The sea area of Changhai County in Dalian City is a typical floating raft aquaculture area, located in Liaoning Province, China, where a key issue in determining the scale and spatial layout of the floating raft aquaculture is the assessment of the impact of aquaculture activities on the hydrodynamic environment. To address this issue, we established depth-averaged two-dimensional shallow water equations and three-dimensional incompressible Reynolds-averaged Navier–Stokes equations for the open sea area described in this paper. The impact of floating rafts for aquaculture on hydrodynamic force was reflected in the numerical model by changing the Manning number, where scenarios with different aquaculture densities were taken into account. Finally, the water exchange rate of the floating raft aquaculture area in the study area was calculated. It was found, through a comparison between the simulated value and the measured value obtained via layered observation, that the two values were in good agreement with each other, indicating that the model exhibits great accuracy. In addition, the calculation results for scenarios before and after aquaculture were compared and analyzed, showing that from low-density to high-density aquaculture zones, the variation in flow rate was greater than 80% at the peak of a flood tide. The water exchange rates of the water body after 1 day, 4 days, and 8 days of water exchange were also calculated, and the results show that they had been reduced by 17.92%, 13.59%, and 1.63%, respectively, indicating that the existence of floating rafts for aquaculture indeed reduced the water exchange capacity of the water body. The model described in this paper can serve as a foundation for other studies on aquaculture in open sea areas, and it provides a theoretical basis for the scientific formulation of marine aquaculture plans and the rational optimization of the spatial layout.

A monthly survey of the feeding selectivity of Ruditapes philippinarum in the Yalu River Estuary in 2020–2021 was conducted using high-throughput sequencing identification and visual grading technology. The results showed that the most-dominant species in the water of the shellfish culture area and in the stomachs of R. philippinarum was Karlodinium veneficum in those years. The selectivity index (E) indicated that R. philippinarum avoided consuming Bacillariophyta, Chrysophyta and Cryptophyta throughout the year and preferentially consumed Dinophyta and Chlorophyta. In 2020, the annual average biomass of Dinophyta, Bacillariophyta, Chlorophyta, Dictyochophyta, Cryptophyta and Chrysophyta in the stomach contents of R. philippinarum was 54:14:16:1:10:4; it was 41:12:28:0:1:17 in 2021. The annual average biomass ratio of picophytoplankton, nanophytoplankton and microphytoplankton in the stomachs of R. philippinarum was 13:48:39 in 2020; it was 14:66:20 in 2021. R. philippinarum actively fed on nanophytoplankton and avoided picophytoplankton. Among the phytoplankton of different sizes and groups that R. philippinarum prefer to feed, chemical oxygen demand (COD) and organic phosphorus (DOP) have a significant negative effect on the nanophytoplankton community, pH has a positive effect on the Dictyochophyta community and COD and the inorganic nitrogen to phosphorus ratio (DI-N/P) have a significant positive effect on the Chlorophyta community.

Floating raft aquaculture has gradually become a mainstream aquaculture model in the waters of Changhai County, Dalian. To quantitatively describe the impact of floating raft aquaculture facilities on the hydrodynamic environment of nearby sea areas, in this study, we took a single floating raft aquaculture structure as the research object and built a numerical prediction model for water flows passing through the floating raft aquaculture structure using a six-degree-of-freedom VOF (volume of fluid) multiphase flow simulation method based on an overset moving mesh system. Then, we verified the numerical model by utilizing oblique hydraulic jumps and water flows passing through a submerged bar. As shown by the findings, the simulated values are in good agreement with the theoretical solutions and measured values, indicating that the model features high precision and great stability. The impact of the raft area on the hydrodynamic force was introduced into the source term of an equation for consideration. In order to further determine the hindering effect of the raft body on the water body, transport equations and the tracer method were used to simulate the impact of floating raft aquaculture facilities on the water exchange performance of nearby sea areas. This study shows that the VOF multiphase flow model can be easily and accurately applied to studies on floating raft aquaculture, which can greatly reduce the limitations of experiments that utilize pure hydraulic models, wherein the impacts of floating raft aquaculture facilities on hydrodynamic force are generally considered simply based on observations, water roughness or the secondary drag force coefficient, thereby effectively improving the scientific understanding of the physical mechanism involved in floating raft aquaculture.