The properties of tilapia sperm and its cryopreservation*

The specific differences between the testis, milt and sperm of six species of tilapia including Oreochromis aureus, O. mossambicus, O. niloticus, Tilapia zillii, O. nilolicus×O. aureus hybrid and red tilapia, Oreochromis sp., were studied. The shape of testis is tubular; the gonadosomatic index varied from 0.07 to 2.71. The pH values of individual milts ranged from 6.2 to 8.2 and the osmolarity from 240 to 380 mOsmol kg−1. The quantity of milt obtained by stripping averaged only about 0.3 ml, and only in the O. niloticus×O. aureus hybrid did it exceed 3 ml. Sperm motility graded from weak to moderate was determined for the stripped tilapia milt. Sperm concentrations ranged from 7.70 × 108 sperms ml−1 in T. zillii to 2.74 × 1010 sperms ml−1 in O. mossambicus. Tilapia sperm was active in various salinity ranges such as 0–5‰ for O. niloticus, and 0–15‰ for O. mossambicus and T. zillii. Extender containing 15% milk and 5% methanol was used to prepare milt mixture before cooling rapidly to −35° C and then at 5° C min−1 to −75° C for storage in liquid nitrogen (– 196° C).Fertility tests on frozen tilapia milt resulted in a fertilization rate of 72.7% (v. control 85.7%) for the 22‐day frozen milt of the O. nilolicus×O. aureus hybrid used to fertilize the eggs of O. honorum, and 93.4% (v. control 90%) for the 304‐day frozen sperm of red tilapia used to fertilize eggs of red tilapia.