|Walther, B. D., S. R. Thorrold and J. E. Olney, Geochemical signatures in otoliths record natal origins of American shad, Trans Am. Fish. Soc. 137: 57-69, 2008|
We used geochemical signatures in otoliths of American shad (Alosa sapidissima) to determine natal origins and estimate rates of straying among river-specific populations along the Atlantic coast of the United States. Stable isotope (d13C, d18O and 87Sr:86Sr) and elemental (Mg:Ca, Mn:Ca, Sr:Ca and Ba:Ca) signatures in otoliths of juvenile American shad from rivers from Georgia to New Hampshire varied significantly, allowing for an average of 91% cross-validated accuracy when classifying individual fish to their natal rivers. We also found significant inter-annual variability in geochemical signatures from several rivers, due largely to differences in δ18O values among years. We then used the ground-truthed geochemical signatures in otoliths of juvenile American shad to identify natal origins of spawning adults in the York River system (Virginia). Approximately 6% of the adults were strays from other rivers. Of the remaining adults, 79% were spawned in the Mattaponi River and 21% were spawned in the Pamunkey River. Our results suggested that while most American shad spawning in the York River were homing to their natal river there was much less fidelity to individual tributaries. Small-scale straying allowed fish spawned in the Mattaponi River to subsidize spawning in the Pamunkey River, which has experienced persistent recruitment failure.