Time series applications in otolith chemistry

Life history scans of fish otoliths are bringing new insight into the structure, connectivity, and movement of fish populations.  Data obtained from such scans, however, posses inherent limitations that have not yet been fully addressed or understood.  We offer a novel approach to analyzing otolith chemistry spectra using time series analyses.  We sampled sagittal otoliths from juvenile Atlantic Croaker (Micropogonias undulatus) with laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to obtain elemental concentrations of major elements.  We hypothesized that the three-dimensional structure of otoliths would affect trace element chemistry spectra obtained using laser ablation. To test this hypothesis, we ablated two trenches of different depths on each otolith.  We performed spectral analyses on these data to investigate the effects of ablation depth, including differences in periodicities, temporal variability, and phase shifting between trenches.  Because of the inherent temporal nature of otolith chemistry, we were able to relate significant features in the spectra to time-specific events in the fish life history.  Although traditional parametric tests reveal significant differences in means and variances between the two depths, time series analysis elucidates more nuanced results. Our conclusions highlight the necessity of applying appropriate statistical tests for the data being analyzed.