Scanning proton microprobe analysis of strontium in an arctic charr,Salvelinus alpinus,otolith: implications for the interpretation of anadromy

Synopsis Scanning proton microprobe imaging has been used to map the distribution of strontium (Sr) in an arctic charr, Salvelinus alpinus, otolith. X-ray maps show that Sr variation follows an oscillatory zoned pattern which correlates with optically resolved zones (annuli). The internal region of the otolith shows optical zonation but no significant Sr content. Comparison of the optical image with the Sr X-ray map provides evidence of when the fish entered a high Sr environment. This may be interpreted as when the fish migrated from fresh water to a marine environment; this particular fish first migrated to sea in its eighth year. Micro-PIXE point analysis was used to determine the abundance of Sr in annuli along a radius from the nucleus to an outer edge; Sr levels in the internal region were about 10 ppm and variation in the outer zoned region was between 62 and 175 ppm. Detection limits for Sr are in the 1–2 ppm range. Other trace elements, such as transition elements, were found to be present in some growth regions in the 1–20 ppm range. Proton beam analysis is a non-destructive analytical technique capable of preserving the spatial integrity of trace-element data in otoliths such that element distribution may be linked to the growth structure of the otolith.