What key factors must be emphasized in healthy farming of Ezo scallops?
Since the 1980s, floating-raft farming of Ezo scallops has developed rapidly in Changhai County, Liaoning Province. At present, the farming area has reached more than 100,000 mu, and it has become an important local famous, special, and high-quality marine aquaculture species, occupying a very important position in the development of the marine fishery economy. Although Ezo scallop farming technology has been rapidly promoted, during operations, due to negligence in certain technical links, the mortality rate during farming has continued to rise, and the specifications, quality, and fullness of commercial scallops have declined somewhat, thereby affecting the economic benefits of the industry. Based on years of investigation, research, and on-site surveys, the author analyzed many problems currently existing in Ezo scallop farming and believes that the key issue for the sustainable and healthy development of this industry is that attention must be paid to promoting the healthy farming model for Ezo scallops.
I. Main problems existing in Ezo scallop farming
1. The layout of farming floating rafts is too dense. From the layout of floating rafts at sea, the spacing between raft units is basically about 5 m. Driven by profit, some farmers add floating rafts between existing raft units. Some, in order to increase the length of raft bodies, block almost all the flow channels and navigation channels between rows, and the floating rafts at sea appear uneven in length and highly irregular, with the short ones about 70 m and the long ones even reaching about 200 m. One floating raft is equivalent to the number of shellfish hung on 2 floating rafts in the past, which not only affects the normal flow of seawater and blocks navigation channels, but also plants hidden safety hazards.
2. The stocking density of seedlings is too high. In Changhai County, large-scale mortality in Ezo scallop farming has occurred since 1984. After years of investigation and research by experts, multiple causes were analyzed, including excessive farming capacity and the primary productivity of the farming sea area having reached its limit. Therefore, it should be clearly recognized that the carrying capacity of the sea area is limited. At present, the farming net cages used are basically 15 to 20 layers of trays, and from the start of third-stage temporary rearing, the seedling density per tray layer actually reaches about 300 grains, and the overwintering temporary rearing density is also nearly 100 grains. This practice of blindly increasing stocking density not only affects the growth rate of individual Ezo scallops and affects product quality, but also easily causes phenomena such as seedlings biting each other to death and an increase in deformed scallops, increases the mortality rate of seedlings, and reduces the fullness of commercial scallops. Once sea area pollution is caused, the loss will be even more severe.
3. The farming species is single. A few years ago, shellfish farming was mainly based on Zhikong scallops or bay scallops. After mortality occurred in the above 2 major species, farmers now turned to farming the single species of Ezo scallop. Once problems occur in Ezo scallop farming, there is no other substitute species to supplement it. This single-species farming method is unfavorable to the future development of farmers. From the investigation, this year the Zhikong scallops and bay scallops of individual farmers have grown very well. Therefore, in the absence of new substitute species, multi-species farming combining Ezo scallops, Zhikong scallops, bay scallops, and others can be carried out.
4. The risk of Ezo scallop mortality accidents still exists. In Changhai County, artificial seedling rearing and floating-raft farming of Ezo scallops are affected by high temperatures above 23°C in summer, and the seedling survival rate has always been very low, and a scale has been slow to form. In recent years, due to changes in the natural environment and improvements in farming methods, the survival rate of scallop seedlings has increased somewhat. We compared the water temperature measurement conditions in some farming sea areas of the county from 1999 to 2004 and found that summer water temperatures before 2000 differed from those in the past 2 years. For example, from August to September 1999, the highest water temperature reached 24°C to 26°C, while from August to September 2003 and 2004, the highest water temperature was only about 23°C. After comparative analysis, it is believed that the main reason for the increased survival rate of Ezo scallops in recent years is related to relatively lower water temperatures. Therefore, it is suggested that farmers should pay special attention to temperature differences between different locations and seedling transport methods when transporting scallop seedlings, so as to avoid weakening the resistance of seedlings due to stimulation from excessive temperature differences between the two places and causing death when farmed in unsuitable environments, and so on.
II. Several key factors that must be emphasized
1. Seize the season with suitable temperature to divide seedlings and grow them early. The Ezo scallop is a cold-water shellfish, with a suitable growth temperature range of 5°C to 20°C, and the most suitable growth water temperature is 10°C to 15°C. Within the suitable temperature range, the metabolism of Ezo scallops is vigorous, producing positive effects on respiration and excretion, movement and feeding, digestion and growth, and gonadal development and reproduction. Therefore, when the water temperature reaches above 5°C and below 20°C, seedlings should be divided and grown early, and after recovering from transplanting, they can promptly enter a rapid growth period.
