Baseline:
Zhenhai Bay is an estuary of shallow water where saltwater and freshwater meet in the southwest of Taishan, Guangdong. The bay is shaped like a trumpet, with a width of about 16.3km, a depth of 27km, the narrowest point of 1.6km, and an area of about 100 km2. The annual water temperature in the bay ranges between 15-32?, the salinity of seawater is about 8?-25?, and the type of sediment is silt. There are concentrated and contiguous natural mangroves in the upper reaches of the bay, with an area of about 117.61hm2. It is the largest mangrove wetland park in the Pearl River Delta (Guangdong Zhenhai Bay Mangrove Wetland Park). The unique offshore estuary ecosystem and the terrain advantages of natural havens support the development of marine industries in the bay, mainly aquaculture and tourism. At present, the central and southeastern waters of the bay are the main producing areas of Hong Kong oysters with a long history and nationwide fame, and the local oyster aquaculture culture has been passed down from generation to generation; the southwest side of the downstream estuary is the local coastal tourist resort (Guan Jionghui et al., 2019).
The total area of Taishan oyster aquaculture, mainly in Zhenhai Bay, is 6,200 hectares, with an annual output of 85,300 tons and an output value of 1.5 billion yuan. Most families in the communities in Shenjing Town, Wencun Town, and Beidou Town along Zhenhai Bay engage in oyster aquaculture as a source of livelihood. The local traditional oyster aquaculture practice can be divided into four stages, including seedling cultivation, mid-cultivation, growing, and fattening. During the seedling picking stage (spring, summer, and autumn), local farmers hang the cement boards (cakes) in strings to the seedling picking area in the bay. The seedlings are cultivated by hanging in the temporary holding area when the number and size of the attached seedlings (about 5mm or more) reach the specifications, and the seedlings can enter the next stage when their shells are as high as 1-3cm. In the mid-cultivation stage, farmers mainly use rafts and a small amount of piling to cultivate juvenile oysters. When the height of the oyster shells reaches 8cm, they can be moved to the growing area for cultivation. In the growing stage, farmers use rafts, longlines, or piles to raise oysters, and the oysters can be harvested when they grow above 15cm. In the autumn and winter (August-December), before oysters are sold to the market, local farmers usually choose sea areas or salty enclosures with rich bait for fattening, and in winter and spring (October-April), when the oysters are full of soft tissues, they will be sold to the market considering the market conditions.
However, due to the influences of reclamation (ponds), the input of land-based pollutants, and the tailwater of coastal pond aquaculture, the ecological environment of Zhenhai Bay has been degraded to a certain extent: the destruction of mangrove ecosystems and biodiversity, and the declined quality of the water environment (pollutants include inorganic nitrogen and active phosphate, and water quality are in the third and fourth categories). In addition, the high-load oyster aquaculture activities in Zhenhai Bay, which may not be the primary reason for the deterioration of the ecological environment though, have also brought negative effects to its ecological environment. The local resource-dependent oyster aquaculture industry also suffers from the degraded water environment and has unstable natural seedlings, longer oyster breeding cycles, poor fatness, and occasional large-scale oyster deaths.
In recent years, the government has paid more and more attention to the construction of marine ecological civilization. During the "13th Five-Year Plan" period, in order to rectify water pollution in coastal waters and improve the quality of the coastal ecological environment, governments at all levels in Guangdong Province have carried out the construction of urban water treatment facilities, cleaning and rectification of sewage outfalls into the sea, treatment of tail water from aquaculture ponds, and artificial planting of mangroves within their management areas. Although phased results have been achieved, the main nitrogen and phosphorus pollutants entering the sea still need to be further reduced, and the service function of the offshore ecosystem still needs to be improved. The general challenges faced by the ecological governance of coastal waters are also reflected in Zhenhai Bay. During the "14th Five-Year Plan" period, Guangdong Province continued to follow the action strategies used during the "13th Five-Year Plan" period and also listed Zhenhai Bay as one of the construction goals of the 15 beautiful bays, focusing on ecosystem protection and restoration, and strengthening the management and control of critical ecological spaces, to build it into an ecologically friendly and beautiful bay ("Guangdong Province Marine Ecological Environmental Protection "14th Five-Year Plan").
