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

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 colle cted from males that had been stimulated to increase semen output by treatm ent with a GnRH analogue. The semen extender consisted of a modification of a diluent that has been used to cryopreserve salmonid semen with 10% propy lene glycol added as a cryoprotectant. The semen was frozen in liquid nitro gen vapour and stored in liquid nitrogen. Freeze rates inside the straws we re measured by a thermocouple. In a preliminary fertility trial, there were no significant differences in egg fertilization rates and hatch rates betw een freshly collected semen and semen frozen in 1.7 ml straws. A larger tri al 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 f or these groups were 64.7, 59.2 and 54.4%, respectively, and the hatch rate s 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 treatmen ts. The freeze 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 C. T he 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 f or frozen-thawed yellowtail flounder semen. (C) 1999 Elsevier Science B.V. All lights reserved.