2. Change shore-based seedling division to sea-based seedling division to shorten dehydration time. Shore-based seedling division is to transport Ezo scallops from temporary rearing rafts to the shore for seedling division, then transport them to the raft area for hanging and farming. The round-trip transport time is long, and it easily causes seedlings to be dehydrated for a long time. In addition, the shore water temperature is relatively high and impacted by wind and waves, and sand particles and impurities on the shore easily enter the seedling shells, greatly affecting seedling growth. Sea-based seedling division is to divide seedlings while hanging them in the sea, greatly shortening the dehydration time of seedlings, and the mantle is also less likely to be affected. Practice has proved that sea-based seedling division operations can not only increase the survival rate of seedlings, but also be more conducive to the normal growth and development of seedlings.
3. Clean net cages promptly, remove impurities, and prevent seedling contamination. Used net cages should be cleaned thoroughly and impurities removed before loading seedlings; new net cages should be soaked in seawater before use; old net cages that are already seriously contaminated or new net cages with peculiar smells must not be directly used for loading seedlings, so as to prevent seedling contamination. In addition, when dividing seedlings, dead shells and impurities should be removed completely to avoid polluting the water body and causing a vicious cycle.
4. Choose suitable temporary rearing sea areas. Ezo scallop seedlings have relatively strict requirements for temporary rearing sea areas. The outer rows have relatively large wind and waves and rapid currents, and are not suitable for temporary rearing seedlings. The inner rows have poor water flow, relatively large floating mud, and scarce food organisms, and are likewise unfavorable for seedling growth. Farmers with the means should choose deep-water sea areas with clear water, slow currents, high transparency, relatively little floating mud, no strong wind and waves, and abundant food organisms for temporary rearing seedlings, so as to create a good growth environment for Ezo scallop seedlings. Choosing a suitable temporary rearing sea area is the first-choice condition for cultivating seedlings with strong constitutions and vigorous vitality.
5. Rationally utilize farming sea areas and clear illegal occupation of raft area waterways or navigation channels. Row spacing must not be less than 60 m to ensure smooth water flow in the raft area, and floating rafts should be rationally arranged.
6. Increase the spacing between raft units. Farmers should remove the floating rafts added between rafts. The spacing between raft units must not be less than 8 m, and floating rafts should be rationally arranged within the sea area use range specified by confirmation of rights.
7. The number of hanging strings should be reasonable. The number of hanging strings should be reasonably arranged according to the water flow conditions and safety level of the sea area. Generally, the distance between farming cages must not be less than 1 m.
8. The stocking density of seedlings should be appropriate. For third-stage temporary rearing, each tray layer must not contain more than 100 grains of seedlings; for overwintering temporary rearing, each tray layer must not contain more than 40 grains of seedlings; for growing one-year-old scallops, each tray layer must not contain more than 20 grains of seedlings; for farming large-size adult scallops, each tray layer must not contain more than 13 grains of seedlings.
9. Increase law enforcement efforts, promptly correct unscientific and irregular operations, protect the ecological environment of farming sea areas from the overall perspective, and reduce sea area pollution, so as to improve the survival rate of seedlings and promote rapid seedling growth.
10. Continue to implement the healthy farming model and maintain water area ecological balance. Diseases and mortality still threaten Ezo scallop farming. Disease prevention and control should focus on prevention. At the same time, in view of the current situation of water quality pollution in farming waters and water bodies approaching and reaching eutrophication, research and promotion should continue on economically viable species suitable for local growth, changing from single-species farming to a farming pattern combining multi-species shellfish and algae farming.
11. Strict precautions during high-temperature periods to ensure seedlings safely pass the summer. In recent years, because summer water temperatures have been relatively lower than in previous years, people have almost forgotten the seedling farming method of sinking rafts into deep water layers to prevent high temperatures, and have ignored the fact that Ezo scallop farming is limited by high temperatures. Therefore, farmers should promptly adjust the farming water layer for Ezo scallops according to changes in water temperature in their respective farming areas. When the water temperature reaches above 23°C in August to September in summer, measures should be taken early to control the floating rafts with movable floats to low-temperature locations below 7 m in water depth, so as to ensure that scallop seedlings safely pass the summer.