Factors Influencing the Nutritional Characteristics of Mytilus galloprovincialis from Different Culture Areas in the Haizhou Bay

Mytilus galloprovincialis has high-protein and low-fat contents, as well as medicinal and therapeutical values. It is one of China's most important mussel breeding species, with strong adaptability, mature and straightforward breeding technologies, and high yield. It mostly feeds on small plankton, with main nutrients varying seasonally with different growth stages, and the individual biochemical components of the same species also differ geographically. The change of bait is also an important factor in changing the volatile substances of this shellfish. The mussels' nutritional flavor characteristics have been a concern of national scholars. However, the nutritional and flavor characteristics of M. galloprovincialis in different cultural areas have not been reported yet. Haizhou Bay is the main M. galloprovincialis aquaculture area in China, with an annual output of more than 150 000 tons and a value of 300 million yuan. Mussels cultivated in five sites (H1 to H5) in this region were collected once every month, from January to March. H1 is one nautical mile off the coastline; H2 is 3, H3 is 7, H4 is 13, and H5 is 21 nautical miles off the coastline. The physical and chemical factors of these water environments and the protein, fat, total sugar, and glycogen contents within the soft tissues of the sampled mussels were measured. The results showed that the M. galloprovincialis nutrient content varied among sampling time and sites and was directly related to the gonad development. The H1 condition index increased continuously from January to March, and the mussels' highest condition index in other areas was found in February. The highest total sugar content of mussels collected in all regions was observed in February, while for the H4 and H5 areas, it decreased significantly in March and was 62.7% and 61.6% lower than that in February, respectively. These changes were mainly caused by M. galloprovincialis entering its breeding season in March. The total content of 16 amino acids and the content of flavorful amino acid at H5 were significantly higher than those in other regions in January (P < 0.05). However, these amino acid levels decreased significantly and became the lowest in March for the mussels sampled in the H5 region. From January to March, the glycogen content in the mussel mantle increased firstly and then decreased, while its highest level was found in mussels collected in H5 in March. Most of the cultivated M. galloprovincialis are sold in winter. Here we observed that the gonad development and nutrients and flavor amino acids levels of the offshore mussels were different from those of the nearshore mussels, and the offshore mussel completed the material and energy storage earlier and entered the breeding period sooner, suggesting that the best marketing time is different for the studied areas. Differences in the mussels´ nutrient reserve were mainly related to temperature and feed density. The water temperature and the Chlorophyll-a (Chl-a) concentration tended to increase with the offshore distance rise, leading to the rapid development and early marketing time of M. galloprovincialis in the studied offshores. In February, mussels in the H5 area were rich in nutrients and flavor materials; therewith, they could be marketed one month earlier than in other areas. Early listing of the offshore cultured mussel can prolong its market supply time and reduce the competitive pressure caused by centralized listing. In addition, the offshore cultured mussel has a good market potential because of its short breeding cycle, wide breeding space, and good taste. In recent years, thanks to the development of internet technology, online sales have become an important sales channel in all industries. This way avoid the third-party vendor, wherein the producer can sell directly to customers with higher unit prices and a vast market. Combined with the advantages of offshore mussel cultivation, farmers can use online sales to increase their income further. With the increase in the density of offshore aquaculture in the Haizhou Bay, harmful algal blooms threaten the safety of shellfish products. Environmental problems in nearshore areas are the main factors hindering the favorable and productive development of the M. galloprovincialis industry. Therefore, the development of offshore aquaculture has great potential. Compared with the nearshore sea, the offshore sea has more extensive space, frequent water exchange, and larger environmental capacity. In order to improve the output of mussel aquaculture, New England began to transfer the aquaculture area from the nearshore to the offshore sea. Although the cost and technology needed for the offshore culture are significant, the comprehensive benefits of the offshore culture are still considerable. The offshore aquaculture development is conducive to optimizing the spatial structure of the marine industry, improving the overall efficiency of food production, alleviating its resource and environmental pressures, and cultivating new marine products with technological and industrial advantages. This study may provide supportive data for optimizing the spatial layout planning and harvesting time of M. galloprovincialis offshore cultivation