MUHAMMET ALTUNOK1, ANDREAS MÜLLER-BELECKE2, GABRIELE HÖRSTGEN-SCHWARK2
1University of Ege, Faculty of Fisheries, Turkey
2Georg-August-Universität Göttingen, Institute of Animal Husbandry and Genetics, Germany
Cryopreservation of sperm can be a vital tool in gamete and broodstock management of cultured fish species. A stable supply of sperm for optimal utilisation in hatchery production and laboratory experiments can be ensured. Rare genetic material can be conserved and transported in an economical way and experimental materials for advanced studies can be made more accessible. Cryopreservation of sperm is used increasingly in the production of fish with wide variability of fertility rates obtained with frozen-thawed sperm depending on the species and procedures employed. Cryo-straws were most frequently used to package sperm samples, however many researchers pelleted sperm samples on dry ice. In this study different procedures were optimised for Nile Tilapia (Oreochromis niloticus) sperm cryopreservation with focus on the comparison of the two packaging methods, pellets and 0.25ml-cryo-straws. Before each optimisation experiment fresh sperm was collected by artificial stripping of a random sample of five males (Lake Manzala strain, Egypt). Different diluents, cryoprotectants, freezing and thawing procedures were tested. Successful cryopreservation and thawing was verified by microscopic examination of sperm motility and by comparison of fertilisation- and hatching rates of artificially stripped and inseminated tilapia eggs. Promising results were obtained with the following procedures: sperm was diluted 1:5 with the extender containing 15% milk (3.5% fat, homogenised) and 5% methanol in 0.6 M sucrose solution. Freezing in straws was realised by floating them 5cm above of the surface of liquid nitrogen before submerging them into the liquid nitrogen. Packaging with pellets was practised on dry ice before depositing the pellets in liquid nitrogen. Pellets and straws were thawed in 0.1 M NaHCO3 before microscopic inspection and fertilisation experiments were conducted in comparison to fresh sperm. In replicated trials, the mean hatching rates after fertilisation with straw-frozen sperm corresponded to 85% and with pellet"=frozen sperm to 59%, if compared with the fresh sperm fertilisations (100%). Packaging by pellets represented the more time efficient method, whereas packaging by straws eased the handling of small amounts of sperm and allowed tagging and recognition of individual samples.
Keywords: Aquaculture, cryo-straw, cryopreservation, Oreochromis niloticus, pellet, sperm