The Yalu River Estuary Coastal Wetland National Nature Reserve ( Figure 1 ) is in Donggang city (E123°34′20″~E124°07′28″, N39°40′06″~N39°58′24″), Liaoning Province, covering an area of 10,100 hectares. It contains land, beach and ocean ecosystems and forms complex and multiple ecosystem types, including reed wetlands, marshes, lake basin bogs, tidal marshes, and estuaries. There are 7 large rivers in the reserve. Among them, the substrates of the Yalu River and the Dayang River in the intertidal zone are suitable for annelid and mollusk growth. The quantity of nutrients and organic matter carried by these two rivers offers sufficient food to benthic organisms. The reserve is one of the significant shellfish production areas in Liaoning Province (Li et al., 2014). The main shellfish culture method used in this area is beach cultivation, and there is a small amount of mixed aquaculture of shellfish, shrimp, and jellyfish above the coastline.

The Yalu River Estuary Coastal Wetland National Nature Reserve has the characteristics of zonal distribution along the coastline in Donggang city, Liaoning Province, from east to west. This wetland intertidal zone is a breeding base of the harbor and serves as a beach in Donggang city. As a result, we obtained data from Donggang city’s shellfish culture statistics to represent the breeding conditions of the Yalu River Estuary Coastal Wetland.

The data on the varieties, culture area and yield of cultured shellfish from 2015 to 2019 were from the Liaoning Province Fishery Statistics Annual Report (Liaoning Ocean and Fisheries Bureau, 2015-2019). The Rst (dry weight ratio of soft tissue) and Rs (dry weight ratio of shell) of Grassostrea gigas, Scapharca suberenata, Sinonovacula constricta and Ruditapes philippinarum were measured in the coastal wetland of the Yalu River Estuary in 2018. The sample collection method was based on national guidelines (Standardization Administration of the People’s Republic of China, 2007). Shellfish were collected from 4-8 plots (25×25×20 cm) at each station, and the shellfish were screened by using a sieve with a pore size of 0.37 mm and a diameter of approximately 40 cm. The selected shellfish were put into sealed bags and placed in cold storage. The relevant parameters were determined according to the Chinese industrial standard (Ministry of Natural Resources, People’s Republic of China, 2021). Thirty samples were randomly selected, and the shell length, shell height and shell width of each species were measured by a Vernier caliper. The shellfish samples were weighed by an electronic balance (wet weight). Furthermore, the wet weight of the shell and soft tissue was weighed. Shell and soft tissue were washed with distilled water. Then, their surfaces were dried, and their wet weights were determined. The dry weight, dry-wet ratio and mass ratio were recorded after drying at 60°C for 48 h. The dried samples were then ground into powder and sieved to 80 mesh (178 μm), and the rate of carbon content was determined by an elemental analyzer. Additionally, the instrument parameters were set according to the manufacturer’s instructions, and the samples were embedded based on the sample requirements. Moreover, three parallel samples were fixed for each sample, and average values were calculated. Note that the parameters for Meretrix meretrix were taken from the references (Tang et al., 2011; Ge et al., 2022). A flow chart of the method in this paper is presented in Figure 2 .

Research shows that from 1999 to 2008, through the harvest of farmed shellfish, the amount of carbon removed from China’s offshore every year is 700,000 to 990,000 t (an average of 860,000 t), of which 670,000 t of carbon was removed from the ocean in the form of shells (Zhang et al., 2005; Tang, 2011). From 2015 to 2017, the average annual carbon removal of maricultured algae and shellfish in Liaoning Province was about 27,700 tons, equivalent to the reduction of 1108,200 million tons of CO₂ (Cao et al., 2019).

