Otolith Aging and Geochemical Analysis for Migratory Ecology: Brief Review and a Case of the Japanese Eel

As the fish grows, the otolith forms incremental rings, generally on a daily or yearly basis, and has therefore long been used in fisheries science. At least 30 different elements are known to be incorporated into accreting otoliths. As the otolith is a non-cellular tissue, elements/stable isotopes once deposited undergo almost no change throughout the fish’s life. Thus, analyzing the otolith rings and elemental/stable isotopic composition along its growth axis will allow us to identify individual age, growth and migration patterns. The effectiveness of otolith elemental/stable isotopic composition as a natural tag has been extensively verified and demonstrated in various fish species living in rivers, estuaries, and the ocean. This talk focuses on summarizing the types of factors that are responsible for variations in otolith composition and our perspectives for the potential novel marker that has possibility for use in the estimation of migration routes in the oceanic regions, and introducing a case of the application of otolith aging in the large demographic survey on the migratory fish, catadromous Japanese eels, to monitor the traits of the population with the declining trend of the stock.