Projections of nutrient supply to a wakame (Undaria pinnatifida) seaweed farm on the Sanriku Coast of Japan

Abstract

AbstractWakame (Undaria pinnatifida) aquaculture is one of the major fishery industries on the Sanriku Coast of Japan. Low concentrations of nutrients in the autumn, when the germinated wakame sporophytes are installed at aquaculture facilities, induce defective growth, resulting in withering. To determine the characteristics of the nutrient variations in this region, we conducted hydrographic measurements off the coast of Iwate Prefecture. The measured results showed that in the autumn, vertical mixing pumped nutrients from the deeper layers to the mixed layer, where nutrients had been depleted during the summer. Based on these results, we developed a one‐dimensional model incorporating vertical mixing by atmospheric cooling to project the vertical distribution of the nutrient concentrations. By conducting ensemble calculations, we created a probability projection calendar for the nutrient supply up to 50 days ahead. This information was provided to wakame farmers before they installed the germinated sporophytes. The projected probability calculated from the initial conditions on 11 October 2016 was 0% until 25 October and gradually increased to 85% on 30 November. The nitrate concentration detected by an ultraviolet nitrate sensor moored at a wakame farm revealed that the nutrient concentrations reached a critical threshold on 18 November. The projected probability on that date was 75%; therefore, the projection was successful. The observed temporal evolution of the temperature, salinity, and nutrient concentrations was well reproduced by our model. The information obtained using this model is expected to reduce the risk of wakame withering and improve the efficiency of wakame production.