Mechanisms leading to the decline in Pacific oyster Crassostrea gigas seedlings in Matsushima Bay, Japan

In 2013, there was a significant decline in the wild seedling yield of the Pacific oyster Crassostrea gigas in Matsushima Bay, Japan. We used hydrographic observational data obtained from 2009–2014 to determine whether hydrographic conditions explained the low yield and the absence of pre-attachment stage larvae. In August 2013, low salinity water was distributed at the surface resulting in a density difference between the surface and near the seafloor of the bay exceeding 10 kg m−3. This difference generated a strong outward driving force of 10.0 × 10−3 N m−3. We hypothesize that the oyster larvae were transferred out of the bay before they could reach the pre-attachment stage. The low salinity distribution resulted from increased precipitation associated with stagnation of a stationary rain front. In addition to the outflow driven by freshwater inputs into Matsushima Bay, freshwater derived from the Naruse River, located on the east of the bay, flowed into the bay and formed the horizontal circulation with increased water exchange resulting in the acceleration of the outflow of oyster larvae. The influence of freshwater from the Naruse River has likely increased in the bay as a result of topographical changes caused by the 2011 Tohoku earthquake.