Shiga Prefecture Fisheries Experiment Station, Hikone, Shiga 552-0057, Japan
This study was conducted to examine the relationships among the effects of population density, mean standard length, plankton biomass, water temperature and precipitation on the first ascending day of ayu Plecoglossus altivelis altivelis and the number individuals ascending the Ado River, Lake Biwa. Effects of four factors on fluctuations of the first ascending day (Y1) in the river were recognized by multiple regression analysis. The factors were as follows: number of ayu caught by a set net in March (X1), mean standard length of ayu in mid-March (X2), plankton biomass in mid-March (X3), and water temperature in Lake Biwa in mid-March (X4). In addition, effects of four factors on fluctuations in the number of ascending individuals of ayu (Y2) in the river from April to June were recognized. The factors were as follows: X1, X2, X3, and amount of precipitation from April to June (X5). The following equations were obtained by multiple regression analysis of the factors except X5 for the forecast.
Y1=−0.316X1−3.14X2−1.29X3−9.25X4+290 (p<0.001)
Y2=9.09X1+17.5X2+55.3X3−988 (p<0.001)
These equations can be used to forecast the first ascending day and the number of ascending ayu individuals in the river.