Sources and uptake of trace metals in otoliths of juvenile barramundi (Lates calcarifer)

We conducted an experiment to determine if the concentrations of three trace metals (Cu, Sr, and Ph) in the otoliths of juvenile barramundi (Lates calcarifer) were related to their concentrations in the water or diet. Fish were kept in freshwater aquaria for 22 days and either fed diet enhanced with 10 mug g(-1) Cu, Sr or Pb, or had 10 mg l(-1) (Sr and Pb) or 0.5 mg l(-1) Cu added to the water. A control group were then transferred to seawater and maintained for a further 83 days. The otoliths were initially ablated for five trace metals (Li, Cu, Sr, Ba and Ph) with LA-ICPMS. Ablation intervals corresponded to between 7 and 11 days of growth. There was no detectable increase in the otolith concentration of any metal for the fish in the enhanced-diet treatments. There was significant uptake of the three metals added to the water (Cu, Sr and Pb). Three (Sr and Pb) or six fish (Cu) were reanalysed with the ICPMS in high-sensitivity mode enabling 5 mum wide ablations to be made at intervals equal to every day or second day during the experiment. These analyses showed that there was considerable variation in uptake rate among individual fish and that the concentration fluctuated widely before an equilibrium concentration was reached after about two weeks exposure. Analysis of the otoliths of the control group of fish that were transferred to seawater showed that the concentration of Li, Sr and Ba in the otoliths varied between the periods of fresh and seawater residency. These variations were related to changes in the concentration of these metals in the water. The fluctuations in Sr concentration were similar to those seen in wild diadromous fish and not as great as those measured in the fish kept in Sr-enhanced water. Sr concentration in the otoliths was strongly negatively correlated with Ba concentration in the experimental fish, but not in wild fish that migrate from the sea into freshwater. Sr/Ca, Ba/Ca and Cu/Ca ratios in the otoliths were positively correlated with water ratios. The partition coefficients of Sr and Ba were similar to published values for marine fish. The concentrations of Li and Pb in the water were below the detection limit in some treatments so no comparisons could be made for these metals. When taken in conjunction with the results of other recent studies of trace metal uptake in otoliths. our data indicate that under experimental conditions, otolith concentrations of several trace metals are related to water concentrations. However, these patterns do not simply extrapolate to the natural environment where the mechanisms involved appear to be more complex. (C) 2001 Elsevier Science B.V. All rights reserved.