Abstract
An underwater line transect was used every May and October, from 2006 to 2012, to assess the population size of ayu, Plecoglossus altivelis altivels, in the Nahari River. Estimated numbers of ayu ranged from 145,000 to 1,540,000 in May, and from 55,000 to 420,000 in October. The mortality rates of ayu from May to October varied from 30 to 76 ％. The mortality rate was not correlated with the May population size of ayu, fishing intensity, precipitation, and the maximum of turbidity. In contrast, the mortality rate was positively correlated to low levels of turbidity （20-50 mg/L）. It was suggested that the long-term of the turbidity increases the mortality rate of ayu.
