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Cryogenic gene banking is a prime necessity for conserving the genetic constitutes of vulnerable, endangered and critically endangered native and exotic fish species. Cryogenic freezing of fish gametes is the first step to develop such a gene bank. In the study, attempts have been taken to develop a cryogenic freezing protocol of spermatozoa of Mohashol, Tor tor as it is considered as a critically endangered fish species by IUCN. Juvenile and brood fish of Tor tor were collected from the Someshwari River of Netrokona district and domesticated in captive condition at Bangladesh Agricultural University campus. During breeding season, sperms were collected from hormone induced males by stripping. The milt was very sticky. The concentration of sperm was 4.47x10 10 cells per ml and pH was 8.6±0.5. To evaluate the toxicity of cryoprotectant, sperm was incubated with DMSO and methanol at 5, 10 and 15% concentrations for a period of 5 to 55 min. The motility of sperm decreased with the increase of cryoprotectant concentration (5, 10 and 15%) and 10% DMSO with Alsever's solution could be the best combination for cryopreservation of T. tor. sperm at 10 min (83.0±2.74).

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