Investigation of Water Surface Fluctuation and Flow Velocity Inside a Floating Closed Flexible Membrane Fish Cage Under Forced Oscillation Experiment

Abstract A floating closed fish cage is required for environmental issues such as SDGs (Sustainable Development Goals). The flexible membrane fish cage generally exhibits a reduced motion response. However, the sloshing-induced fluctuations in the internal water remain unclear, despite their critical importance for fish welfare and waste and bait removal in closed fish cages. In the present study, we investigated the water flow velocity, water surface level changes, and fish school behavior in a closed flexible fish cage by forced oscillation experiments. The target fish cage is circular, representing a real fish cage with dimensions of 25 m in diameter, a sheet height of 12.5 m, and a free board of 2.5 m. A 1/50 scale model was designed and attached to a forced oscillation device. To analyze the internal water movement in waves, we reproduced pitch motion using the forced oscillation device. We measured pitching moment, and flow velocity at a depth of 6 cm and 12 cm below water surface, and water level fluctuations on both sides and in the center of the fish cage. 23 zebrafish Danio rerio were placed inside the fish cage, and their behavioral characteristics were observed. As a result, the pitching moment exhibited an increasing trend with increasing pitch oscillation amplitude, peaking at a pitch oscillation period of 1.05 s. Simultaneously, the internal water surface exhibited pronounced and highly nonlinear fluctuations at the period of 1.05 s. The internal water level at the sides followed a similar trend in both magnitude and change tendency, showing symmetry. The water level measured at the center was significantly lower than that on the sides. Within the measurement range, the flow velocity increased as the oscillation amplitude increased, reaching a maximum of 10.8 cm/s. Despite the complexity of zebrafish fish school behavior under forced oscillation conditions, the fish tended to aggregate in the deeper parts of the net cage. Furthermore, the presence of fish had minimal impact on the pitching moment results. Overall, these findings are expected to provide references for the design and operation of the closed fish cage system.