Increasing or decreasing? - Current status of the Japanese eel stock

Abstract Although Japanese eel, Anguilla japonica, is listed as endangered on the International Union for Conservation of Nature Red List of Threatened Species, a previous stock assessment of the Japanese eel estimated that the age 1+ stock of the species has been recovering since 1990. To help understand the population dynamics of wild Japanese eels, eel fisheries in coastal and estuarine areas in Japan were investigated, because inland fisheries cooperative associations have conducted eel stocking that is presumed to bias the fisheries data in inland waters of Japan. We conducted a questionnaire survey of 1509 fisheries cooperative associations in coastal and estuarine areas of Japan, where the possible bias of eel stocking is expected to be relatively low. Based on the questionnaire, exploitable eel (yellow and silver eels) catch and effort data from 2003 to 2018 were obtained from six fisheries cooperative associations. A standardized catch per unit effort (CPUE) was developed using a generalized linear model for each fisheries cooperative association. The results indicated that the standardized exploitable eel CPUE was significantly lower in four of the six associations. As for the other two associations, significant relationships between CPUE and year was not found. Additionally, of the 227 fisheries cooperative associations that confirmed they catch eels, 1.3%, 5.3%, 21.6%, and 71.8% answered that the eel stock is “relatively increasing,” “stable” “relatively decreasing,” or “decreasing,” respectively, with no association stating that the eel stock is “increasing.” Moreover, reported glass eel CPUE in nine prefectures in Japan was significantly lower in both periods of 1977–2018 and 2003–2018. Because the dataset used in this study is likely to be one of the best currently available, the wild Japanese eel stock appears to be currently declining in Japanese coastal and estuarine waters, where the bias of eel stocked eels on stock dynamics is thought to be relatively low.