Potentials of short term and long term cryopreserved sperm of the African giant catfish (Clarias gariepinus Burchell, 1822) for aquaculture
To service the growing demand for male African giant catfish (Clarias gariepinus) broodstock for aquaculture in Nigeria, and to conserve valuable genetic resources, we improved both short-term (in deep freezer at -35°C) and long-term cryopreservation (in liquid nitrogen at -296°C) of catfish sperm. Catfish sperm cryopreserved at -35°C using three different types of cryodiluents composed of 10% methanol, glycerol or dimethylsulphoxide (DMSO) with 15% non-permeating cryoprotectant (skim milk) and 75% phosphate-buffered saline (PBS) as extender was evaluated at day 1 to 28. Sperm motilities on thawing for the two cryodiluents that contained glycerol and DMSO were the same (p > 0.05) while those for the cryodiluent that contained methanol were significantly lower (p < 0.05). Sperm cryopreserved in DMSO and glycerol provided hatching at least 50% of catfish eggs. Liquid nitrogen (LN2) was with the same 3 cryoprotectants and one new extender based on glucose and skim milk (Ginzburg fish Ringer, GFR). Two thawing procedures were tried: 1) In the first trial, one set of sperm was frozen with different cryodiluents with extenders in the freezer at -10°C and 2) Stored for up to 8 months in LN2. Sperm with up to eight months cryopreservation from the first trial when thawed at room temperature at 27°C for 15 min gave 0 - 6.25% fertilization rate (FR). The sperm stored for two months in the second trial when rapidly thawed at 35°C for 5 min gave better results. The FR of 94% was achieved with a cryodiluent of DMSO with 5% glucose solution and PBS when the cryopreserved sperm was thawed at 35°C. Also, DMSO in combination with PBS resulted in a higher FR than either glycerol or methanol. However, sperm cryopreserved with glycerol had the lowest motility rate in the 2 trials (p < 0.05). The use of cryopreserved sperm did not significantly affect (p < 0.05) the survivability of the fry. The use of cryopreserved sperm of the African catfish for fry production will be especially beneficial in Nigeria where reliable sperm supply is a major bottleneck in the catfish aquaculture industry.
Key words: Cryopreservation, sperm motility, Clarias gariepinus, African giant catfish.