The use of energy dispersive X-ray spectroscopy to detect strontium marks in fish otoliths

Otolith marking provides a reference point for otolith growth patterns by validating the temporal significance of growth increments. This widespread method is primarily implemented using fluorescent dyes. The incorporation of a trace element that appears naturally in otoliths offers an alternative approach. Diplodus annularis and Serranus scriba otoliths were marked with an intramuscular injection of SrCl2 diluted in 0.9% sterile saline solution (55 mg Sr ml-1 saline solution), given at a dose of 100 mg Sr kg-1 fish. At 277 to 366 days after marking, the fishes showed little or no mortality and experienced growth in length and weight. All of the otoliths analyzed showed a clear Sr mark detected with backscattered or secondary electron imaging during a scanning electron microsope (SEM) analysis. The mark was confirmed by Sr analysis using an X-ray detector and an energy dispersive spectrometer system with the SEM. The otoliths continued to grow after the mark, and background Sr values in this new growth were much lower than at the mark. This method was feasible and yielded good results. However, the method’s cost may limit its applicability to experimental studies.