Limited diversity in natal origins of immature anadromous fish during ocean residency

Abstract

Variable migration patterns can play a major role in promoting diverse life history traits among populations. However, population and stage-specific movement patterns are generally unknown yet crucial aspects of life history strategies in many highly migratory species. We used a natural tag approach using geochemical signatures in otoliths to identify natal origins of 1-year-old anadromous American shad ( Alosa sapidissima ) during ocean residency. Otolith signatures of migrants were compared with a database of baseline signatures from 20 source populations throughout their spawning range. Samples were dominated by fish from only two rivers, while all other potential source populations were nearly or completely absent. These data support the hypothesis that American shad exhibit diverse migratory behaviors, and immature individuals from populations throughout the native range do not all mix on northern summer feeding grounds. Rather, our results suggest populations of anadromous fish are distributed heterogeneously at sea in the first year of life and thus may encounter different ocean conditions at a critical early life history stage.