Key Technical Points for Disease Prevention and Control in Artificial Scallop Seed Rearing

During artificial seed rearing, scallop larvae are highly susceptible to bacterial and fungal infections, which cause mass mortality.

Among the common bacterial diseases are Vibrio species; one type, Vibrio anguillarum, specializes in decomposing organic matter and can break down the larval velum within a very short time. This symptom occurs especially within 5-7 days after egg collection, spreads extremely fast, and can kill all larvae within 6-8 hours. At present, once disease caused by this pathogenic bacterium is detected, there is no treatment plan available; it can only be prevented. Prevention methods: strengthen the physical condition of broodstock during conditioning, transfer tanks daily during conditioning to thoroughly eliminate the source of the bacterial disease, and transfer tanks frequently during larval rearing (once every 3 days); after tank transfer, use potassium permanganate to disinfect and clean the tank bottom.

Pseudomonas and Aeromonas bacteria are normally hidden in seawater, bottom sediment, tank-bottom pollutants, and can also be found on the body surface or in the digestive tract of animals. Once conditions are suitable, they multiply in large numbers and invade the tissues of scallop larvae or juveniles, becoming pathogenic bacteria. Infected larvae sink to the tank bottom, their velar cilia become short and uneven, their activity weakens, and they soon die in large numbers. Under the microscope, ulcers and even decomposition can be seen in the larval tissues. The therapeutic effect for this disease is not significant. Prevention methods: remove metabolites and residual bait from the rearing tank bottom to prevent bacterial proliferation.

During larval rearing, a type of mold and fungus often infects the foot muscle part of the larvae. This condition is often found after the larval shell length reaches 168 microns. The pathogen infection is slow. Under the microscope, it is very clear that as the larvae grow and develop, the shell length and shell height continuously increase, and the internal organs also develop normally, but the foot muscle, especially the sole part of the foot, begins to ulcerate and loses its crawling function. In the later stage of eyed larvae, the probability of occurrence is high; the velum begins to shrink and degenerate, while the foot muscle ulcerates, causing the larvae to sink and die.

At present, there is still no good method for treating bacteria in seawater. Taking preventive measures is the main approach: frequently and thoroughly clean the tank bottom, disinfect the tank bottom, and transfer tanks frequently. In addition, the most effective method is to disinfect seawater with ultraviolet light.