We successfully discriminated wild and cultured anguillid eels without artificial tagging. For the purpose of population restoration of anguillid eels, stocking has been widely conducted in the European Union (EU) and Japan, although the net benefits of stocking remain unclear. To evaluate the effectiveness of eel stocking for population restoration, the survival, growth, escapement, and reproduction of stocked eels should be tracked. In this study, we explored the potential of using otolith oxygen and carbon stable isotope ratios as a natural tag to discriminate between wild and cultured eels, because cultured eels are often stocked into natural rivers and lakes. A discrimination model was developed based on wild (n = 95) and cultured (n = 314) Japanese eels as a training dataset. The results of accuracy estimation based on leave-one-out cross-validation were 96.8%. We then tested the applicability of the model to stocked–recaptured eels (n = 20); 100.0% were successfully identified as cultured eels, indicating that these eels spend their early continental phase in aquaculture ponds before stocking. This method could be widely applied to assess the effectiveness of eel stocking by determining the proportion of stocked eels among those captured from rivers, coastal areas, or spawning grounds.

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The excess radiocarbon produced by nuclear bomb testing in the atmosphere in the 1950s–1960s (bomb‐14C) is used as a tracer in the surface ocean, extending our understanding of geophysics and biogeochemical cycles. However, there is no bomb‐14C record for the high‐latitude western North Pacific Ocean because of the paucity of long‐lived marine calcifying organisms equivalent to reef‐building corals. The shells of Stimpson's hard clam, Mercenaria stimpsoni, potentially provide such a record because the clam's lifespan is very long (>100 years). We analyzed 14C in six live‐caught M. stimpsoni shells from the western North Pacific (39.4°N, 142°E) and report, for the first time, the bomb‐14C record with robust calendar ages based on annual growth increments. The value was constant in 1934–1952 (Δ14C = −66‰), with a sudden increase in 1959, a peak in 1974 (107‰), which was 60‰ lower than that of the Kuroshio Current, a gradual decline after 1974, and a current value of 16–18‰, which is ∼10‰ higher than the atmospheric value. The bomb‐14C values are between the Kuroshio Current (the northwestern subtropical gyre) and Oyashio Current (the Western Subarctic Gyre) values, suggesting that the Tsugaru Current, downstream from the Kuroshio Current, mixes with the Oyashio Current after passing through Tsugaru Strait.

Recent degradation and loss of tropical estuaries due to human impact have made necessary the protection of essential habitats for estuarine fishes. A better understanding of estuarine use patterns contributes greatly to identifying essential habitat conditions. This study presents quantitative reconstruction data of the salinity history experienced by individual estuarine fishes using an otolith oxygen isotope. δ 18 O otolith was determined from the kuhliid Kuhlia rupestris and pomacentrid Chrysiptera cyanea for use as freshwater and marine end-member references, respectively. The salinity histories of 3 species of estuarine mangrove fishes, often dominant in tropical East Asian estuaries, viz. the apogonid Fibramia amboinensis, the pomacentrid Neopomacentrus taeniurus and the terapontid Terapon jarbua, were examined. The δ 18 O otolith ranges of −5.4 to −2.3 ‰ for F. amboinensis and −3.7 to −2.3 ‰ for N. taeniurus corresponded to 6 to 30 psu and 19 to 30 psu, respectively, when compared to the 2 extreme salinity values of the reference fishes. Terapon jarbua, in contrast, exhibited a δ 18 O otolith range of −3.0 to −1.8 ‰, corresponding to 25 to 34 psu. Field observations in other studies have indicated that F. amboinensis and N. taeniurus may be resident species, hovering near mangrove roots and tolerating a variable salinity environment, whereas T. jarbua may be a vagile species that can move to areas of preferred higher salinity conditions. Such behavioral ecologies are largely consistent with the salinity histories reconstructed from δ 18 O otolith data. These results suggest that F. amboinensis and N. taeniurus may be estuarine resident fishes, whereas T. jarbua may be a marine migrant.

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