Suitable oyster culture density in Oginohama Bay, Miyagi, Japan

We investigated regular oyster dietary conditions, and shellfish and periphyton growth from 2013 to 2014 in Oginohama Bay, Miyagi Prefecture, Japan. We calculated phytoplankton biomass in the aquaculture areas minus the total phytoplankton filtration by oysters and periphyton. We estimated that the suitable oyster culture density is when that value is >0. The mass balance of phytoplankton biomass for food availability increased from January through April, and decreased from May to June. Although the mass balance of phytoplankton biomass fluctuated after June, no significant changes were seen until December. Therefore, the risk of a decrease in oyster food was thought to increase after June. The mass balance of phytoplankton biomass in both low (360 ropes x 25masters x 10ind./master) and high (400 ropes x 30 masters x 15ind./master) culture density (approximately 450 oyster longline facilities) was always >0 after the tsunami. The number of longline facilities before the 2011 tsunami (approximately 1,100) did not negatively impact the oyster food supply if culture density was low. However the mass balance of phytoplankton biomass under high culture density before the tsunami was calculated as negative from June to August and from October to November. We speculate that an increase in the number of longline facilities and high-density culture would result in a decrease in oyster food. Food availability under low culture density before the tsunami (1,118 longline facilities x 360 ropes x 25 masters x 10 ind./master) was the most suitable, because the mass balance of phytoplankton biomass was >0, and oyster density was the highest of the four conditions.