Semen collection and preservation in African catfish, Clarias gariepinus

Stock improvement using quantitative and molecular genetics is an essential part of nowadays production of farm animals and fish. To achieve this in aquaculture, germplasm of both parental sexes should be obtained in a life-saving manner. In captivity, male African catfish, Clariasgariepinus , do not release semen under abdominal massage and have to be sacrificed to obtain sperm from the macerated testes. Of course, this is regarded as a major constrains by the catfish farming sector. Against this background, the research of the present thesis had a two-pronged approach and aimed (a) to induce semen release and facilitate stripping of semen under abdominal massage, and (b) to optimize protocols for cryopreserving semen of the African catfish. To facilitate hand-stripping of semen, several maturational hormones that increase plasma gonadotropin levels and drugs that stimulate contractions of the reproductive tract, such as oxytocin, were tested. The response to some of these treatments was compared between normal males and males that possessed undeveloped seminal vesicles - a possible block of the sperm flow during abdominal massage. Based on the results, it is unlikely that catfish males kept in captivity are not strippable because of a lack of gonadotropin surge. Fertile semen was hand-stripped from males that possessed undeveloped seminal vesicles but not from normal males, suggesting that seminal vesicles actually block the sperm flow during hand-stripping. However, stripping was possible only after treatment with pituitary extract. Oxytocin may play a role in sperm transport in catfish, but more research is needed to optimize dose and latency time. To optimize protocols for semen cryopreservation, different cryoprotectors, cooling rates and temperatures at which plunging into liquid nitrogen occurred, were evaluated. Catfish semen showed good tolerance to freezing and thawing. Hatching rates similar to the fresh semen were obtained with semen frozen in 10% methanol, at a cooling rate of -2, -5 or -10ºC/min to -40ºC and plunged into liquid nitrogen as soon as semen temperature reached -38ºC. Samples plunged into liquid nitrogen from a semen temperature above -30ºC or below -50ºC produced decreasing hatching rates. Post-thaw semen could be diluted at least 200 times without loosing fertilization capacity. Cryopreservation of semen is a valuable tool for selection and conservation of genetic diversity in catfish species.