Assessing Movement of Fish Through Spectral Analysis of Otolith Life History Scans

ASSESSING MOVEMENT OF FISH THROUGH SPECTRAL ANALYSIS OF OTOLITH LIFE HISTORY SCANS Renee Reilly Hoover Old Dominion University, 2012 Director: Dr. Cynthia M. Jones The ability to accurately measure movement timing across environmental gradients is fundamental for testing hypotheses in marine ecology that deal with ingress, egress, and migration of fish. Timing and patterns of movement have been estimated using life-history scans of the chemical signatures encoded in fish otoliths (ear stones). I provide a quantitative approach to examining life history scan data using spectral analysis, which retrospectively measures the movement timing for individual fish. Sagittal otoliths from juvenile Atlantic croaker (Micropogonias undulatus) and adult black sea bass (Centropristis striata) were sampled using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). For Atlantic croaker, spectral analyses of the data estimate the timing of ingress at 68 days on average using strontium and 85 days using barium. Based on the inflection points of their nonlinear mixing curves, these data reveal entry and subsequent movement up-estuary. Moreover, I use spectrally-derived estimates to show that growth rates did not drive ingress timing for these samples. These data thus lend no support to the critical-size hypothesis in this instance. I additionally hypothesized that the three-dimensional structure of otoliths could produce sampling artifacts in the results of laser ablation scans. To test this hypothesis, I ablated two trenches of different depths on each otolith, performing spectral analyses on these data to investigate the effects of ablation depth, including differences in periodicities and temporal variability between trenches. The means of the two trench depths were significantly (t=l 14.25, p<0.0001). From shallow to deep trenches, variance decreased. Peaks or poles are very useful indicators of the frequency and periodicity (seasonal effect) when working with spectral densities. In this case, peaks in spectral density shifted in absolute value an average of 32 days. These results highlight the necessity of considering the depth of ablation when conducting life history scans. I examined the periodic movement of a coastal fish population by quantifying the inshore-offshore migration patterns in black sea bass. I sampled adult black sea bass from two portions of the population's residency as a means of making inferences about this expected life history pattern, using differences in space as a proxy for differences in time. I estimated the movement timing between inshore and offshore environments using the aforementioned spectral analysis techniques. Mean timing of this periodic movement was estimated at 365 days using strontium and 455 days using barium. The otolith edge chemistry between the inshore and offshore samples was significantly different in both strontium (t=33.48, p<0.0001) and barium (t=7.59, pO.OOOl), with offshore samples consistently exhibiting much higher barium signals. These results offer preliminary evidence to the hypothesis of barite-rich waters being supplied to the offshore coastal shelf via submarine canyon upwelling processes.