Challenges in assessments of Japanese eel stock

Abstract

Although Japanese eel, Anguilla japonica, is one of the most important fish species in East Asia, only one study has assessed Japanese eel stocks to date. That study estimated the stock status and dynamics of this species mainly based on Japanese fishery data on recruiting glass eel, growing-phase yellow eel, and maturing silver eel. That study also used data on biological characteristics, such as the growth, maturity and natural mortality of both Japanese eel and European eel, A. anguilla, to develop an age- and sex-structured production model. However, Japanese fishery statistics for glass eel are not an appropriate data source for stock assessment because there are non-negligible illegal and unreported glass eel catches occur in Japan. Moreover, yellow and silver eel fishery data in Japan are biased by those pertaining to stocked individuals, although most of the data are for eels originating from Japan as opposed to other countries and regions within the species distribution range. In addition to these problems in data collection, the biological parameters might be biased by contamination from stocked individuals. Therefore, it is difficult to assess the stock status of Japanese eels using an age- and sex-structured production model based on official Japanese eel fisheries statistics. The legality and traceability of glass eel fisheries and eel trading should be improved to enable appropriate stock assessment in future, and data on catch per unit effort (CPUE) in areas where eel stocking is not conducted should be collected throughout the species distribution range.