The "wooden fish" is a percussion instrument made of a hollow wooden block originally used by Buddhist priests to beat rhythms when chanting scriptures. It is said that fish don't close their eyes when sleeping to remind the chanting monks to concentrate on when they are doing.
Origin of wooden fish(mokugyo):
Many years ago, a Chinese Buddhist went to India for scriptures. On his way, he was blocked by a wide flooded river with no bridge or boat nearby. At that moment, a big fish swam up to carry the Buddhist across the river. When they reached the middle of the river, the fish said: "Because I have committed a crime, I have been sentenced to live in this river for many years. I am told that you will spare no efforts to go to India for scriptures, so I have come here to help you to atone my misdeeds. Should you meet Sakyamuni (founder of Buddhism), please ask him when I can become Bodhisattva".
Being anxious to cross the river, the Buddhist accepted the fish's proposal without hesitation. Having spent 17 years in India, the Buddhist went back to China, taking the scriptures with him. On his way back, he once again came to the former river, which was dangerously flooded. As he was worrying about his predicament, the big fish appeared once again to give him a hand. In the middle of the river, it asked the Buddhist: "You have been in India for many years. Did you ask Sakyamuni when I could become Bodhisattva?" The Buddhist replied, "Ah, sorry! I forgot." Hearing this, the fish got angry and bumped the Buddhist and his scriptures into the water. A fisherman who happened to be passing nearly helped the Buddhist out of the river, but, unfortunately, his books were ruined.
The Buddhist came home, full of anger. He said to himself: "It is the fish that made my 17 years of efforts wasted." He then had a statue of a fish's head carved from wood. Recalling his misfortune, the Buddhist beat the wooden fish with a wooden hammer. To his surprise, each time he beat the fish, the fish opened its mouth and produced a Chinese character. The Buddhist became ecstatic and beat the wooden fish frequently. A few years later, character by character, he was able to retrieve what he had lost in water from the wooden fish's mouth.