The decrease of the population of Japanese eel (Anguilla japonica) is of great concern. Four participants (China, Japan, Korea, and Chinese Taipei) have begun working to manage the Japanese eel, and draw a Joint Statement up in 2014, in which they mentioned to restrict the initial input of glass eels taken from the wild into aquaculture ponds (80% of the 2013--2014 level; 21.7 ton in Japan, baselessly). (1) Here we estimated a rough standard on the supply of seed for aquaculture based on the past almost-unexploited population status using the time-series of the fisheries catch, the initial input of seeds, and the production of aquaculture. (2) Based on a Leslie Matrix model, we tested whether the decrease of the eel population was caused by the decrease of the birth rate or by the decrease of the survival rate using the time-series of the catch of seeds and yellow eels. These correspond to seeds fisheries and environment destruction, respectively. Which one causes population to decrease?