Year-to-year density fluctuations in clams Ruditapes philippinarum linked to wind-induced disturbance during juvenile to subadult stages

Year-to-year fluctuations in the densities of bivalve populations are often large, hindering short-term forecasting and control of their densities. Such hindrances would be mitigated by elucidating the (1) life stages and (2) environmental factors that strongly affect year-to-year density fluctuations. These were examined for the asari clam Ruditapes philippinarum on an intertidal sandflat near the mouth of the Midori River, western Japan, from 1992 to 2015. Analyses of the densities of juvenile (≈0-2 mm in shell length), subadult (≈5-15 mm) and adult (20-30 mm) clams collected twice a year and environmental data (sand grain size, air temperature, salinity, discharge of the Midori River and wind speed as an indicator of wave height) revealed the following results. (1) The fluctuation in adult density during one year (year j ) to the following year (year j +1 ) was mainly explained by the fluctuation in the subadult/juvenile density ratio during the previous years (year j -1 to year j ). (2) This fluctuation in subadult/juvenile density ratio showed a significant negative correlation with the fluctuation in wind speed during the same period (year j -1 to year j ). With increasing fluctuation in wind speed, the fluctuation in the disturbance of spatial distribution patterns for juveniles and subadults also increased. These results indicate that increased wind speed leads to increased disturbance of the sandflat and decreased juvenile and subadult survival rate, which in turn leads to decreased adult density.