Detailed Explanation of Key Technical Points for Intensive Rearing of Broodstock During the Gonadal Transition Period

After the beginning of spring, as water temperature rises, broodstock gonadal development gradually transitions. Especially from early spring to mid-May before spawning, broodstock gonadal transition is extremely rapid, and the ovary coefficient can reach 15%-20%. During this period, doing a good job of intensive rearing management during the broodstock transition period can promote smooth spawning; otherwise, gonadal development will stagnate or even regress. The intensive rearing techniques for broodstock during the transition period are as follows.

1. Timely pond separation. To understand the growth and development status of broodstock and facilitate the arrangement of induced spawning and reproduction, broodstock should be separated into ponds when the water temperature is 10°C. Before pond separation, do a good job of disinfecting the small ponds, and transfer broodstock reared during the overwintering period in larger ponds into small ponds with a water surface area of 667-1330 sq m (1-2 mu). Broodstock with good growth and development and broodstock with poor growth and development should be stocked separately, at a density of 120 kg-150 kg per 667 sq m of water surface. The broodstock placed in the same pond should complete induced spawning in one operation. In this way, it is convenient both for carrying out pre-spawning intensive rearing of those broodstock with poor growth and development, and for avoiding disturbing other broodstock that have not yet been induced to spawn when catching broodstock for induced spawning. During pond separation, broodstock with injuries or disease should be treated with medication.

2. Rational feeding. After the beginning of spring, broodstock need large amounts of nutrients for gonadal development. Silver carp broodstock ponds should be given more human feces, and bighead carp broodstock ponds should be given more livestock manure, applying 300 kg-500 kg of organic fertilizer per 667 sq m. The amount of topdressing should be small and frequent, so that the pond water is both "lively" and "fertile". Stop or reduce fertilization 15 days before induced spawning. For grass carp and black carp broodstock, feed mainly with green feed, supplemented by concentrated feed. In the early stage, feed with grain sprouts, malt, bran cake, etc., and feed green feed such as lettuce leaves and ryegrass as early as possible. Near the induced spawning period, grass carp broodstock with mature gonadal development noticeably reduce their food intake, or even stop feeding. Black carp broodstock are mainly fed snails and mussel meat, supplemented by soybean cake and compound feed.

3. Scientific water management. After the beginning of spring, the pond water depth should be 1 m, which helps water temperature rise and promotes the reproduction of food organisms. Broodstock ponds should be frequently flushed with new water to keep water quality fresh. Silver carp and bighead carp broodstock ponds should be flushed once every 10 days, and grass carp and black carp broodstock ponds once a week. After that, gradually shorten the interval; in the half month before spawning, silver carp and bighead carp broodstock ponds should be flushed once every other day, and grass carp and black carp broodstock ponds should be flushed every day. Flushing with new water should preferably be done in the afternoon, 3-5 hours each time, increasing the pond water depth by 15-20 cm. Broodstock ponds should be splashed with quicklime water once every week, at 10 kg-15 kg per 667 sq m. Timely adjust the pH of the pond water, maintaining pH at 7-8.5, to promote fish egg development.

4. Disease prevention. To prevent bacterial fish diseases in broodstock ponds, use 1 g of bleaching powder, 5 g of Shajunwang, 25 g of quicklime, and 0.3 g of strong chlorin per cubic meter of water. To prevent parasitic fish diseases, splash 0.8 g of trichlorfon, 0.5 g of copper sulfate, and 0.6 g of Qiangli Shachongling per cubic meter of water; or mix 0.5 g of copper sulfate and 0.2 g of ferrous sulfate with 200 kg of water, and splash the whole pond once every half month.