From 2015 to 2019, the total carbon sequestration quantity of cultured shellfish was approximately 110,500 tC along the coastal wetland of the Yalu River Estuary, and the annual average carbon removal was approximately 22,100 t. Based on the annual average carbon removal of maricultured shellfish of 166,800 t for Liaoning Province (Yin et al., 2022), the annual carbon sequestration of cultured shellfish in the coastal wetland of the Yalu River Estuary comprised 13.25% of the total provincial level. The higher the yield of cultured shellfish was, the greater the carbon sequestration (Yue and Wang, 2012; Zhang et al., 2013). From 2015 to 2019, the shellfish species cultured in the coastal wetland of the Yalu River Estuary were mainly R. philippinarum. Their culture area accounted for 66.67~81.87% of the total area, their yield accounted for 76.60~83.30% of the total yield, and the carbon sequestration accounted for 79.07~85.41% of the total. The second main species of cultured shellfish were S. constricta and S. suberenata. The culture area of S. constricta accounted for 1.53~2.72%, the yield accounted for 10.74~12.39%, and the carbon sequestration accounted for 8.91~10.08%. In addition, the culture area of S. suberenata accounted for 12.6~23.81%, the yield accounted for 1.67~7.77%, and the carbon sequestration accounted for 1.68~8.53% ( Figure 3 ). To the extent of the yield of the unit culture area and quantity of carbon sequestration, in the past few years, the unit area yield of S. constricta was the highest and increased continuously each year, except for G. gigas in 2019, which reached a quantity of 39.63 t/hm² in 2019 ( Figure 3 ). Nevertheless, the unit area yield and the amount of carbon sequestration of R. philippinarum were lower than those of S. constricta, although its culture area and yield were the highest ( Figures 3 , 4 ). This result is consistent with the results of previous studies showing a linear relationship between the production of marine shellfish aquaculture and the carbon sequestration (Yue and Wang, 2012).

In conclusion, among the cultured shellfish of the Yalu River Estuary Coastal Wetland, R. philippinarum had the highest proportion of yield and carbon sequestration of the total amounts, and S. constricta had the second highest proportions ( Figure 3 ).

Due to the lack of samples of externally attached organisms and sediments during sampling, the carbon sequestration calculations for these two types of samples were not included in this study. Although this result does not fully reflect the entire process of carbon sequestration of cultured shellfish (Wen et al., 2022), as the most important ecologically relevant product of coastal wetland culture in the Yalu River Estuary, the carbon sink capacity of cultured shellfish directly reflects the carbon sink importance of ecological products in the region.

The carbon sink conversion ratio of cultured shellfish is the ratio of carbon sequestration to yield and is one of the most important values for measuring the carbon sink capacity of aquaculture (Yue and Wang, 2012; Shao et al., 2018). From 2015 to 2019, the average carbon sink conversion ratio of different cultured shellfish in the coastal wetland of the Yalu River Estuary was, in descending order, S. suberenata, R. philippinarum, G. gigas, S. constricta and M. meretrix ( Figure 5 ). The results showed that S. suberenata had the strongest carbon sequestration capacity. In contrast, the carbon sequestration capacity of shellfish in Liaoning Province was ranked in descending order as S. constricta, R. philippinarum, M. meretrix, G. gigas and S. suberenata (Yin et al., 2022), indicating that the same cultured species exhibited different carbon sequestration capacities in different breeding environments. However, the total carbon sequestration of S. suberenata in the Yalu River Estuary Coastal Wetlands is not incredibly high, mainly due to the limitation of a lack of yield per unit culture area. The average carbon sink conversion ratio of cultured shellfish farmed in coastal wetlands was 10.92 ( Figure 5 ), which was not only higher than the average for Liaoning Province (Shao et al., 2019b) but also higher than the average for other marine areas (Shao et al., 2018; Hong et al., 2022), indicating that the Yalu River Estuary Coastal Wetlands have a locational advantage in deploying carbon sink fishery activities.

Therefore, we determined that improving the yield of cultured shellfish or increasing the variety of shellfish with high-level carbon sequestration would be an important way to enhance the carbon sink capacity of cultured shellfish in coastal wetlands.

Between 2015 and 2019, the carbon sink value of cultured shellfish in the coastal wetlands of the Yalu River Estuary was approximately 63.71 million yuan, with an annual average of 12.748 million yuan. The average annual carbon sink value of farmed shellfish in Liaoning Province calculated using the same parameters was 1600 million yuan (Yu et al., 2020). Comparatively, the carbon sink value of cultured shellfish in the Yalu River Estuary Coastal Wetlands was approximately 7.97% of the average level in the province. Moreover, shellfish aquaculture in the coastal wetlands of the Yalu River Estuary was mainly beach cultivation, which had little impact on the ecological landscape and caused little damage to the coastal wetland ecosystem.

