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In recent years, many fishermen working in bay scallop ponds have experienced poor results. While climate change plays a role, the main issues are related to technical practices. To improve the economic returns from raising seedlings in ponds, it's essential to focus on enhancing seedling quality and survival rates. Here are some key considerations:
First, the pond bottom should be a soft mud surface, and the edges must be well compacted. Otherwise, strong winds can cause the water to become cloudy, clogging the mesh of the seedling bags. The size of the pond should match the number of seedlings. If the density is too low, resources are wasted; if it's too high, feed shortages may occur later. Maintaining good water quality and proper fertilization can boost scallop growth and increase survival rates. Typically, the water transparency should be around 40 centimeters, with a yellow-brown or yellow-green color. The dominant food species include *Chaetoceros*, *Dinobryon*, *Skeletonema*, *Cylindrotheca*, and *Chlorella*. If there's an overgrowth of dinoflagellates, it’s important to change the water or move the seedlings to a new pond.
Second, the timing of juvenile release should not be too early. Once they are placed in the pond, if the water temperature remains below 13°C for too long, their metabolism slows down, and growth is delayed. If they detach, reattachment becomes difficult. Cold air outbreaks can cause them to close their shells, which negatively affects growth and survival.
Third, selecting high-quality juveniles is crucial. It’s important to inquire about the source, probiotics used, incubation time, culture conditions, drug usage, and feeding species. Conducting microscopic exams, visual inspections, swing tests, and cooling tests can help evaluate attachment, feeding activity, vitality, and size—ensuring the seedlings are healthy before placement.
Fourth, seedling bags should be hung in a low-light zone between 30 to 40 cm deep. Regularly washing off the mud inside the bags helps maintain water quality and allows for better exchange of food and oxygen. Using a large tube brush or a long-bristled hard brush is recommended to thoroughly clean the mesh and remove debris that might block the holes. Traditional methods using soft brushes are often ineffective. This lack of proper cleaning leads to poor water exchange, making it difficult for the larvae to get enough food and oxygen. Additionally, seedlings should be moved out of overcrowded bags to prevent stress and weakness. Prolonged stays in plastic pots can also lead to drying and make it hard for them to return to the seedling bags.
Fifth, starting from the third day after the juveniles are placed in the pond, water changes should be done daily at 10%, increasing to about 20% in the middle and later stages. Water changes help prevent water aging, keep temperatures stable, and promote water flow, aiding in the exchange within the seedling bags. During rainy days, the water level should be raised as much as possible, and seedling bags should be lowered to avoid sudden salinity changes that could harm the juveniles. In the first three days after transfer, more than 30% of the water should be changed to mimic natural sea conditions, improving the juveniles’ ability to adapt when they eventually go to sea.