Feasibility of Integrated Oyster-Sea Cucumber Raft Culture

China is a major aquaculture country, with both the world´s largest aquaculture production and area, especially oyster aquaculture which accounts for 80% of the global production. However, its ascent has had numerous negative implications, requiring the development of more environmentally friendly aquaculture methods. The ecological farming model is gradually being acknowledged and encouraged as it is the result of in-depth investigations of marine ecosystems. "Integrated Multi-trophic Aquaculture" is a well-known environmentally friendly aquaculture model. The application effects have been outstanding, as it boosts high output per unit area, improves the marine environment, and improves material utilization. Filter-feeding shellfish are raised at levels that create considerable biodeposition. Particulate matter is transferred from the upper to the lower layers of the water body. Organic matter accumulates on the seafloor in the form of biological sediments, which badly influences the substrate environment, including releasing ammonia nitrogen, increasing dissolved oxygen consumption, and altering seabed biodiversity. Previous research suggests sea cucumbers absorb large amounts of organic matter-rich sediments, reducing the nutrient load caused by coastal shellfish and fish aquaculture. Therefore, a novel sustainable farming model based on the principle of multi-trophic integrated farming could be developed by using sedimentary sea cucumbers to feed on the biological sediments produced by filter-feeding shellfish. The purpose of this study was to investigate the possibility of an oyster-sea cucumber raft integrated culture. Sea cucumbers were stocked in oyster breeding cages. A raft-style integrated oyster-sea cucumber culture was attempted to improve the breeding method. This method allows oyster biological sediments to be utilized in situ, reducing oyster breeding density and maintaining economic benefits. This comparative culture experiment of integrated oyster (Crassostrea gigas)-sea cucumber (Apostichopus japonicus) raft culture utilized Sanggou Bay and Guazichang as representative oyster farming locations. We stocked C. gigas in the odd-numbered layers and A. japonicus in the even-numbered layers of the oyster cages in polyculture. Even-numbered layers had three levels of chassis: common aquaculture plate, holeless aquaculture plate, and holeless aquaculture plate with non-knot nets. The aquaculture plate was the first variable in the experiment. The stocking density of A. japonicus in each of the even-numbered layers were separated into three levels, 1, 2, and 4 ind./plate. The second variable in the experiment was stocking density of sea cucumbers. The experiment was simultaneously conducted at both sea locations. Therefore, the experimental design consisted of a three-factor and three-level experiment with a total of 18 treatment groups. During the experiment, we examined to content of: chlorophyll a, particulate organic matter, PO43–-P, NO2–-N, NO3–-N, and NH4+-N in both sea locations. The survival rate, growth performance, and condition of C. gigas and A. japonicus were compared. There was no significant difference in the contents of chlorophyll a or particle organic matter between the two marine areas (P > 0.05). There were significant differences in the four nutrient salt contents between the two locations (P < 0.05). There were no significant differences in the individual oyster weight or condition between the two locations (P > 0.05). Only the low-stocking density sea cucumbers grew, with individual weights over 25% higher than that of the high-density individuals. Individual sea cucumber weights and survival rates in the low density treatment groups were considerably higher than those in the high density treatment groups (P < 0.05). The performance of the holeless aquaculture plate considerably exceeded the common treatment group (P < 0.05). The holeless aquaculture plates with sea cucumbers at a density of 1 ind./plate achieved the highest results in this study. The chlorophyll in the sea area of Sanggou Bay remained mostly unchanged in this experiment. However, the chlorophyll in the water region of Guazichang reduced when compared with that of previous data. With the recent rapid growth in the oyster industry in Rushan City, the oyster output may have reached or possibly exceeds the area´s aquaculture capacity. We advise the oyster breeding density in the Rushan sea area to be reduced to lower the breeding risk for farmers while also promoting the breeding industry´s long-term viability. In this experiment, there was no significant difference in oyster growth across the treatment groups, indicating that the integrated oyster-sea cucumber raft culture mode can lower oyster density and reduce environmental impacts. Simultaneously, breeding high-value sea cucumbers compensates for the loss of breeding income induced by the lower oyster breeding density. When compared to bottom-seeded sea cucumbers, this raft cage mode has a higher level of safety and ease of harvest. This method can be used to replenish oyster growing zones with a high density of oysters to boost the aquaculture industry´s health and long-term development.