Evaluation of seasonal variations on the growth features and removable carbon sinks of cultured mussels Mytilus coruscus in Gouqi Island

Marine fishery carbon sinks are an important component of blue carbon. Fishery activities such as shellfish farming are considered among the most promising carbon sink activities. However, precise assessment of carbon sinks in shellfish farming still requires further investigation. To explore the potential of carbon sink in shellfish farming, this study focused on the raft-cultured mussels Mytilus coruscus near Gouqi Island. We examined the seasonal variations in their growth characteristics and the community features of the epiphytic animals, as well as their correlation with environmental factors. Based on the biomass carbon density (BCD) of the cultured mussels and epiphytic animals, we evaluated the removable carbon sink. The results revealed seasonal, age, and regional differences in the tissue moisture, fatness, and gonad index of the mussels. Tissue moisture exhibited the following patterns: spring > winter > autumn > summer, age 1 > age 2+, and the center area > the periphery area, showing opposite trends to both fatness and gonad index. Additionally, the gonad index followed the pattern of age 4 > age 3 > age 2 > age 1. There are significant seasonal differences in the carbon content of the tissues and the shells, as well as the wet weight ratio of the tissues. Redundancy analysis (RDA) showed a positive correlation between tissue moisture and ammonia nitrogen. Carbon content of the tissues, fatness, and gonad index were positively correlated with environmental factors such as chlorophyll a, temperature, and total nitrogen. A total of 153 species were identified among the epiphytic animals, belonging to the phyla Coelenterata, Mollusca, Annelida, Arthropoda, Echinodermata, Nemertina, and Chordata. The main classes included Polychaeta, Malacostraca, Bivalvia, Gastropoda, Anzhozoa, Thecostraca, and Ascidiacea. Seasonal differences were observed in the abundance and biomass of epiphytic animals, with the highest biomass detected in spring, primarily consisting of Anthopleura midori, Amphibalanus reticulatus, Modiolus comptus, Menippe rumphii, and Halosydna brevisetosa. RDA analysis indicated that the biomass of the Malacostraca animals was positively correlated with chlorophyll a, temperature, total nitrogen, total carbon, salinity, and nitrite, while the biomass of Ophiuroidea, Anzhozoa, and Thecostraca animals was positively correlated with ammonia nitrogen and nitrate content. The BCD of the mussels was highest at 129.98 g C/kg in autumn and lowest at 108.29 g C/kg in spring. The BCD of epiphytic animals was highest at 6.85 g C/kg in spring and lowest at 1.38 g C/kg in summer. By harvesting the mussels and epiphytic animals, the maximum BCD that can be removed from the water can reach 135.34 g C/kg. In 2021, farming M. coruscus in Gouqi could harvest approximately 3.02×10⁴ tons of carbon, generating an economic value of carbon sink about 10.49 million yuan. Our results provide an important theoretical basis and practical significance for maximizing the removable carbon sink capacity through farming M. coruscus, as well as for deepening the understanding and evaluation of the carbon sink effects of shellfish farming.