In summer, if you go to upper reaches of a river, you may see many anglers who have very long fishing rods. Probably, they are Ayu fishing. Ayu is famous in Japan. Most Japanese know Ayu live clean rivers at least. 
Ayu have many relations. First, we eat Ayu. For esample, Sioyaki, Tempura, Segosi, Sabazusi and Uruka. But Ayu may have parasitic which are called Yokogawakyuutyuu. So if you eat Ayu, you should not eat it raw. In addition to this, it is Ayu fishing. There are caught by decoy and fry-fishing. But there are closed season for fishing from May to November and we must buy license fishing. And goods are very expensive.
Ayu’s scientific name is Plecoglossus altivelis altivelis. They belong to Osmeriformes like smelt and Japanese smelt. They are related to koukakugyorui.
Ayu fry live in sea and downriver. They eat plankton and insect which live or fall in the water. They hatch from two weeks and they are clear in body so we can their beating heart and air bladder. When they become ten millimeters, they begin to be fragrant like watermelon or Oriental melon. On April and May, they get color and the teeth become like spit. And they go up river.
Adult Ayu live in middle and upper reaches of rivers and eat algae. They are gray and green. Young Ayu swim together but bigger Ayu make territory in places where there are many diatom. This territory is one meter in every direction and if others enter the territory, they deliver an attack by bodily impact. In fall, they become orange and black. It is nuptial coloration. From February to September they swim down river. Then they lay eggs in shallows where there are many pebble stones and sand in group and die. So they end their lifetimes in one year.
Much Ayu live in Kochi rivers because Kochi’s river are clean. Rivers which Ayu live in Kochi are Shimanto River, Yoshino River, Niyodo River, Monobe River and Kagami River. So if you are interested in Ayu, go to the upper reaches of a river and fish Ayu.
Ayu have a relation with people from old times. But wild Ayu are decreasing by people impact. Ayu cannot live in muddy rivers because algae cannot photosynthesize in muddy rivers. Releasing Ayu into the river is done in Japan. It is good thing. But I don’t like this because it may mix alien species and origin. So I think we must keep clean river. If wild Ayu are not seen river, it is very lonely.