Use of cryotubes for the cryopreservation of tambaqui fish semen (Colossoma macropomum)

Tambaqui (Colossoma macropomum) is a freshwater fish of great importance to aquaculture in several South American countries. Recent studies have developed a protocol for semen cryopreservation in 0.25 and 0.5 mL straws; however, this technique has limitations for fingerling production at a large scale due to the high fecundity of tambaqui. The main objective of this study was to evaluate the feasibility of using cryotubes (1.6 and 4.5 mL) for tambaqui semen cryopreservation. Semen samples were diluted in freezing solution (5% glucose solution, 10% methylglycol, 5% egg yolk), stored in 1.6 and 4.5 mL cryotubes, frozen in liquid nitrogen vapor at −175 °C and transferred to a cryogenic container at −196 °C. The cryotubes were thawed in a water bath at 60 °C for 70 or 90 s and the motility (total motility - TM; progressive motility - PM; curvilinear velocity - VCL; straight line velocity - VSL and average path velocity - VAP) and the viability of sperm were evaluated. There was no significant difference in sperm motility and viability post-thawing between 1.6 and 4.5 mL cryotubes, except for TM (47% and 40%, respectively). Thawing for 90 s provided better results, being used in fertilization trials. Although the fertilization rate did not differ between the cryotubes (41-45%), it was significantly lower than that for fresh semen (74%). A strong positive correlation was observed between the sperm motility and fertilization rate (r = 0.69-0.89). We conclude that 1.6 and 4.5 mL cryotubes have high potential for tambaqui semen cryopreservation when thawed for a minimum time of 90 s at 60 °C.