Some well-managed shellfish and algae aquaculture systems (involving species, density, facilities, site selection, etc.) have not only minimal negative environmental impacts but also great potential for ecosystem services, such as improved water quality, habitat provision, etc. When commercial or subsistence aquaculture practices provide direct ecological benefits to the environment and have the potential to produce net positive environmental outcomes, it is referred to as remedial aquaculture (The Nature Conservancy, 2021). There are large-scale oyster aquafarms in Zhenhai Bay, and the bay is facing the challenges and needs of ecological environment governance and sustainable development of oyster aquaculture. Restorative oyster aquaculture practices and implementation promotion paths suitable for local communities with economic, ecological, and social benefits, which are developed by combining local environmental characteristics, aquaculture communities, and government development goals, will provide effective supplementary action strategies for the construction of beautiful bays in Zhenhai Bay.

Problems:
Although large-scale oyster aquaculture has brought good economic benefits to aquafarmers in the past, the deterioration of the water environment has limited the sustainable and stable growth of aquafarmers' economic benefits. Aquafarmers in the province reported that the growth rate of oysters had decreased, the fatness had decreased, and there had even been large-scale death of oysters, which had increased the risk of aquaculture.
The scale and density of aquaculture exceeding the aquaculture capacity of the bay is an important limiting factor for the sustainable development of domestic oyster aquaculture and the primary reason that oyster farms have a certain degree of negative impact on the ecological environment. Zhenhaiwan is no exception. Oyster aquaculture over aquaculture capacity produces a large amount of excrement and leads to insufficient water exchange, which causes water deterioration and substrate environment in the aquaculture area, further disturbs the inhabited marine species.
At present, the government actions involved in oyster aquaculture areas mainly focus on the delineation of aquaculture waters, the mudflat planning, the issuance of sea use licenses and aquaculture licenses, subsidies and incentives for the upgrading of aquaculture facilities into green and environmentally friendly floating balls, and water quality monitoring to designate shellfish aquaculture and production areas. The aquaculture management system based on aquaculture capacity has not been implemented. Also, because the current oyster aquaculture practice is relatively clean compared to shrimp and crab aquaculture in ponds and fish aquaculture in offshore cages, the green model for single-species aquaculture of oysters is rarely promoted by the government. Efforts have been concentrated on the treatment of tailwater in aquaculture ponds and the development of land-based intensive aquaculture and deep-sea cage aquaculture while removing offshore cages.
Implementing oyster aquaculture practices and management systems based on aquaculture capacity is a crucial measure for the coordinated development of oyster aquaculture and the ecological environment. Although the research on aquaculture capacity has been gradually carried out in China as early as the 1990s, and its research and evaluation methods have been advanced so far, in the actual practice and management of oyster aquaculture, only a few areas take the aquaculture capacity into consideration. Challenging to implement guidelines of aquaculture capacity is the result of the interaction of multiple socio-economic facts of oyster aquaculture. For example, the number of aquafarms is large, and most of them are operated dispersedly on a small-scale (low degree of intensification); the water quality and bait organisms that oyster aquaculture relies on for survival are typical public pond resources, which are fluid; most aquafarmers' pursuit of short-term economic benefits prompts them to scramble to expand the scale and density of aquaculture (although the aquaculture output may not meet expectations); government management policies and regulations need to be improved and strengthened. Currently, the issue that the aquaculture scale exceeds the aquaculture capacity has received the government's attention. In 2019, the "Several Opinions on Accelerating the Green Development of Aquaculture Industry" jointly issued by ten ministries and commissions, including the Ministry of Agriculture and Rural Affairs, proposed to "carry out aquaculture capacity assessment and reduce the total amount of aquaculture in areas where the aquaculture scale exceeds the carrying capacity of water and mudflats", which shows that aquaculture management based on aquaculture capacity will be the general trend of aquaculture development in the future.
