The Long-Term Fluctuation and Recent Characteristics in the Number of Ayu Plecoglossus altivelis altivelis Caught in the Shimanto River

Abstract

The long-term fluctuation of the population of ayu Plecoglossus altivelis altivelis was analyzed from the catch abundance in three markets (upper, middle and lower site) along the Shimanto River during 1977-2009. The catch number showed a similar fluctuation in the three markets, and the decline of catch after 2003 was noticeable. However, there were some differences among the three markets; that is, the long-term decline of catch was most remarkable at the lower site, and the annual catch variability was the largest at the middle site. The results of analysis of the catch number in the three markets suggested that a strong catch and poor rainfall in October would cause a decline of ayu stock in the following season. Therefore, the catch number of the early-spawning population, should be controlled considering the amount of rainfall in the spawning season (especially in October), for successive preservation of the native ayu stock in the Shimanto River.