The Effect of Freezing Rate on the Survival of Cryopreserved African Sharptooth Catfish (Clarias gariepinus) Spermatozoa

Cryoprotective agents were evaluated to find the optimal concentration of the cryoprotectant and most suitable combination of solution and cryoprotectant. A cryoprotective agent composed of 4% glucose and 9% glycerol yielded the best results. It was established that the optimal freezing rate is dependent on the composition of the cryoprotective agent. Maximal survival of catfish spermatozoa (60%) occurs at 5°C min^-1 and faster and slower freezing rates result in poor survival or no survival at all. Incorporation of an isothermal holding period into the freezing rate led to remarkable increase (20-30%) in sperm survival when Me₂SO was present in the cryoprotective agent. Cryoprotective agents containing glucose also showed improved survival when a three phase freezing rate was used. These results lead to the conclusion that the presence of an isothermal holding period in the freezing rate is beneficial for the cryoprotective action of Me₂SO and glucose.