Analysis of Death Causes in Scallop Farming and Preventive Measures

1 Death causes

1.1 Excessive farming density This is a commonly existing problem. Farmers single-mindedly pursue yield per unit area, and most adopt high-density farming methods, placing 100-120 seedlings per layer of net cage, or even more. At present, most scallop farming methods use the hanging-down type, which has poor water exchange capacity, and large amounts of excrement cannot disperse in time, depositing on the seabed around the farming rafts and decomposing, producing harmful substances (such as H2S). When wind and waves increase, these harmful substances are stirred up and poison the scallops. On the other hand, excessively high farming density causes a sharp decrease in feed organisms, and because of the lack of feed, the scallops' disease resistance and ability to withstand environmental deterioration decline, leading to mass death of scallops.

1.2 Impact of wind and waves The Zhejiang sea area has many typhoons, concentrated in August-October, when scallops lack byssus. Due to the impact of wind and waves, the scallops roll and collide inside the cages and become entangled with each other, causing injury and death.

1.3 Impact of reproduction Because most scallop seedlings currently come from artificial seedling nurseries, the seedling emergence period is early, and the growth time in the sea area is long. By August and September, the gonads can mature and enter the autumn reproduction period. At this time, the organic matter and energy used for growth decrease, so the scallops' physique is relatively poor. When the "autumn tiger" weather appears in the Zhejiang sea area (referring to abnormal sudden temperature changes in autumn), the water temperature in shallow sea areas changes greatly, causing mass death of scallops. In our investigation, we found that larger individuals with well-developed gonads had a relatively high mortality rate.

1.4 Sea area pollution Due to the discharge of urban sewage and the increase in dumping from wharves and ships, the sea area is increasingly polluted. Some farming areas have low transparency and large amounts of oil pollution, causing water quality to deteriorate, and the scallop mortality rate is relatively high.

2 Preventive measures

2.1 Adopt suitable farming density Before farming, a suitable farming density should be selected according to conditions such as sea area flow speed, water transparency, number of feed organisms, seedling quality, and farming level. For seedlings with a size of 1 cm, it is appropriate to stock 60 per layer of cage. When the seedlings grow to 2-3 cm, divide the seedlings in time, stocking 20 per layer; and design a reasonable farming layout, keeping the cage spacing at 60-80 cm.

2.2 Strengthen grow-out management During the farming period (especially during typhoon season), regularly check the safety of the farming facilities; promptly remove floating mud and fouling organisms on the net cages and shells, and make full use of dissolved oxygen and feed organisms in the water.

2.3 Adjust the farming water layer in time Normally, the farming water layer can be controlled at about 3 m. During the summer high-temperature period, typhoon season, and periods with large water temperature differences between summer and autumn, sink the same cage by 1-2 m, and fasten the attached stones, or move it to an outer area with deeper water for farming.

2.4 Implement shellfish-algae intercropping and rotation farming to create a good ecological environment The photosynthesis of algae can increase the dissolved oxygen content of the water and improve the growth environment of scallops; while the excrement of scallops can provide nutrients for algae growth; at the same time, it also makes full use of farming space and improves comprehensive economic benefits.