The objectives of the present work were to study the ovulatory response of silver carp when induced to breed using carp pituitary powder as an ovulation stimulator at various hours of the day. Furthermore, an attempt was carried out to determine the natural latency time, i.e. degree-hours hoping that it would be possible to employ it in carp hatchery procedures. Therefore, two experiments were carried out; in the first, the response to hormonal stimulation and the reproductive performance of female carps received a decisive injection with a spawning stimulator at different day times: 04.00 p.m., 08.00 p.m. and 11.00 p.m. was evaluated. In the second experiment, an estimate for the latency time in terms of degree hours was determined by placing pre-ovulating females with fertilizing males and measuring the time elapsed till the natural spawning occurred. The relevance of this measured time was then tested in artificially induced spawning females groups.
The results revealed that percentage of ovulated females was higher (100%) in the group received the decisive pituitary injection at 04.00 p.m.* than those received it at 08.00 p.m. and 11.00 p.m. (87.5% and 93.8%, respectively). Injection time, however, had no effect on weight and quality of the obtained eggs when assessed in terms of percentages of fertilization, survival rates and percentage of deformed larvae, Nevertheless, combining results from ovulation and incubation success (spawning effectiveness coefficient) showed that injection time affected egg quality. The present results showed that the best overall quality of the sexual products was produced when female silver carps received the decisive injection at 04.00 p.m. when compared to those received it at either 08.00 p.m. or 11.00 p.m. The measured natural latency time was approximately 7.4 h which is equivalent to 188.4 ±1.99 degree hours at a constant water temperature of 25.5 °C, although the latency time of artificially spawned females was 199±1.74 degree hours under the same conditions. The presence of males with the spawning female groups significantly (PO.05) increased the percentage of ovulated females from 92-2% to 100% and decreased the average latency time of the spawning females by approximately 24 minutes.