Mercury and methylmercury in seafood from Maluku, Eastern Indonesia and New South Wales, Australia -considering food risk and trophic levels

Mercury (Hg) is a known toxicant, and human exposure—primarily through fish consumption—is a significant health concern. While regulations aim to reduce risk, activities like artisanal and small-scale gold mining (ASGM) can increase local contamination. This study analysed seafood from fish markets in areas with no known Hg sources (New South Wales, Australia – 20 species) and those near ASGM activities (Ambon – 14 species and Namlea – 12 species, Maluku Province, Indonesia). We measured total Hg and methylmercury (MeHg) in muscle and skin tissue. Species at higher trophic levels generally showed higher Hg concentrations, although no consistent correlation between trophic level and Hg or MeHg was found. In Australia, blacktip sharks ( Carcharhinus limbatus , 1377 ng/g ww) and silver gemfish ( Rexea solandri , 809 ng/g ww) exceeded seafood Maximum Levels (MLs). In Namlea, barred queenfish ( Scomberoides tala , 982 ng/g ww) and grunters ( Terapon theraps , 601 ng/g ww) also exceeded MLs. High individual variation was noted within species. Grunters, mid-trophic species, showed particularly high Hg in Namlea, suggesting a local contamination source. Generally, skin samples had lower or similar Hg levels than muscle, except in Namlea grunters, where skin Hg was much higher (1877 ng/g ww). MeHg made up between 20% (e.g., mackerel tuna from Namlea) and 100% (e.g., blue swimmer crab from Ambon) of total Hg, depending on species. These findings raise food safety concerns for certain species and provide baseline data for future monitoring in both Australia and Indonesia. • Species with higher trophic levels generally had higher Hg • To avoid health risk, consumption of grunters and barred queenfish should be limited • Consumption of black tip shark should be limited to the recommended guideline • Ratios of MeHg to Hg varied between species