Environmental Conditions and the Role of Plankton Community as Prey for Oysters in Oyster Farming Areas in Oginohama Bay (A Branch of Ishinomaki Bay), Northern Japan

Abstract

Aquaculture of the Pacific oyster Crassostrea gigas is one of important fisheries industries in Japan. Since production of cultured oysters mainly depends on the natural productivity of the ecosystem, information on the mechanism of the lower trophic ecosystem in oyster farming areas is essential to sustain the high productivity of oyster aquaculture. In the present study, characteristics of environmental conditions, primary production and the grazing loss by microzooplankton in an oyster farming area in Oginohama Bay (Miyagi Prefecture), Japan, were clarified by partly comparing with data in Hiroshima Bay. Then, based on accumulated data, carbon energy flow from primary production through the planktonic food chain to oysters were estimated. In Oginohama Bay, occurrence of phytoplankton blooms in spring and autumn, low concentration of nutrients in early summer, and high abundance of microzooplankton in early spring and late summer were observed. Among environmental parameters in the bay, the annual mean temperature, nutrient concentration and abundances of prey organisms for oysters were lower and salinity was higher than the values in Hiroshima Bay. Primary production in Oginohama Bay was high in spring and autumn, and low in summer and winter. Low primary production in summer was probably due to low nutrient concentrations. Grazing loss of phytoplankton by microzooplankton often exceeded the primary production in summer. The annual production of oyster and plankton components and estimated energy flow between them showed that the transfer efficiency from primary production to oyster production was ca. 4% and also implies that the energy flow from microzooplankton to oysters is an important pathway as well as that from phytoplankton.