Metal bioaccumulation and oxidative stress in the southern sand flathead (Platycephalus bassensis, Teleostei) across an urbanised embayment

Urbanised embayments worldwide are increasingly affected by anthropogenic release of metals and other contaminants into their surrounding waters, posing risks to both marine and human health. This study assessed metal bioaccumulation and associated sublethal effects in the benthic fish Platycephalus bassensis across eight sites in Port Phillip Bay, Australia. Metal concentrations were quantified in sediments and fish tissues (muscle, liver, gills) and traditional biomarkers, including biometric indices (condition factor, hepatosomatic and gonadosomatic indices) and detoxification enzymes (superoxide dismutase and glutathione S-transferase) were used to evaluate sublethal effects associated with tissue metal burdens. Overall, metal concentrations in sediment and fish tissues varied among sites, with higher values occurring in areas associated with intensive industrial, urban, and shipping activity. Sediment metal concentrations were generally below ANZG default guideline values (DGV-low), except for nickel, which exceeded the DGV-low value at two locations. Mercury concentrations in fish muscle (0.05-0.68 mg/kg wet weight) exceeded food-safety guidelines of 0.5 mg/kg wet weight at these locations, indicating a potential health risk to consumers. Zinc concentrations in liver tissue showed a strong positive association with superoxide dismutase activity and a negative association with hepatosomatic index, while hepatic cadmium showed a moderate negative association with glutathione S-transferase activity. Despite elevated mercury levels, no mercury-linked biomarker responses were detected. These findings highlight the limited sensitivity of traditional biochemical biomarkers in mildly contaminated ecosystems, defining an important threshold for their effective application in environmental monitoring.