Geochemical Signatures in Otoliths Record Natal Origins of American Shad

—Population connectivity is a critical component in the life history dynamics of anadromous fishes and in the persistence of local populations. We used geochemical signatures in the otoliths of American shad Alosa sapidissima to determine natal origins and estimate rates of straying among river-specific populations along the U.S. Atlantic coast. Stable isotope (dC, dO and Sr:Sr) 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 interannual variability in the geochemical signatures from several rivers, due largely to differences in dO values among years. We then used the ground-truthed geochemical signatures in the otoliths of juvenile American shad to identify the natal origins of spawning adults in the York River system in Virginia. Approximately 6% of the spawning adults collected in the York River were strays from other rivers. Of the remaining fish, 79% were spawned in the Mattaponi River and 21% in the Pamunkey River. The combined results of this and other recent studies suggest that although 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 could allow fish spawned in the Mattaponi River to subsidize spawning in the Pamunkey River, which has experienced persistent recruitment failure. Anadromous fishes often display complicated migration patterns that present challenges to investigators seeking to understand the relationships among movements, life history traits, and population dynamics. Unresolved questions include the degree of homing to natal rivers and the effects of fishing pressure directed at small, tributary-specific stock components. Although significant work has gone into addressing these questions, direct tests of hypotheses concerning natal origin and migratory behavior are difficult with traditional tagging techniques (Dingle 1996; Thorrold et al. 2002). Most information on anadromous migrations comes from mark–recapture studies that apply a tag to a fish and attempt to reconstruct a route once that tag is recovered (Dadswell et al. 1987; Hendry et al. 2004). Although the tags employed are becoming increasingly sophisticated (e.g., Block et al. 2005), this approach can only yield information about movements subsequent to tag application after the fish reaches some minimum size (Webster et al. 2002). As a result, traditional tags are unable to provide data about early life history movements and spawning origins of fishes, both of which are crucial aspects of population dynamics (Metcalfe et al. 2002). The use of natural geochemical tags in animal tissues and hard parts provides an alternative marking technique for species that are difficult to tag using conventional approaches (Rubenstein and Hobson 2004). Recently, fish otoliths have been shown to be particularly useful natural tags (e.g., Thorrold et al. 2001). Otoliths are paired calcareous structures in the inner ear of fishes that are formed by the sequential addition of inert layers of calcium carbonate, usually in the form of aragonite, from birth to death (Campana and Nielson 1985; Campana 1999). The composition of otolith aragonite reflects, at least to some degree, the chemistry of ambient waters at the time of deposition (Bath et al. 2000; Walther and Thorrold 2006). Thus, otoliths from fish spawned in chemically distinct waters will record unique signatures reflective of those habitats and continue to record movements between distinct waters over their lifetimes. The combined use of isotope ratios and elemental concentrations can allow fine-scale geographic discrimination of freshwater habitats. A pronounced * Corresponding author: [email protected] 1 Present address: Southern Seas Ecology Laboratories, School of Earth and Environmental Science, Darling Building, University of Adelaide, Adelaide, South Australia 5005, Australia. Received February 12, 2007; accepted July 27, 2007 Published online January 3, 2008 57 Transactions of the American Fisheries Society 137:57–69, 2008 Copyright by the American Fisheries Society 2008 DOI: 10.1577/T07-029.1 [Article]