Onnenuma is one of the representative lakes in Nemuro, along with Kazarenko.
Onnenuma is a brackish lake with a circumference of 15 km, surrounded by a dense forest of Ezo spruce, and connected to Nemuro Bay.
At low tide, a sandbar extends offshore, and the scene of people gathering clams, hokki-gai shells, and other shellfish is a typical scene at Onnenuma.