Characteristics of surface hardness of spawning riverbed of Ayu (Plecoglossus altivelis altivelis) in Lake Biwa basin

In the Lake Biwa basin (Japan), the number of laying eggs in Ayu, Plecogossus altivelis altivelis, has decreased recently, especially the number in 2017 was very small. Therefore, improving the spawning environment of Ayu is an urgent issue. As Ayu prefers soft riverbed for spawning, in order to restore the favorable spawning environment of Ayu, it is necessary to quantify the spawning riverbed hardness. In previous studies, quantification of the spawning riverbed hardness of Ayu was carried such as Hasegawa type soil intrusion meter, but there was a problem in accuracy and reproducibility. In this study, we conducted quantitatively evaluate the hardness of spawning riverbed of Ayu by using Yamanaka type soil hardness tester which has not been used as it is. The field survey period was conducted in September 2014 and the field survey area were in the four Lake Biwa inflow rivers: Echi, Ado, Shiotsuoh and Ane Rivers. We surveyed both the spawning riverbed at head of rapid riffle area and the non-spawning riverbed at flat riffle area on dried parts near edge of water of sandbars in these rivers. As a result of the survey, median value of the spawning riverbeds hardness was 0.22 kgf/cm2 and the non-spawning riverbeds hardness was 1.40 kgf/cm2. Statistical analysis of riverbeds hardness and occurrence of egg laying by mixed effect logistic regression model formula showed the tendency that the egg production probability became 40% or more when the riverbed hardness was 1.0 kgf/cm2 or less from the model formula. Finally, we discussed the usefulness and the issues of quantification of monitoring about surface hardness of spawning riverbed of Ayu by using Yamanaka type soil hardness tester.