Suppose fisheries carbon sink projects are included in the China Certified Emission Reduction (CCER) mechanism in the future. In that case, shellfish aquaculture in the Yalu River Estuary Coastal Wetlands can be an effective way to help achieve carbon peaking and carbon neutrality goals.

At the same time, The Yalu River Estuary Coastal Wetland National Nature Reserve, as a trading entity for carbon sink property rights in marine fisheries, can receive over 10 million yuan annually for wetland restoration and maintenance, ecological replenishment, wetland ecological monitoring, etc. in protected areas, which is basically equivalent to the central government wetland subsidy funds received by Dandong in 2021 (Liaoning Forestry and Grassland Bureau, 2022), providing strong financial support for achieving coordinated development of fishery economy and ecology.

In terms of aquaculture species, R. philippinarum was the largest contributor to the annual average carbon sink value, accounting for approximately 84.36%, followed by S. constricta, at 9.65%. On average, S. constricta had an annual carbon sink value per unit aquaculture area of 1,537 yuan/hm², which was second only to that of G. gigas. The culture of M. meretrix had the lowest contribution to the average annual carbon sink value, at only 0.94%. G. gigas has been cultured since 2018, and the breeding scale is generally very small. Despite its high average annual carbon sink value per unit area, it has contributed little to the cultured shellfish’s total carbon sink value ( Figure 6 ). At present, the quantity of shellfish larvae from R. philippinarum in the coastal wetlands of the Yalu River Estuary accounts for more than 80% of the total amount of larvae (Yu, 2022), and there is still potential to adjust its culture structure from a carbon sink point of view. Consequently, an in-depth quantitative study of the carbon sequestration mechanism of cultured shellfish in coastal wetlands and a further accurate accounting of the carbon sink value of cultured shellfish may provide a foundation for enhancing the carbon sink value of fisheries, formulating a coastal wetland resource protection plan, and maximizing ecosystem service functions.

Based on Tapio’s decoupling model, the decoupling relationship between the carbon sink of cultured shellfish in the Yalu River Estuary Coastal Wetland and the seawater economy of the region is shown in Table 3 , and there are six different types of elasticity characteristics that define the relationship: strong decoupling, weak decoupling, recessive decoupling, expansive negative decoupling, strong negative decoupling, and expansive coupling. Among them, R. philippinarum had the most ideal state of decoupling between the carbon sink and the economic growth of cultured shellfish. As the main aquaculture species in the coastal wetlands of the Yalu River Estuary, although R. Philippinarum is not optimal in terms of yield per unit area and carbon sequestration, it is yet the optimal species for the coordinated development of carbon sink fisheries economy and ecology in response to the national blue carbon strategy.

The carbon sinks of M. meretrix and S. suberenata had a more ideal decoupling state except in 2016. In contrast, the carbon sink of S. constricta with the economic growth rate was in the least ideal state or undesirable state, except in 2016. The carbon sink of G. gigas also showed an undesirable state.

In general, the elastic characteristic frequency of different cultured shellfish was similar, although it may be caused by the accuracy of data measurement or diversity of data sources, that still indicating that the decoupling relationship between the cultured output value and the shellfish carbon sink was not close and that the overall decoupling state was relatively unstable. Especially with the increased cultivation area and yield of S. constricta and G. gigas, their decoupling elasticities represent expansive coupling and strong negative decoupling, respectively, indicating that although they have a strong carbon sink capacity, the environmental costs of culturing these species are higher than the economic benefits.

Therefore, for S. constricta and G. gigas, it is necessary to optimize the aquaculture production process, and reduce unnecessary energy consumption and carbon emissions for promoting the decoupling of these types of shellfish aquaculture from carbon emissions. It is necessary to take the advantages of high carbon sequestration capacity shellfish varieties to continuously improve carbon sink capacity for the farmed shellfish in the Coastal Land of the Yalu River Estuary.