Ontogenetic Variation in the Trophic and Mercury Levels of Japanese Anchovy in the High Seas of the Northwestern Pacific Ocean

The aim of this study was to explore the connection between growth and feeding ecology and mercury (Hg) levels in Japanese anchovy (<i>Engraulis japonicus</i>). We measured the amounts of Hg and stable carbon and nitrogen isotopes in the muscle of 143 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. The results showed that there were significant differences (<i>p</i> < 0.05) in the δ¹³C and δ¹⁵N values of Japanese anchovies across all body length groups. As individuals grew, δ¹³C tended to decrease first and then increase, and δ¹⁵N tended to gradually increase. The standard ellipse corrected area showed an increasing and subsequently decreasing pattern with growth. It reached its greatest value (0.80) in the 111–120 mm group. Compared to the body length group of 91–120 mm, the niche overlap decreased for the 121–140 mm group in Japanese anchovy. Hg levels increased gradually with body length. Linear regression models revealed a positive correlation between Hg levels and δ¹³C in fish. Hg levels increased gradually, while δ¹⁵N remained relatively constant in the 7–9‰ range. In our study, a distinct shift in diet was observed for Japanese anchovy with increasing body length, and the differences in diet among life stages could be responsible for the changes in Hg levels.