Trace Metal Contamination and Biomarker Responses in Fish from a Mining-Impacted River Basin in Cebu, Philippines

Mining activities in Toledo City, Cebu, have created extensive mined-out lands and left a legacy of metal contamination in surrounding aquatic systems. This study assessed trace metal concentrations and molecular biomarker responses in fish from the downstream section of the Sapangdaku River and adjacent Tañon Strait coastline. Pore water and sediments were analyzed for Cd, Cr, Cu, Pb, and Zn using FAAS and ICP-MS, while biomarker assays measured metallothionein (MT) induction in liver tissues and genotoxicity through the assessment of micronuclei (MN) and nuclear abnormalities (NA) in erythrocytes. Results revealed that Cu was the dominant pollutant in pore water (mean 173 µg/L; peak 831 µg/L at Station 3), exceeding the U.S. EPA and Philippine DENR criteria by more than tenfold. Sediment indices (Igeo, CF, PLI) confirmed very high contamination from Cu, with Pb and Zn contributing moderately, while Cd and Cr remained near background levels. Fish analyses showed substantial bioaccumulation of Cu and Zn, particularly in benthic feeders (Mugil cephalus, Eubleekeria jonesi), which also exhibited the strongest biomarker responses. MT levels reached 299 µg/g in M. cephalus, while MN and NA frequencies were strongly correlated with Cu concentrations (R > 0.98, p < 0.05). NA were consistently more frequent than MN, suggesting their greater sensitivity as early markers of genotoxic stress. These results demonstrate that legacy mining continues to exert significant ecological stress on the Sapangdaku River system. The combined use of chemical analyses and biomarkers provides valuable baseline data, with MT, MN, and NA emerging as sensitive indicators of metal contamination and ecological risk.