Mariculture structure adjustment to achieve China's carbon neutrality and mitigate climate change

China is responsible for the biggest shellfish and macroalgae production in the world. In this study, comprehensive methods were used to assess the CO₂ release and sequestration by maricultured shellfish and macroalgae in China. Through considering CaCO₃ production and CO₂ release coefficient (Φ, moles of CO₂ released per mole of CaCO₃ formed) in different waters, we find that cultured shellfish released 0.741 ± 0.008 Tg C yr^-1 through calcification based on the data of 2016-2020. In addition to calcification, maricultured shellfish released 0.580 ± 0.004 Tg C yr^-1 by respiration. Meanwhile, shellfish sequestered 0.145 ± 0.001 and 0.0387 ± 0.0004 Tg C yr^-1 organic carbon in sediments and shells, respectively. Therefore, the net released CO₂ by maricultured shellfish was 1.136 ± 0.011 Tg C yr^-1, which is about four times higher than that maricultured macroalgae could sequester (0.280 ± 0.010 Tg C yr^-1). To achieve carbon neutrality within the mariculture system, shellfish culture may need to be restricted and meanwhile the expansion of macroalgae cultivation should be carried out. The mean carbon sequestration rate of seven kinds of macroalgae was 174 ± 6 g m^-2 yr^-1 while some cultivated macroalgae had higher CO₂ sequestration rates, e.g. 356 ± 24 g C m^-2 yr^-1 for Gracilariopsis lemaneiformis and 331 ± 17 g C m^-2 yr^-1 for Undaria pinnatifida. In scenario 0.5 (CCUS (Carbon Capture, Utilization and Storage) sequesters 0.5 Gt CO₂ per year), using macroalgae culture cannot achieve China's carbon neutrality by 2060 but in scenarios 1.0 and 1.5 (CCUS sequesters 1.0 and 1.5 Gt CO₂ per year, respectively) it is feasible to achieve carbon neutrality using some macroalgae species with high carbon sequestration rates. This study provides important insights into how to develop mariculture in the context of carbon-neutrality and climate change mitigation.