Estuarine ingress timing as revealed by spectral analysis of otolith life history scans

Abstract

The ability to accurately measure the timing of migration is fundamental in testing hypotheses in marine ecology that deal with migration and movement of fish populations. Timing and patterns of movement in larval and juvenile fish have been estimated using life history scans of the chemical signatures encoded in their otoliths. We provide a quantitative approach to analyzing life history scan data using spectral analysis, which retrospectively measures the timing of ingress for individual fish. Saggital otoliths from juvenile Atlantic croaker ( Micropogonias undulatus ) were sampled using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Spectral analyses on these 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, we use these spectrally derived estimates to show that growth rates did not drive ingress timing for our samples. These data thus lend no support to the critical-size hypothesis in this instance.