Numerical simulation of dissolved aquaculture waste transport based on water circulation around shellfish and salmon farm sites in Onagawa Bay, Northeast Japan

The Marine Environmental Committee (MEC) ocean model was applied to Onagawa Bay to examine the bay-scale water circulation and to evaluate the current configuration of aquaculture facilities that were devastated by the 2011 Tohoku earthquake. In this study, the drag forces of aquaculture facilities and the blocking effects of both stocked fish and shellfish were integrated to MEC ocean model. The model was validated with long-term observations regarding tide level, flow velocity, and water quality. In summer, the remote water discharge from the Kitakami river significantly affected the density-driven currents at the surface in Onagawa Bay, whereas, in winter, wind stresses by the prevailing monsoon altered the tidal level fluctuations as well as influenced the surface water circulation. The model also revealed the drag forces of the aquaculture facilities exerted a limited influence on the bay-scale water circulation. The dispersal of dissolved aquaculture waste was further studied by a tracer method. The degree of waste accumulation was geographically different in Onagawa Bay, and a reallocation of the aquaculture facilities between the southern bay and the western coast area may mitigate the accumulation of aquaculture waste. Overall, a further ecosystem study on material circulation is required to incorporate the effects of interactive processes between farmed species and sediments below and/or the water column around aquaculture facilities.