GROWTH CHARACTERISTICS, CHANGES OF MORPHOLOGY AND BODY COMPOSITION OF HONGKONG OYSTERS DURING OFF-SITE FATTENING

In order to provide the basis data for further study of carbon and nitrogen utilization in integrated culture pond ecosystem, the growth characteristics, morphology and body composition changes of Hongkong oysters during off-site fattening were studied. Hongkong oysters Crassostrea hongkongensis (2-year-old) with an initial weight of 68.00 g were obtained from Maowei Sea, Qinzhou, Guangxi, and were fattened on floating rafts for 44d in a fish-shrimp polyculture pond in Huguang Town, Zhanjiang. The oyster samples were collected during fattening regularly, and the morphological indicators, growth indicators, gross composition of soft tissue, carbon and nitrogen stable isotope values (δ¹³C and δ¹⁵N) of oyster samples were determined, and the correlation of these indicators were analyzed. The results showed that, after 44d fattening, the shell height, soft tissue quality, meat yield and fat content of 2-year-old oysters were significantly increased (P<0.05), the protein and ash contents were significantly decreased (P<0.05), while the shell length, shell width, body mass and moisture content did not showed significant difference (P>0.05). The δ¹⁵N, δ¹³C of the soft tissue and shell δ¹³C values of oysters all decreased significantly (P<0.05), and the δ¹⁵N and δ¹³C values of the soft tissue were significantly different from the initial samples after 16 days of fattening (P<0.05), which indicating that oysters could quickly utilize the abundant food sources of fattening pond, and obtained rapid grow of soft tissues. During the fattening process, the meat yield of oyster was significantly positively correlated with the mass of soft body (P<0.05), extremely significantly negatively correlated with shell δ¹³C, δ¹⁵N, δ¹³C, N% and C% of soft body (P<0.01), and had a extremely significant negative correlation with shell length, shell width, shell height and body mass (P<0.01). However, there was no significant correlation between the meat yield of oyster and the shell length, shell width, shell height and body mass (P>0.05). During the fattening process, C% of oyster shell was positively significant correlated with N% and C% of soft body (P<0.05), and extremely significant positive correlated with δ¹⁵N of soft body (P<0.01); while C% of soft body tissue was extremely significant negative correlated with meat yield (P<0.01), significantly positive correlated with shell C% and δ¹³C of soft body (P<0.05), extremely significant positive correlated with the soft body δ¹⁵N and the soft body N% (P<0.01). These correlations indicated that monitoring changes in these indicators can better determine whether fattening oysters are adapted to the fattening environment and have access to sufficient food for rapid growth. The rapid growth of oysters during the fattening process resulted in a significant decrease in the carbon content of oysters, which must be taken into account when calculating the carbon sequestration benefits of oysters. In addition, the results show that the ash content and carbon and nitrogen stable isotope characteristics of Hongkong oyster are easily affected by short-term off-site fattening, this influence must be considered when using inorganic elements and stable isotopes traceability technology for geographical origin traceability of Hongkong oyster.