How to Improve the Survival Rate of Scallop Seedlings in Nursery Rearing

Suitable ponds for nursery rearing The pond should preferably have an east-west oriented rectangular structure, with a sandy or hard bottom, a water storage depth of more than 1.5 m, a salinity below 35‰, and an unpolluted external seawater source; before filling the pond with water, the bottom sludge layer must be thoroughly removed and the pond fully sun-dried; it is prohibited to stock large-sized benthic fish or other species requiring heavy feeding in nursery ponds, so as to prevent pond water pollution.

Focus on good water quality regulation 10-15 days before the scallops are put into the pond, fill with 60 cm-80 cm of water and apply fertilizer to enrich the water, applying nitrogen fertilizer at 3.5 g per cubic meter of water; the amount of phosphorus fertilizer depends on conditions, generally about 1/10 of the nitrogen fertilizer, and the feed density should be controlled at 50,000-200,000 cells/mL. 4-5 days before stocking the seedlings, gradually raise the pond water to the maximum water level, with a transparency of 70 cm-80 cm being suitable; if transparency is too high, when the temperature is relatively high around noon, promptly apply additional inorganic fertilizer using the method of small amounts; if transparency is too low and feed density is too high, several days before the scallop seedlings are put into the pond, the water exchange volume should be appropriately increased, so that the water quality is clear but not lean, and fertile but not aged. Before releasing the seedlings, the physical and chemical indicators of the nursery pond water quality must be strictly monitored to ensure all indicators are within the specified ranges; after the seedlings are distributed, the daily water exchange volume should reach more than 20%. If the water color changes abruptly, indicating water quality aging, this is a sign of scallop seedling mortality, and at this time a thorough large water exchange should be carried out or the pond should be changed promptly for nursery rearing.

Select high-quality juvenile scallops When seedlings are taken out of the hatchery for nursery rearing, carefully select juveniles with uniform size, firm attachment, suitable density, and attachment substrate placement time within 15 days. The water quality conditions of the hatchery should be basically the same as those of the nursery pond; generally, when the pond water temperature is stable at 13°C, they can be put into the pond for nursery rearing; the larval rearing water temperature must not be higher than the specified water temperature. The combination of unicellular algae feed varieties during seedling rearing should be reasonable, and the supply should be sufficient, to ensure uniform size and healthy, strong bodies.

Appropriately increase the size of juveniles when taken out of the hatchery Generally, the size of juveniles when taken out of the hatchery is 500 microns; increasing the size to 700 microns-800 microns can greatly improve the nursery survival rate.

Strictly follow the operating procedures for bagging and transporting seedlings Bagging personnel should receive pre-job technical training, and movements during operation should be proficient, shortening the bagging time as much as possible; use polyethylene thread to hang the attachment substrate to the bag opening, avoiding multi-layer rolling, stacking and friction that may injure the seedlings. It is suitable to transport seedlings in the early morning or at night; during transport, attention should be paid to sealing, moisture retention and cooling, and the transport time should be shortened as much as possible.

Reasonably determine the nursery rearing density Generally, no more than 100 million 500-micron juveniles should be reared per mu of usable water surface, and no more than 30,000 seedlings should be placed in each bag. As the size of juveniles taken out of the hatchery increases, the number of seedlings per bag should be appropriately reduced.

Use double-layer mesh bags and round mesh bags for nursery rearing

Nursery rearing with double-layer mesh bags When seedlings are taken out of the hatchery, put the juveniles into 20-mesh-30-mesh polyethylene mesh bags, with an outer 40-mesh-60-mesh mesh bag. About 10 days after being put into the pond, the seedlings in the bags have grown; remove the outer bag, and put the juveniles on the outer side of the inner bag into 40-mesh bags for temporary rearing, which not only removes attachments but also disperses the seedlings. The nursery survival rate with double-layer mesh bags can generally reach more than 30%.

Nursery rearing with round mesh bags Round mesh bags are made by sewing 20-mesh and 40-mesh polyethylene mesh sheets into a cylindrical shape, with a bottom diameter of 200 mm and a height of 280 mm-300 mm, with a cylindrical plastic frame inside, 185 mm in diameter and 60 mm in height. Round mesh bags have many advantages: supported by the plastic frame, they have a larger nursery space, water flows smoothly, allowing the seedlings to move freely, increasing feeding opportunities, and avoiding death caused by stacking, friction, and mutual biting among juveniles; the nursery survival rate with round mesh bags can reach more than 40%.

Strengthen routine supervision measures Fully tighten the floating ropes, appropriately increase the weights on the seedling bags, reduce the swaying of mesh bags, persist in pond patrols, and prevent mesh bags from becoming tangled as well as broken frames, broken ropes, and falling weights; as long as mesh holes are not seriously clogged, there is no need to brush the nets. When necessary washing is required, simply lift and dip them in the water to reduce damage to the seedlings.

Separate seedlings in a timely manner and send them to the sea for grow-out The nursery rearing time in ponds should not be too long; generally, after 20 days-25 days of nursery rearing, when the juvenile size reaches about 3 mm, the seedlings can be separated and sent to the sea, entering the intermediate grow-out stage. The time for sending them to the sea should be chosen in calm weather, avoiding dryness and direct sunlight, and it is best to carry it out in the morning and evening.