Determination of the best post-ovulatory stripping time in female rainbow trout Oncorhynchus mykiss

Considering few oocytes in female rainbow trout, Oncorhynchus mykiss, factors affecting egg quality are more important. Since post-ovulatory stripping time has been reported the most important factor affecting egg quality, the present study was performed to understand the best stripping time in female rainbow trout. Egg quality measurement was done through assessing the eyeing and hatching rates of eggs. One hundred females expected to ovulate in the near future were selected from the brood stock and stored at 2 ºc. Then serial judgments of ovulation were performed at 3-day intervals for 30 days. Ten groups of fish, each comprising of three females ovulating simultaneously, were held in separate tanks until fertilization. Other ovulated females were removed from the brood stock. All the females were fertilized at the end of the 30th day. So there were 10 groups presenting 10 different retention times until artificial fertilization. The eyeing and hatching rates for eggs fertilized immediately after ovulation (0-3 DPO) were 37.6 % and 31.5% (Mean ± SEM), respectively, which were significantly lower than those for eggs fertilized thereafter. Apart from these initially low values, the average eyeing and hatching rates were 85.15 % and 68.55 % during the post-ovulation period of 30 days.  Significant relationships were observed between the retention time and both the eyeing and hatching rates. The present study therefore demonstrates that immediately stripping after ovulation leads to lower egg survival and so, it seems necessary to retain oocytes in the abdominal cavity for a short period time after ovulation. Also, the best post-ovulatory stripping time was assessed at least 60 degree-days in female rainbow trout.