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

Semen from yellowtail flounder 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 was a modification of a diluent that had previously 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. Freezing rates inside the straws were measured using a thermocouple. In a preliminary fertility trial, there were no significant differences in egg fertilization rates and hatching 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; the hatching 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 freezing rate to -15 degrees C was slower for 1.7 ml straws than for 0.25 ml straws but was slightly faster from -15 degrees to -45 degrees ..