舞根湾に蘇った干潟におけるア サリの出現と動態

摘要

The Great East Japan Earthquake on March 11, 2011, destroyed many artificial structures, such as seawalls and landfills, on the coast of northeastern Japan, and as a result it restored wetlands in many places. To estimate the ecological and economic values of a restored tidal flat in Moune Bay, Miyagi Prefecture, Japan, we investigated the age structure, growth, and biomass of the Manila clam, Ruditapes philippinarum. We collected the clams at 21-23 sampling stations(20 × 20cm)on the mudflat about every six weeks between August 4, 2012, and July 24, 2013. Size-frequency distribution of the clam based on shell length(SL)on August 4, 2012, was mainly divided into three cohorts. A new cohort was detected in autumn of 2012, although its frequency was low. Year-classes(age groups)were estimated for these cohorts after considering the growth pattern of each cohort and the number of clams that recruited. We concluded that the population consisted of 2011-year-class clams that recruited massively to the tidal flat after the earthquake and 2010-year-class that were stranded by the tsunami. After tracking the growth rate of the clams, we estimated that the clams would reach a commercial size(SL 30 mm)in four years after recruiting to the mudflat. The density of the clams on July 24, 2013, was 381 individuals/m2. Our data indicates that the mudflat would become highly productive as long as the physical environment of the tidal flat, such as the ground level, continued to recovery naturally.