Effects of diet and habitat on Hg levels in Japanese anchovy in the high seas of the northwestern Pacific Ocean.

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 δC, δN and M (P < 0.01). As individuals grew, δC and δN tended to gradually increase, but M 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 δO in fish. Hg levels increased gradually with increasing δN. 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.