, have been developed. cDNAS encoding the subunits have been cloned. They are applied now for Northern blotting and in situ hybridization. GnRHs were fully characterised (a specific catfish-GnRH and chicken-GnRH-II). Specific antibodies against these peptides were raised and the cDNAS encoding the hormone precursor molecules were cloned and used for respectively immunocytochemical localisation and radioimmunoassays, and in situ hybridisation. The importance of the two GnRH forms for gonadotropin release was studied. Chicken-GnRH-II appears to be 10 to 100 times more potent than catfish GnRH, probably due to its higher receptor affinity. Catfish GnRH, however, is present in the brain and pituitary about 100 times more than chicken GnRH-II. Steroid hormone synthesis by ovaries, testis and seminal vesicles was analysed. The sex steroids that play a role in the negative feedback control of gonadotropin release were identified (11 -keto-testosterone and testosterone) and their interaction with hypothalamic dopamine metabolism was demonstrated as one of the possible mechanisms of action. Several steroid conjugates from the seminal vesicles were shown to have pheromonal activities, involved in reproductive behaviour. They induce under certain physiological conditions attraction between conspecifics and synchronization of ovulation.