Biological productivity evaluation at lower trophic levels with intensive Pacific oyster farming of Crassostrea gigas in Hiroshima Bay, Japan

Enclosed seas are suitable for bivalve farming due to high primary production, which provides food sources. The impact of oyster farming on the biological productivity of lower trophic levels was evaluated in Hiroshima Bay. The area-weighted mean primary production in the estuary (northeastern bay; NB) was 1.1 to 2.1 times higher than that of the offshore area (southwestern bay; SB) in all four seasons. In contrast, the area-weighted mean secondary production by net zooplankton in the NB was lower than that of the SB, except in August. The areaweighted mean secondary production by oysters in the NB was 2.2 to 2.8 times higher than that of the SB in all four seasons, and exhibited a similar spatial pattern to that of the primary production. The primary production was more efficiently utilized and transferred to secondary producers in the SB (16.7% on average) than in the NB (9.4% on average). This study provides guidance for oyster farming in Japan.