Method for preventing harmful aquatic periphyton, and prevention composition

The objective of the present invention is to provide a composition exhibiting a high preventive effect on harmful aquatic periphyton while being safe for fish, animals, and humans, and a method for preventing harmful aquatic periphyton using said composition. In the present invention, a composition containing at least one inhibitor selected from among mitochondria electron transport chain complex III inhibitors, such as fluacrypyrim, is applied to an underwater structure, and harmful aquatic periphyton, such as shellfish, adhering to the underwater structure are brought into contact with the mitochondria electron transport chain complex III inhibitor.