Effects of Sea Water Physical Condition on a Population of the Clam Ruditapes philippinarum.

Physical conditions of sea water strongly influence growth and survival of the clam Ruditapes philippinarum; however, quantitative analysis of this influence has not been fully made. In this paper, effects of some physical condition (vorticity, tidal current, and waves) on population characteristics of the clam (recruitment, growth, and passive emigration) on the tidal flat and shoal of Banzu, Tokyo Bay, are reported. The vorticity (an indicator of shear, such as that caused by tidal fronts or vortices) tended to be larger offshore. Abundant recruits (<1mm in shell length) were observed on offshore shoals. A positive correlation was observed between vorticity during the spring ebb tide and density of clam recruits. This suggests that planktonic larvae of the clam were transported onto the tidal flat during the flood tide, then settled on shoal bottoms where a large shear was induced during the ebb tide. A higher condition factor value and lower sharpness index were observed in clams on bottoms where the tidal current velocity was higher, suggesting that current velocity positively affects growth of the clam. During winter, most monitored clams seeded on the bottom (about 28 mm in shell length) were found to have emigrated under the strongest wave condition, which was associated with sediment erosion. Hydraulic model test indicates that the presence of layer culture nets decreases wave action. During the last two decades, the number of rows of layer culture structures on the lowest part of the tidal flat has decreased. This may lead to a decline in clam population during winter, due to a decreased capacity for wave buffering.