Influence of the time to egg stripping on eyeing and hatching rates in rainbow trout Oncorhynchus mykiss under cold temperatures

The effects of egg retention time in the abdominal cavity after ovulation on eyeing and hatching rates were studied in rainbow trout, Oncorhynchus mykiss, at cold temperatures of 2 and 8 °C. Two types of experiments were performed: 1) eggs with different retention times were grouped by 3-day intervals until 30 days post-ovulation (DPO) and fertilized with mixed milt from the same sources, and 2) partial volumes of eggs were obtained from individually identified females and stripped and fertilized with mixed milt at 7-day intervals for 35 days. The first experiment showed that the eyeing and hatching rates remained greater than 70 and 60%, respectively, throughout the 30-day experimental period except in the shortest DPO group which exhibited rates of 37.6% and 31.5%. The second experiment clarified that deterioration in egg quality did not occur before 30 DPO at 2 °C and 14 DPO at 8 °C. The eyeing rate also did not show any marked reduction during the experimental period (83%) at 2 °C during the period 28–35 DPO in marked contrast with the sharp drop to 3.6% at 8 °C during the same period. The present study demonstrated that the best stripping time was estimated in the range from 30 to 40 degree-days, and over-ripening of the eggs occurred from 224 degree-days after ovulation at least under cold temperature than in studies reported so far.