A field study examining metal elimination kinetics in juvenile yellow perch (Perca flavescens)

Currently little is known about how and at what rate fish eliminate metals in natural environments. To address this knowledge gap we examined metal elimination kinetics in the field using juvenile yellow perch (Perca flavescens) that were caught in a metal-contaminated lake (elevated levels of Cd, Cu and Zn) and transplanted to cages held within a reference lake. Fish were sampled from the cages over 75 d and changes in metal concentrations were measured in the gills, gut, liver and kidney. In transplanted fish, Cd concentrations decreased most rapidly in the gills and gut, i.e. from organs in contact with the ambient water and food; biological half lives (t1/2) were 18 and 37 d, respectively, for each organ. Longer half-lives were observed in the liver (75 d) and kidney (52 d) for this metal. Elimination of excess Cu by the liver and gut occurred much more rapidly, with estimated half-lives of labile Cu being 8 and 4 d, respectively, for these two organs. In contrast to Cd and Cu, there was little Zn elimination. To compare how the liver handles different metals during elimination, we used a differential centrifugation approach to examine changes in metal concentrations (Cd and Cu) at the sub-cellular level. Consistent with the long half-life observed for Cd at the whole organ level, there was no significant loss of Cd from any of the sub-cellular fractions. Copper, on the other hand, was lost from both the organelle and cellular debris fractions. As these fractions likely contain structures such as lysosomes, we suggest that Cu is depurated from the liver by direct elimination of these sub-cellular vesicles. These field results clearly demonstrate how the liver handles essential (Cu, Zn) and non-essential metals (Cd) differently during depuration.