Estimation of abundance of ayu larvae during seaward drifting at the Shou River

The number of ayu, Plecoglossus altivelis, larvae drifting seaward was inve stigated at the Shou River from 1992 to 1996. The main period of seaward dr ifting of ayu larvae took place from early October to early December and la rvae were mainly collected from 18:00 to 22:00 during the period. The numbe r of ayu larvae drifting seaward during the period was estimated to be 3 to 29 hundred million in a year, and it was supposed that larvae drived from landlocked form, that is Lake Bi Na, occupied 5-15% of the total except in 1994. It is considered that the number of ayu larvae was influenced by the water temperature and the water flow of a river before and after the period of beginning of spawning.