The habitat of freshwater clams found in a drainage ditch connected to Lake Biwa, Japan, and consideration of ditch management approaches for protecting the vulnerable Asian clam, Corbicula leana

Corbicula leana, classified as a vulnerable species in Japan, is a freshwater bivalve. While the species is declining in number across Japan, research into the current population size and cause of decline is lacking, and an investigation into effective conservation methods is overdue. This study focuses on the waterways connecting to Lake Biwa and aims to define approaches to manage clam numbers in the wild through the following: (1) measuring seasonal changes in the number of clams in drainage ditches, (2) examining conditions of the sedimentary environments preferred by the species, and (3) evaluating the influence of predation of the red swamp crayfish Procambarus clarkii on the clam. Over the research period, the number of freshwater clams in the drainage ditches fluctuated and reproductive individuals increased in summer and autumn. We also found that large numbers of the species could be found in environments containing sedimentary materials such as sand. It was also clear from our laboratory experiments that the red swamp crayfish preys on small freshwater clams but we also found that predation pressure varied depending on whether sediment was present. Thus, to manage clam habitats, encouraging silt deposition and developing a means of measuring sediment thickness may be effective, in addition to regularly removing red swamp crayfish from drainage ditches.