Cryopreservation of yellowtail flounder ( Pleuronectes ferrugineus) semen in large straws

Abstract

Semen from yellowtail flounder ( Pleuronectes ferrugineus) was cryopreserved in 0.25 ml and flat 1.7 ml straws. Semen frozen in 1.7 ml straws was collected from males that had been stimulated to increase semen output by treatment with a GnRH analogue. The semen extender consisted of a modification of a diluent that has been used to cryopreserve salmonid semen with 10% propylene glycol added as a cryoprotectant. The semen was frozen in liquid nitrogen vapour and stored in liquid nitrogen. Freeze rates inside the straws were measured by a thermocouple. In a preliminary fertility trial, there were no significant differences in egg fertilization rates and hatch rates between freshly collected semen and semen frozen in 1.7 ml straws. A larger trial compared the fertility of freshly collected semen, semen frozen in 0.25 ml straws and semen frozen in 1.7 ml straws. The mean fertilization rates for these groups were 64.7, 59.2 and 54.4%, respectively, and the hatch rates were 36.7, 52.5, and 42.8%, respectively. The mean fertilization rate for semen frozen in 1.7 ml straws was significantly lower ( P<0.05) than that of fresh semen. There were no other significant differences among treatments. The freeze rate to −15°C was slower for 1.7 ml straws than for 0.25 ml straws but was slightly faster from −15° to −45°C. The freeze rate may have affected the fertilizing ability of semen frozen in 1.7 ml straws or differences in batches of semen could have been the main influence. Overall, fertilization rates and hatch rates were satisfactory for frozen–thawed yellowtail flounder semen.