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Using cormorant to fish is an ancient practice in Japan and China. In Japan, cormorant fishermen puts a tethered steel ring around the bird's neck (to prevent them from swallowing the fish). One fisherman usually handles ten to twelve birds at a time.
In China, the cormorants are more like beloved pets to the fisherman - he would train the bird to catch the fish and release it to him instead of swallowing it.