Impact of the 2011 tsunami on the Manila clam Ruditapes philippinarum population and subsequent population recovery in Matsukawa-ura Lagoon, Fukushima, northeastern Japan

The tsunami caused by the 2011 Tohoku Earthquake with an epicenter off the Pacific coast had a significant effect on Manila clam, Ruditapes philippinarum, populations of northeastern Japan. In this study, the interannual variability of the clam density before and after the tsunami and monthly population dynamics after the tsunami were investigated in a tsunami-impacted brackish lagoon: Matsukawa-ura, Fukushima, northeastern Japan. In the lagoon, the fishery had been stopped after the tsunami, enabling the subsequent natural recovery process of the population to be assessed. Clam density immediately after the tsunami was markedly lower than before, suggesting substantial impact of huge tsunami. The impacts on clam population showed some degree of variation depending on the clam size and station. The smaller individuals, which have shallower burrowing depth and lighter individual weight, were swept away by direct tsunami more severely than larger individuals. Larger individuals locally remained with low densities at some stations shortly after the tsunami. Recruitment of early juvenile clams (<5 mm shell length) was observed in November 2011, indicating that tsunami surviving large-size adult clams obviously contributed to reproduction immediately after the tsunami. Until 2014, the density of juvenile clams rapidly recovered to the equivalent level as before the tsunami. The growth rate of the clam after the tsunami was comparable to the pre-tsunami periods and the juveniles reached almost 30 mm within two years after settlement with rapid growth during spring to early-summer. Summer mass mortalities of juveniles, which were not assumed to occur before the tsunami, were observed in 2012 and 2013. The yearly variation of the juvenile survival rate was a critical factor in the dynamics of the clam population. Our results clearly demonstrate the substantial direct impact of the huge tsunami on the infaunal bivalve species and reveal the high recovery potential of the species.