About 6,000 years ago a rise in sea level on the east coast of Honshu Island formed a cove, and subsequently the narrowing of the cove’s entrance with sand from rivers made it into a brackish lake, namely Hinuma. The seawater flows ten kilometres upstream to Hinuma through Naka River and Hinuma River at high tide, and blends with freshwater. Hinuma provides habitats for many species, including nationally endangered species such as the four-spot midget damselfly (Mortonagrion hirosei) and Steller’s sea eagle (Haliaeetus pelagicus) during important stages of their lives. More than 88 species of birds are observed at Hinuma. In winter, more than 10,000 ducks such as mallard (Anas platyrhynchos) and greater scaup (Aythya marila) migrate to Hinuma to feed and roost. The number of greater scaup wintering at Hinuma is estimated at around 5,000 individuals annually and accounts for more than 1% of its population in East Asia. Hinuma has also long been a fishing site for brackish fishes and clams such as Asian clams, gobies and pond smelts.