| China is a major global shellfish aquaculture country, with a total national shellfish aquaculture output of 16,659,000 tonnes in 2024, of which marine aquaculture accounted for 98.81%. Oysters, as the main farmed species, have an annual output of 6,671,200 tonnes, accounting for 40.52% of the total production of marine shellfish farming. Due to the over-expansion of some traditional aquaculture areas, the ecological carrying capacity has been exceeded and the phytoplankton biomass available for oyster feeding has been reduced, resulting in lower fattening, lower quality and market price, and even higher mortality rate, so exploring new oyster fattening sites and new fattening methods has become an important way to solve the current problems in the industry. In order to investigate the feasibility of C. gigas pond fattening, a self-developed oyster pond culture device was used to enhance the water exchange around C. gigas by using the ‘wind-generated flow’ generated by the natural wind disturbance on the surface of the pond to increase the bait delivery efficiency to provide sufficient bait for the C. gigas, and to try to fertilize the diploid C. gigas, which have poor fattening degree, cultured in the sea area in the shrimp and crab ponds. We attempted to fatten diploid C. gigas with poor fattening in marine culture in shrimp and crab culture ponds. From November 2023 to May 2024 (the experiment was suspended during the winter freezing period), we went to the experimental ponds every 20 days to collect samples, and used the WTW multi-parameter water quality analysis to measure the water temperature, salinity, pH and dissolved oxygen; we used a water collector to collect 2 L of water samples from each of the five points at the four corners of the ponds (5-10 m away from the shore) and the centre of the ponds, and then put them into water sample bottles, and then we referred to the Code of Practice for Marine Surveys, the water samples were fixed with 5% formaldehyde solution on site and brought back to the laboratory for measuring nutrient salts, chlorophyll a and particulate organic matter (POM). Three different sizes (S, M, L) of C. gigas were set up for the determination of growth and physiological indexes, with average wet weights of 54.86 ± 3.26 g, M: 83.81 ± 3.94 g, and L: 127.46 ± 8.64 g, respectively, and were cultured in three mixed groups in oyster pond culture devices. For the physiological indexes, eight replicates and one control were determined for each specification treatment group, and a flow-through device consisting of buffer box, flow-through tank, and rectangular plastic box was used for feeding physiology and respiratory metabolism measurements; during the experimental period, a grapple-type mud collector was used to collect the sediment of ponds underneath and around the aquaculture devices at the aquaculture site, as well as to collect the artificial feed, suspended particles, and feces of C. gigas, which were used to determine the sediment of aquaculture ponds. The results showed that the sediment of the ponds was collected in different months. The results showed that there were no significant differences (P > 0.05) in water temperature, salinity, pH, dissolved oxygen content, chlorophyll a, and particulate organic matter content of the culture ponds in different months, but there were significant differences (P < 0.05) in PO43--P, NO2--N, NO3--N, and NH4+-N in the culture ponds in different months. The differences in fecundity of the three sizes of oysters in the same month were not significant (P > 0.05), but the differences in water filtration rate, droppings rate, oxygen consumption rate and ammonia discharge rate were all significant (P < 0.05), while the differences in fecundity of the same sizes of oysters in different months were significant (P < 0.05), and the differences in water filtration rate, droppings rate , oxygen consumption rate and ammonia discharge rate were all significant (P < 0.05). The energy balance equation showed that all three sizes of oysters used the most energy for growth in May, with the highest percentage of 48.64%; after 6 months of cultivation, the fattening degree of oysters increased by 31.7%, and the fastest growth rate of large-size oysters was 55.1%; Comparing the same batch of oysters from ponds and the same batch of oysters from the sea area in the same period of time, it was found that the survival and fattening degree of oysters from pond culture were higher than that of oyster cultivation in the sea area (P < 0.05); through the analysis of organic matter sources in the pond sediments, it was found that the further away from the aquaculture unit, the higher the contribution of suspended particles, while the opposite was true for artificial feed and feces. The present study showed that C. gigas are more effective for fattening in marine shrimp and crab ponds, and it is recommended to start the pond fattening production in autumn from mid-September to mid-October, and in spring from mid-March to mid-May and to select C. gigas with larger sizes, so that the desired degree of fattening can be reached after 1-2 months. |