In order to promote the application of aquaculture capacity in the management of oyster aquaculture practice, in addition to the improvement of policies and systems, it is necessary to promote the formation of a scientific understanding of the interaction between oyster aquaculture and the ecological environment by local communities and to quantitatively study the practice of farming oysters based on aquaculture capacity. It is necessary to explore multiple ecological benefits of oyster aquaculture practice based on aquaculture capacity and the approach to realizing the value of the ecological products, which will lay the foundation for the implementation of the win-win solution of sustainable oyster aquaculture and synergistic improvement of the bay environment.

Activities:
Activity 1.1
Work with project partners to formulate a preliminary plan for monitoring the status of biodiversity in Zhenhai Bay oyster aquafarms, focusing on fish and aquatic macroinvertebrates;
With the support of the local government and the association, communicate with and cooperate (including monitoring research and community research) with the aquaculture communities in Wencun Town regarding the project, and integrate the aquafarmers' experience and knowledge into the design of the monitoring and research plan to form the final version of the monitoring and research plan;
Implement a monitoring and research plan to form a biodiversity impact assessment report for Zhenhai Bay oyster aquafarms.
Activity 1.2
Integrate the research results of the project partners on the aquaculture capacity of Zhenhai Bay, comprehensively evaluate the impact of Zhenhai Bay oyster aquafarm on the water environment, substrate environment and biodiversity, as well as the relationship between aquaculture capacity and benefits, and invite two or three oyster aquaculture experts outside the project partners to develop recommended oyster aquaculture practices (with potential for restorative benefits) based on aquaculture capacity.
Activity 1.3
In addition to aquafarmers in the aquaculture communities, identify other key stakeholders in the local oyster aquaculture industry, design appropriate survey or interview forms and question lists according to the characteristics of different stakeholders, and form a survey and interview plan for multiple stakeholders ;
Implement the research plan to understand the current situation, development difficulties, and needs of traditional aquaculture, as well as the views of all parties on the changes in the ecological environment of the aquaculture area, reducing the aquaculture density and other issues, and form a research report on the current situation and development difficulties of traditional aquaculture in Zhenhai Bay.
Activity 1.4
Based on the primary aquaculture technical needs and the ecological impact of the aquafarms identified in the community survey, invite two or three oyster aquaculture experts in the province (each time), design and organize one or two interactive and participatory aquaculture technology training sessions, targeting multiple aquaculture communities in Zhenhai Bay, and share and spread the concept of green aquaculture.
Activity 2.1
Organize one seminar on sustainable oyster aquaculture in Zhenhai Bay, invite major aquafarmers, aquaculture associations, aquaculture management departments, and other parties in the bay to participate, share the results of the study on the ecological impact of aquaculture, and the proposed practice plan for oyster aquaculture based on aquaculture capacity (with potential for restoration benefits), and discuss the potential and path of oyster aquaculture practices based on aquaculture capacity in Zhenhai Bay, combined with local aquaculture challenges and potential development opportunities.
Activity 2.2
Based on the knowledge gained at the seminar and the opinions and ideas of all parties, by comprehensively analyzing the potential, key factors, implementation paths, and knowledge gaps of Zhenhai Bay's implementation of restorative oyster aquaculture based on aquaculture capacity, prepare a feasibility analysis report on oyster aquaculture based on aquaculture capacity in Zhenhai Bay;
Share the feasibility analysis report with the local government and aquaculture associations, and through communication and discussion with them, promote the local oyster aquaculture demonstration of 100 mu based on the scientifically appropriate oyster aquaculture density and location recommended in the report.

Outcome:
1. Raise aquafarmers' awareness of environmental protection to help aquafarmers clearly realize that jointly maintaining the ecological environment of the aquaculture waters can ensure the long-term benefits of oyster aquaculture.
2. The aquaculture associations, aquafarmers, and scientific research experts conduct in-depth exchanges and interactive discussions, which deepens the possibility of multi-party cooperation and improves the resources and channels for the local oyster aquaculture community to obtain scientific aquaculture technical guidance in the future.

Form a feasibility analysis report on oyster aquaculture practices based on aquaculture capacity, providing scientific solid support and advice for local government departments to formulate and implement relevant policies or management regulations in the future; and also for local aquaculture associations and communities to explore the multiple ecological benefit potential of oyster aquaculture in the future, and certain information and reference for thinking and developing the oyster aquaculture industry model that integrates three industries.