Hiroshima Bay is the top oyster production site in Japan, producing ca. 60% of the total national production. However, the production has been decreased from its peak production of 30 years ago with fluctuation in recent years. Notably, the lowest collection of seed larvae was recorded in 2017. This study aims to compare the oyster production in recent years in terms of the nutrient load into the bay.
Oyster larvae data cited from the report of Hiroshima Fisheries Promotion Center. The samples were collected from 0–5 m depth at five stations during the same season (June-August) of each year and counted the number for each size category under a microscope. The size categories are small (>90–150 µm), medium (150–210 µm), large (210–270 µm), and settling size (>300 µm). In addition to observing environmental conditions during summer (June to September) of the spawning season in 2016 to 2018, we calculated the material flow through prey-predator interactions each year using a numerical model with 25 compartments.
Under the oligotrophic condition of the bay, particularly the nutrient load was lowest in 2017. There was no significant difference in the number of small-sized oyster larvae, implying that the adult oysters were in good condition to spawn. However, the survival rate of oyster larvae during the small-sized to settling sized development was lowest in 2017. It is concluded that the primary cause of the low collection of seed larvae is insufficient food supply due to low nutrient load to the bay.