As spawning and nursery grounds for fish and shellfish and also a source of feeds for aquacultured bivalves and ascidians, seagrass and seaweed beds are indispensable to coastal fisheries which is the primary industry in the Sanriku Coast. Thus, we conducted studies of impacts by the huge tsunami on 11 March 2011 on seaweed and seagrass beds. In the rias bays along the coastline, the seaweed beds around the mouth and middle of the bays were not impacted by the tsunami, but that the seagrass beds on the sandy bottom in the bay head had been destroyed. However, the seagrass beds are hardily recovering from seeds produced in previous years. While giant seawalls with a long width and a deep bottom are planed and have partly constructed, it is necessary to pay attention not to destroy habitats of seagrass beds which partly overlap space for construction of giant seawalls along the coastline.