Numerical analysis of particulate organic waste diffusion in an aquaculture area of Gokasho Bay, Japan

Particulate organic waste, originating from fish cultured in cages, often brings about negative effects on the structure of bottom sediments. These effects result in deterioration of the aquatic environment. In the present study, a three-dimensional numerical model was coupled with developed submodels of fish cage drag and aquaculture waste diffusion to simulate the hydrodynamic environment and distribution of particulate organic waste. Numerical simulation results showed that fish cages exerted an influence on both the velocity and direction of water current; the reduction of water current velocity was a maximum of approximately 38%. The contribution of fish fecal matter was dominant during winter (ca. 80%), whereas waste fish feed was the main source in other seasons. The distribution of organic waste near the sea bottom indicated that organic waste not only loaded mainly on the sea floor just beneath the fish cages but also diffused towards the coastlines.