The effect of nutrients on the black degree of the dried seaweed produced in nori Pyropia yezoensis aquaculture in Ise bay and Mikawa bay during FY2018 aquaculture season

In nori Pyropia yezoensis cultivation in Mikawa bay, nutrient (PO4-P and DIN) levels were highest at the start of the aquaculture period, and gradually declined at the end of the aquaculture period in spring, until 2002. However, in recent years, a decrease in nutrient levels during autumn has tended to cause a drop in nutrient levels required for high quality nori in early January. This has resulted in the discoloration of nori and a shortened aquaculture period. Here, we investigated the effects of nutrient levels on number of joint sales, black degree, and successful bid price of dried nori produced in Ise and Mikawa bays during the FY2018 aquaculture season. Joint sales were previously conducted up to 11 times; however, due to the reduction in nutrient levels, the shipment ceased sales with the 6th or 7th joint sale in the eastern part of the Chita Peninsula and the islands, and sales concluded with the 8th or 9th joint sale in the southern part of the Chita Peninsula near the bay exit. In contrast, in Nishimikawa (northwest Mikawa Bay) and in the northwest Chita Peninsula, sales of the shipment ended with the 10th or 11th joint sale. Thus, joint sale numbers of the final shipments declined from the rear end of the bay to the bay exit in each bay. Nutrient levels showed a gradual decrease from the rear end of the bay to the bay exit in the latter half of the aquaculture period in each bay. The black degree of the 2nd and 3rd grade dried nori also showed a decline from the rear end of the bay to the bay exit in each bay, and the black degree and successful bid price of the 2nd and 3rd grade dried nori were positively correlated. These findings indicate that nutrient loss during the early second half of the aquaculture period leads to lower bid price and decrease in producer income.