Modeling bottom-up and top-down controls on the low recruitment success of oyster larvae in Hiroshima Bay, Japan

A measured reduction in the phosphorus load in Hiroshima Bay, Japan, is suspected to be the root cause of the declining success of oyster spat collections and rates of oyster production over the past 30 years. The lack of phosphorus leads to an inadequate abundance of phytoplankton as food sources which, along with competition among various filter feeders, might have generated the poor culture conditions. To understand how prey-predator interactions, including those of cultured oysters, are functioning, we developed a prey-predator model. Phytoplankton in different size categories were quantified, with a particular focus on phytoplankton smaller than 5 mu m, which represent suitable food sources for oyster larvae during the planktonic phase. Filter feeding animals that compete with oysters were also identified and counted. Our numerical model consisted of 25 compartments, including inorganic/organic substances, phytoplankton, zooplankton, oysters, other filter feeding animals, and fish. The model outputs reproduced the observed temporal variation of the various parameters well, including the different size categories of oyster larvae from just spawning to settlement. Sensitivity analyses showed that an increase in the dissolved inorganic phosphorus load to 10 times than present value (0.2 mg P m(-3) d(-1)) enhanced phytoplankton production, including that of small-sized phytoplankton, facilitating an increase of 51% in the successful settlement of oyster spat. In conclusion, the recent low success rates in the settlement of oyster larvae appear to be driven by insufficient quantities of the phytoplankton on which they feed. This lack of phytoplankton stems mainly from the reduction in the nutrient load in addition to competition between oysters and various other filter feeders for these food resources.