In Japan, almost 55 % of total yellowtail (Seriola quinqueradiata) production are cultured in fish cages. The square fish cages are typically used for farming yellowtail. The characteristics of the flow field around and inside the stocked fish cage are closely related to the water exchange efficiency. However, the interaction between fluid and fish cage with stocked fish is extremely complex, including fluid and structure, as well as fluid and fish swimming behavior. There are limited literature describing hydrodynamic characteristics between the fluid and fish cage with the stocked fish school. In addition, the on-current swimming pattern of fish schools was found toward the incoming flow in the previous studies, which is different from the circular swimming pattern commonly observed in the actual farming site. In this study, a new self-designed pseudo fish school structure model (PFS) was proposed to reproduce the five circular swimming patterns of farmed yellowtail, and to investigate the influence of fish school behaviors on the flow field inside and around a model square fish cage in laboratory experiments. In experiments, the randomly swimming yellowtails were imitated by standstill fish school structure models C0 and model L0; closely circular swimming yellowtails were imitated by revolving fish school structure model CR; and loosely circular swimming yellowtails were imitated by revolving fish school structure model LR.The results showed that the drag force acting on the square fish cage increased with the increase of the current speed for all fish school swimming patterns, but no clear difference was observed between the fish school swimming behavior patterns. At a maximum current speed of 45.0 cm/s, the drag force of the square fish cage without PFS was 9.8 N. Overall, the drag force of the square fish cage considering the farmed fish behavior decreased by 11.8%, compared to the drag force of the fish cage without PFS. According to the current speed profile through the center line of square fish cage, the current speeds inside and downstream of the fish cage increased almost linearly with increasing current velocities. Compared with the case of the fish cage without PFS, the current speed inside the cage under C0, CR, L0 and LR conditions changed by 10.8%, 7.9%, 65.8% and 28.8%, respectively. In addition, compared to the case of the fish cage without PFS, the current speeds under C0, CR, L0 and LR conditions decreased by 89.8%, 16.3%, 58.2%, and 31.9%, respectively, at 16.0cm downstream from the fish cage, and decreased by 69.2%, 19.4%, 62.7% and 26.3%, respectively, at 63.6cm downstream from the fish cage. Furthermore, the current speed distribution and relative horizontal turbulence intensity distribution inside and around the fish cage under different fish school swimming pattern was discussed. In the future, we will use live fish to conduct experiments to evaluate fish school models.

Flow Field Inside and Around a Square Fish Cage Considering Fish School Swimming Pattern

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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