Discrimination of wild and farmed Japanese eels, Anguilla japonica, using stable isotope ratio of eye lens

Abstract

Although releases of Japanese eel have been conducted to maintain and increase stock levels, the effects on stock maintenance are unknown. Furthermore, there is a risk of disturbing the original distribution of natural individuals or making the proportion of natural and cultured individuals unknown. In addition, existing methods for discriminating between natural and cultured fish lack accuracy and simplicity. Therefore, we aimed to develop a method to discriminate between wild and farmed individuals by analyzing stable isotopes of carbon and nitrogen in the eye lens. Since the lens is formed in chronological order from the inside out, the composition of the eye lens can be compared during the period when there is a difference in food intake to discriminate between wild and farmed individuals. The carbon and nitrogen isotopic compositions of the eye lens were analyzed using samples of farmed individuals before release and wild individuals from rivers without release. The results showed that the distribution of carbon and nitrogen isotope ratios were distinctly different between wild and farmed specimens, suggesting that it is possible to discriminate between wild and farmed specimens. This method can be applied to estimate the contribution of farmed individuals to spawning, the original distribution area of wild individuals, and the ratio of wild and farmed individuals in each river.