Copper/Metal Ratios in the Gills of Rainbow Trout (Oncorhynchus mykiss) Provide Evidence of Copper Exposure Under Conditions of Mixed-Metal Exposure

Previous work has suggested that the ratio of copper residues to zinc in the gills of rainbow trout may indicate short-term exposure to increased levels of waterborne copper. However, the effect of exposure to a combination of increased copper and zinc concentrates in the water column was unknown. We exposed rainbow trout to 8 ± 2 μg L-1, 40 ± 2 μg L-1 and 90 ± 9 μg L-1 of waterborne copper and 21 ± 3 μg L-1, 129 ± 40 μg L-1, and 202 ± 40 μg L-1 of waterborne zinc in a 2-factor experiments and gill copper and zinc residues were examined. Other gill parameters analyzed included the concentrations of calcium, magnesium, sodium, and potassium, liver copper and zinc concentrations and plasma copper, calcium, sodium, and potassium are also reported here. Copper residues in the gill filaments were significantly higher in the highest level of copper exposure (high Cu, 4.06 μg g-1; low Cu 2.41 μg g-1; 0 Cu 2. 01 μg g-1; P = 0.001), whereas no differences were seen in zinc concentrations at any treatment level. Gill sodium and plasma calcium concentrations were also decreased at the highest waterborne copper concentrations. Although copper-zinc ratios in the gills were significantly different between the highest and lowest copper treatments (P = 0.002, F = 6.59), copper-sodium and copper-magnesium ratios were more sensitive to waterborne copper exposure (P = 0.001, F = 17.91 and P = 0.002, F = 15.45, respectively). These copper-metal ratios may be better indicators of copper loading in the water column.