Ontogenetic niche segregation and its implications for mercury levels in Japanese anchovy from the high seas of the northwestern Pacific Ocean as revealed by fatty acid analysis

Japanese anchovy (Engraulis japonicus) is a small pelagic fish that is frequently targeted by commercial fisheries. Japanese anchovy plays a crucial role as a vital link between primary and higher-order consumers. The elemental mercury in Japanese anchovy muscles easily bioaccumulates and is transmitted to top predators. We investigated the variation in the diets and mercury accumulation of Japanese anchovy in the high seas of the northwestern Pacific Ocean. We measured the amounts of mercury and fatty acids in the muscles of 149 Japanese anchovy specimens that were obtained from the open seas of the northwestern Pacific Ocean (39°2' N ~ 42°30' N, 154°02' E ~ 161°29' E) between June and July 2021. The results revealed that the monounsaturated fatty acid (MUFA) and polyunsaturated fatty acid (PUFA) contents significantly decreased and then increased with the growth of Japanese anchovy (p < 0.001). The standard ellipse-corrected area (SEAc) calculated from the fatty acid profiles showed a decreasing and subsequently increasing pattern with anchovy growth. Trophic niche overlap reflects the degree of overlap in diet composition among species. Compared to the niche overlap among the 91-105, 106-120 and 121-135 mm groups, the niche overlap of Japanese anchovy between the 121-135 and 136-150 mm groups was lower. This change may indicate a dietary shift in Japanese anchovy with a body length of 120 mm. Mercury levels were positively correlated with C22:6n3, C20:4n6 and C20:1n9 contents and negatively correlated with C16:1n7 contents. We suggest that the proportion of higher-trophic zooplankton in the diet increased with the growth of Japanese anchovy, and that zooplankton was likely an important cause of the increase in mercury levels.