Characteristics of the flow field inside and around a square fish cage considering the circular swimming pattern of a farmed fish school: Laboratory experiments and field observations

The management of marine aquaculture requires an accurate assessment of environmental impacts to achieve an environmentally sustainable system. To study the influence of farmed fish schools on the flow field around a fish cage, a pseudo-fish-school structure model was developed to simulate the circular swimming patterns of farmed fish in laboratory experiments. The drag force acting on the fish cage and current velocities inside and around a modeled square fish cage were investigated. A field survey was also conducted. As a result, the non-uniformity and complexity of the flow field of a stocked cage were observed in laboratory experiments. Circular swimming patterns were found to have obvious influences on the flow field distribution. The drag force of the square fish cage decreased by 11.8% because of the presence of the fish-school structure. Based on the field measurements, the averaged velocity inside and downstream of the full-scale fish cage was reduced by 46.2% and 57.8%, respectively. An acceleration was also observed in the current velocity below the fish cage. The vertical mixing of seawater may be because of the water blockage resulting from the cage and the circular swimming pattern of farmed fish schools. The findings presented in this study can be used as benchmarks for numerical models in assessing the potential environmental impact of fish farm sites. • A pseudo-fish-school structure model is proposed to simulate the circular swimming patterns of a fish school in laboratory experiments. • Considering farmed fish behavior, the drag force of a square fish cage decreased by 11.8% compared with an empty fish cage. • The movement of the fish-school structure model had a strong influence on the flow field of the square fish cage. • An increase in current velocity in the bottom layer of water beneath the fish cages was observed through field measurements.