Migratory Patterns of American Shad (Alosa sapidissima) Revealed by Natural Geochemical Tags in Otoliths

Benjamin Walther, Ph.D., 2007
Simon Thorrold, Advisor

In an assessment of river-specific signatures in American shad (Alosa sapidissima), stable isotope and elemental ratios in otoliths of juveniles produced accurate natal tags from 12 rivers. The database was expanded to include 20 rivers from Florida to Quebec, encompassing all major spawning populations. Regressions between otolith and water chemistry for those rivers where both were collected showed significant relationships for Sr:Ca, Ba:Ca, d18O, and 87Sr:86Sr ratios but not for Mg:Ca or Mn:Ca. Cross-validated classification accuracies of known-origin juveniles averaged 93%. Adults returning to spawn in the York River were classified according to their otolith composition. Only 6% of spawners originated from rivers other than the York, supporting the hypothesis that most American shad spawn in their natal river. Of remaining spawners, 79% originated from the Mattaponi River and 21% from the Pamunkey River, suggesting less fidelity to individual tributaries. Otolith signatures were also used in mixed-stock analyses of immature migrants in the Gulf of Maine. Mixed-stock compositions were dominated by fish from the Shubenacadie and Hudson rivers, with an increasing proportion of Potomac River fish over time. In contrast to results from adult tagging studies, southern stocks were virtually absent. These data suggest ontogenetic shifts in migratory behavior.