Can otolith microchemistry be used to identify spawning stocks and characterize the life history of Hickory Shad (Alosa mediocris)?

ECU Author/Contributor (non-ECU co-authors, if there are any, appear on document)
Christopher Ryan Hill (Creator)
East Carolina University (ECU )
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Abstract: Highly migratory and diadromous fishes present an array of challenges to fisheries managers. This is particularly true when stocks extend across management borders and occupy multiple management jurisdictions. It is valuable for managers to be able to separate fish populations into sub populations and spawning populations because it allows them to allocate sustainable harvest regulations. One popular method of achieving this is the use of otolith microchemistry. Otoliths are paired mechanosensory structures found in the teleost inner ear that help with hearing, balance, and environmental orientation. The same biogeochemical properties that allow otoliths to serve their respective physiological functions allow researchers to make quantitative inferences concerning population diversity, movements, and various other aspects of life history. Here, the species of interest is the Hickory Shad (Alosa mediocris (Mitchell 1814). The Hickory Shad is an anadromous clupeid found in Atlantic coastal systems. During the spring, Hickory Shad migrate inshore to spawn in freshwater where they are a popular target of a multimillion-dollar sport fishery. While the Hickory Shad represents a significant economic asset, little is known about its life history. The current management strategy for Hickory Shad groups it with three other native anadromous clupeids, and management decisions rely on the assumption that Hickory Shad and American Shad (Alosa sapidissima) may have similar life histories. However, the extent to which this represents the actual life history is unknown. For instance, it is assumed that like American Shad, Hickory Shad exhibit natal homing. Yet expression of natal homing has never been confirmed in Hickory Shad The overall goal of this study was to determine if otolith microchemistry could be used to discriminate spawning stocks of Hickory Shad. Hickory Shad were captured in 26 locations within 18 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 to dorsal edge through the otolith core, resulting in a time resolved model of the environmental exposure history of each fish. Hickory Shad captured in the same locations frequently had similar element profiles distinct from other capture locations, which immediately suggested natal homing. To test this hypothesis quantitatively, a combination of Bayesian inference and unsupervised learning techniques were used to estimate the natal river element signature of each fish and determine if it was similar in Hickory Shad captured in the same location. A hidden Markov model was fit to the strontium profile of each otolith to identify the initial transition between the natal freshwater river and euryhaline environments, and average element ratios within the first regime were assumed to be estimates of each natal river element signature. Since the origin of each natal river signature was unknown, a Gaussian mixture model was used to estimate the number of mixture distributions (i.e., clusters) present in the data and assign Hickory Shad to each cluster under a probabilistic paradigm. In most cases, between 50% and 100% of Hickory Shad captured in the same location were assigned to the same cluster, indicating that they had similar natal watershed element signatures. A Chi-Square test confirmed that there was a significant relationship between capture location and cluster assignment (p<0.01). These results provide the first piece of evidence that Hickory Shad do exhibit natal homing, and provide an important inferential baseline for further characterization of the rate of natal homing. While these results provided strong evidence of that Hickory Shad exhibit natal homing, they were not able to quantify the spatial extent of natal homing and straying, which in this case would require knowledge of the spatiotemporal variability of element ratios in the spawning rivers. Therefore, the second objective of this study was to quantify element variation in each capture location. Water chemistry data were not available for the capture locations in this study, so elements deposited on the edge of Hickory Shad otoliths were used as a proxy. Element ratios on the ventral otolith edge (~30 µm of absolute distance) were compared across capture locations. Hickory Shad captured in five locations had distinct ratios of and one or two elements, and these differences were minute. Based on knowledge from previous literature and several empirical observations, I concluded that the edge of Hickory Shad otoliths did not reflect the ambient element ratios of the capture location, which was likely a function of a rapid spawning migration. Overall, the results of this study suggest that otolith microchemistry may be a valuable tool for identifying spawning stocks of Hickory Shad, but the information it can provide may be constrained by a number of physiological, ecological, and life history traits that need further refinement. Our results provide strong evidence that Hickory Shad exhibit natal homing, and element signatures that are incorporated early in life may be useful for further characterizing the rate of natal homing and straying. Results also suggest that otolith element signatures produced later in life may not provide accurate descriptions of environmental exposure histories, and otolith regions produced beyond the first year of life may not be as useful for stock discrimination.

Additional Information

Language: English
Date: 2020
Hickory Shad, otolith, otoliths, otolith microchemistry, stock identification

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