Determination of the best post-ovulatory stripping time for the common carp (Cyprinus carpioLinnaeus, 1758)

Abstract

To determine the best time interval between ovulation and controlled egg stripping in female common carp (Cyprinus carpio), ovulated eggs were retained at 20°C inside the fish body until 12–14 h post-ovulation (HPO). The eggs of four females were stripped with two-hour intervals after ovulation for 12–14 HPO and fertilized with mixed milt obtained from three males. Eyeing, hatching and eyed-egg mortality rates as well as the occurrence of larval malformations and ploidy anomalies were considered as indices to assess egg quality. The results indicated that throughout 4–6 HPO the eyeing and hatching rates remained approximately 90 and 80% of the initial rates, respectively. Although not significantly different (P > 0.05), the viability rates (eyeing and hatching percentages) increased to about 7.5% of the initial rates at 2–4 HPO compared with 0–2 HPO. Thereafter, the eyeing and hatching rates decreased over time linearly and finally dropped to 11 and 2.5% in eggs fertilized at 12–14 HPO, respectively. Eyed-egg mortality and larval malformation rates did not display any marked increase in eggs fertilized up to four hours after ovulation but thereafter increased significantly; more than 50% of eyed eggs died, and approx. 20% of hatched larvae were malformed at 8–10 HPO. Post-ovulatory oocyte ageing did not affect the ploidy level of the larvae. Based on the results obtained in this study, common carp egg quality is maintained inside the fish body for at least four HPO. The best post-ovulatory stripping time was estimated to be within 2–4 HPO and therefore we recommend that the eggs be retained inside the fish body for this period of time after ovulation. The complete loss of egg viability also occurs 12–14 h after ovulation.