Essential characteristics of aquatic environment for ayu habitats

Abstract

 We investigated essential factors of the establishment of stocked ayu, Plecoglossus altivelis by going underwater with a snorkel to count the number of ayu individuals and measure the microhabitat in 25 rivers in Hokkaido, Honshu, and Kyushu. In our analysis, three factors were selected as explanatory variables for the number of ayu observed: 1) smaller river width, 2) higher frequency of boulders (over 25 cm in major axis) on the river bed, and 3) higher frequency of exposed boulders. These factors should be taken into account when making management decisions on stocking sites. The number of ayu individuals observed by snorkeling also positively correlated with catch per unit effort of “Tomozuri” angling in the Naka River. Therefore, the number of individuals counted by underwater observation may be an appropriate index of the abundance of ayu as a target for angling.