Effect of post - ovulatory oocyte retention in abdominal cavity on egg quality and its histological changes in Caspian brown trout (Salmo trutta caspius)

Determination of the accurate time of egg stripping after ovulation has been regarded as one of the most important factors in the successful reproduction of Caspian brown trout(Salmo trutta caspius). To determine the best time for egg stripping after ovulation and over- ripening of the oocyte in the Caspian brown trout, the eggs of fish were retained in the coelomic cavity of females at 7 ± 6°C over 40 days of post- ovulation(DPO). Eggs were stripped at 10 day intervals in 4 stages and then were fertilized with the mixed milt of males. The obtained results showed that the eyeing rate decreased from 90.65± 6.28% in newly ovulated eggs (0-10 DPO)to 0.67±1.34% in over-ripened eggs(30-40DPO). Also, the hatching rate decreased from 86.33 ±6.28% in newly ovulated eggs to 0.49±0.98% in over-ripened eggs. However, larval abnormalities remained constant for 30 days after ovulation. The histological study in the newly ovulated egg showed that the yolk consisted of homogenous tissue having the perivitelline space diameter with no considerable difference. At over-ripening stage, the yolk became hetero generous, while chorion diameter did not change. Also the perivitelline space diameter varied among different areas. The micropylar apparatus also showed no change during the over- ripening stage. The present study demonstrated that the best time to strip Caspian brown trout eggs post- ovulation was up to 10 DPO at 7± 0.6°C. Same histological changes occurred in oocyte at over- ripening but it showed no effect on the micropyle.