An investigation of chronological differences in the deposition of trace metals in the otoliths of two temperate reef fishes.

Abstract

Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) was used to determine broad scale chronological differences in the elemental composition of otoliths from two temperate reef fishes, Parma microlepis (Gunther) (Pomacentridae) and Achoerodus viridis (Steindachner) (Labridae). Estimates of a range of microconstituents (<10 μg ion per g of otolith) were obtained, including the low level constituents Ba, Co and Mn. The removal of otolith tissue by either acid digestion or diamond saw allowed the chemical composition of the centre (=early life of fish) and edge (=later life of fish) of otoliths to be accessed without detectable contamination. ICP-MS used with either of these methods can, therefore, access chronological information stored in the otoliths of fishes. Significant differences in elemental levels (e.g., Mn) were found between the otoliths of the two species of fishes and between study sites. For both Parma microlepis and Achoerodus viridis, the centre of the otolith contained less Ca than the edge. Chronological differences in trace metal composition were found when data were normalised to sample weight (e.g., Ba and Co in A. viridis), but these differences were not observed for data normalised to the Ca concentration of the otolith sample. The merits and problems associated with normalising ionic concentrations to weight, Ca concentration, or protein concentration are discussed with the conclusion that a greater understanding of the binding affinities of all the components of otoliths (organic, as well as inorganic matrices) is needed to be in a position to select either of these internal standards as appropriate for specific ions. Despite this, ICP-MS was found to be a powerful tool that can be used to determine microconstituent fingerprints in otoliths among locations and between comparable chronological sections (e.g., cores).