Estimating origins of migrating American shad with otolith chemistry

Origins of migrating individuals can be estimated using otolith chemistry provided a comprehensive baseline database of distinct natal signatures can be assembled.  American shad (Alosa sapidissima) and other anadromous species are ideal candidates for this approach to stock identification given their residence in a wide range of chemically distinct freshwater habitats.  I discuss the assembly of river-specific signatures using a suite of measurements from water samples and juvenile otoliths of fish collected in freshwater spawning habitats along the Atlantic coast.  This database comprises 20 rivers from Florida to Quebec, including all major extant spawning populations of Atlantic shad over the current species range.  This represents the largest geographic range covered by a database of otolith signatures to date.  Multivariate isotope and elemental signatures allowed an average of 93% of known-origin juveniles to be accurately classified.  This database was used to estimate the origins of migrating juvenile fish in the Gulf of Maine and Bay of Fundy, revealing unexpected mixtures and an absence of many populations.  Otolith chemistry thus has the potential to resolve population structure and migratory patterns of highly migratory shads and herrings, even at juvenile stages.