Abstract
The yearly records of the catch and the number of liberated fry were analysed statistically for chum (Onchorhynchus keta) in Hokkaidô, red salmon (Oncorhynchus nerka) in Alaska and trout (Oncorhynchus rhodurus) in Lake Biwa. In the cases of chum and red salmon, the coefficients of partial correlation (rXYn, Xn) were calculated, where X denotes the annual catch in a certain year, Xn that before n years and Yn the number of yearly liberated fry before n years. In the case of trout, the index of yearly catch of liberated fish was estimated from the yearly records of the number of liberated fry and the age composition of the catch, the mortality rate being assumed. The correlation coefficient between the yearly catch and the index of yearly catch of liberated fish was calculated. And the effect of the liberation on the catch was calculated from the slope of regression line. The results obtained are as follows: -
When one hundred millions of fry are liberated yearly, the increase in size of catch is evaluated as in the next table: -
Kind of fish Increase in size of catch in millions
Chum in Hokkaidô 0.46
Red salmon in Alaska 7.30
Trount in Lake Biwa 0.42
