A quantitative approach to investigate natal philopatry in Hickory Shad (Alosa mediocris) using otolith chemistry

Abstract

The goal of this study was to understand if Hickory Shad exhibit natal homing. Hickory Shad were captured in eighteen major rivers along the known spawning range. LA-ICP-MS was used to quantify seven elements (Mg, Mn, Cu, Zn, Sr, Ba, and Pb), along a continuous transect that ran from the ventral edge of the otolith through the core to the dorsal edge of each otolith, resulting in a time resolved model of the environmental exposure history of each fish. Hidden Markov models were used to estimate a natal river element signature for each Hickory Shad. The true number of natal rivers present in the dataset were unknown, so a Gaussian mixture model was used to estimate the number of mixture distributions and classify hickory shad based on natal river signatures. Five clusters were identified; Hickory Shad captured in the majority of rivers were assigned to one of three or more clusters, while those captured in a few rivers had unique element profiles and were assigned the same cluster at high rates (70%–90% of samples per river). A Chi-Square test confirmed that there was a significant relationship (p < 0.01) between capture location and cluster assignment. Overall, the results of this study suggest a high rate of straying from natal rivers through most of the Hickory Shad spawning range, and that some localized populations may return to natal rivers at a higher rate. This study provides the first piece of quantitative information about the rate at which Hickory Shad exhibit natal homing, and provides an important inferential baseline for future studies to understand their life history strategies and population dynamics.