Environmental factors mediate mercury exposure in fish with diverse feeding habits in the Beibu Gulf

Tropical and subtropical oceans, including the Beibu Gulf, a key fishing area in China, receive a substantial share of global mercury (Hg) deposition, yet the exposure levels and influencing factors in marine organisms remain underexplored. This study analyzed total and methylated Hg (THg and MeHg) concentrations in 650 fish from 10 species across 24 sites in the eastern Beibu Gulf in April and August 2022, along with concurrent surface water parameter measurements. The significant variations in fish Hg content across species were primarily driven by feeding habits, with benthic feeders exhibiting notably higher Hg levels than nekton and zooplankton feeders. Among 13 seawater parameters, temperature and pH were found to significantly influence Hg concentrations across species, with these effects varying by feeding guild. As sea temperature rose, MeHg levels increased in nekton-feeding fish but decreased in zooplankton-feeding fish, whereas in benthos-feeding species, MeHg levels showed a significant negative correlation with pH rather than temperature. Hg levels in Upeneus sulphureus and Johnius belangerii pose moderate and low risks for human consumption, respectively, with long-term consumption potentially reducing neonatal IQ by 0.45 points and increasing the risk of fatal heart attacks by 2.68 cases per 100,000 person-years in adults.