Post-tsunami Oyster Feeding Environment in Nagatsuraura Bay for Three Years

Abstract

We investigated water temperature, Chlorophyll a (Chl a) concentration, and oyster growth from April to December in 2014, 2016, and 2017 in Nagatsuraura Bay, Miyagi Prefecture, Japan. We also calculated the remaining phytoplankton biomass by subtracting total filtration of oysters in aquaculture areas. We estimated that suitable oyster culture density is when that value is positive. Water temperature was high in summer, exceeding 20 °C from mid-June to mid-September. Chl a concentration gradually increased from April through July, and remained high from July through September, before decreasing after October. Maximum Chl a exceeded 9 µg/L in the water column. In Nagatsuraura, which is a closed bay, the periods of phytoplankton bloom from Spring forward were longer than in open or semi-open bays. Therefore, Nagatsuraura oysters grow more quickly than those in open or semi-open bays. Oyster growth varied with the season. Although periodic weight loss by egg-laying was predicted from July to October, oyster weights tended to increase easily during this season compared with other periods. The mass balance of oyster feed in the aquaculture was calculated based on phytoplankton amount > 0 in most periods. As values were phytoplankton biomass in an aquacultural area > amount required by oysters, we inferred that Nagatsuraura is not deficient in oyster food. With the post-tsunami retirement of many fishery employees, and reduction in the number of oyster culture companies, the total quantity of oysters in the aquaculture grounds has decreased. As a result, the aquaculture grounds are not currently overcrowded.