Effect of Laser Ablation Depth in Otolith Life History Scans

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, possess in-herent limitations that have not yet been fully addressed or understood. For example, several investigators have noted delays in otolith elemental uptake that do not appear to reflect habitat exposure. We hypothesized that the 3-dimensional structure of otoliths may produce sampling artifacts in the results obtained from laser ablation scans. To test this hypothesis, we sampled sagittal otoliths from juvenile Atlantic croaker Micropogonias undulatus with laser ablation inductively coupled plasma mass spectrometry to obtain elemental molar ratios of a common environmental marker (barium). We ablated 2 trenches of different depths on each otolith and performed spectral analyses on the data to investigate the effects of ablation depth, including differences in the periodicities and temporal variability between trenches. The mean barium concentration between the 2 trenches was significantly different (t = 114.25, p \u3c 0.0001). From shallow to deep trenches, variance decreased; the standard error about the means was reduced from 0.609 to 0.086. Peaks in spectral density, which estimate the ingress timing for this species, were shifted in absolute value an average of 32 d. Our results highlight the necessity of considering depth of laser ablation when conducting life history scans