We studied the factors affecting decline of seaweed beds by unmanned aerial vehicle （ UAV） survey, diving-survey and past findings along south-west coast of Hokkaido, Japan. It was considered that the feeding damage by sea urchin, the lack of seaweed seed such as zoospore, germ and the shortage of stable substrate were main factors of decline of seaweed beds, in addition to the increase in water temperature and the lack of nutrient. In order to restore the seaweed beds, it is necessary to act not only artificial formation of seaweed beds with stones and concrete blocks but also density management of sea urchin, and transplantation of mother seaweed and seaweed seed. In addition, it is very important to reduce the management cost of seaweed beds, define the enforcement standard and update the techniques against environmental changes.
