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The aim of this study was to explore the effects of diet and habitat on the Hg levels of Japanese anchovy at different growth stages. We measured the amounts of Hg and stable carbon and nitrogen isotopes in the muscle and stable carbon and oxygen isotopes in the otoliths of 54 Japanese anchovy specimens obtained from the open seas of the Northwest Pacific Ocean (39°2'N ∼ 42°30'N, 154°02'E ∼ 161°29'E) between June and July 2021. Body length had a significant effect on δ 13 C bulk , δ 15 N bulk and M oto (P < 0.01). As individuals grew, δ 13 C bulk and δ 15 N bulk tended to gradually increase, but M oto tended to gradually decrease. The variation in the niches of Japanese anchovy at different growth stages showed that the standard ellipse-corrected area subsequently decreased with growth. It reached its smallest value in the 136-150 mm group, and there was no overlap between the 136-150 mm group and the other groups. The GAM results showed that Hg levels tended to decrease first and then increase with growth. There was a positive correlation between Hg levels and δ 18 O oto in fish. Hg levels increased gradually with increasing δ 15 N bulk . In our study, there may be a gradual shift in the diet of Japanese anchovy from phytoplankton to prey at higher trophic levels, and the depth of seawater in which the predators feed gradually increased with growth. Changes in diet and habitat were probably the main reasons for the increase in Hg levels., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Guanyu Hu reports was provided by Shanghai Ocean University College of Marine Living Resource Sciences and Management. Guanyu Hu reports a relationship with Shanghai Ocean University College of Marine Living Resource Sciences and Management that includes: funding grants. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)