Abstract
Environmentally-derived elemental and isotopic ratios in the otoliths of anadromous river herring (alewife, Alosa pseudoharengus, and blueback herring, A. aestivalis) were used to distinguish among groups of fish at spatial scales ranging from sub-populations within a watershed to populations from throughout the species' ranges. These ratios were also used to understand early life migrations and habitat use within and among populations. Sub-populations within the Hudson River, NY were accurately distinguished (> 95%), and populations from the Hudson River and Long Island, NY were distinct from each other and outgroups at varying distances, but accurate classification was dependent on the inclusion of oxygen isotopic ratios. Populations from Maine to Florida showed strong separation based on otolith signatures excluding (~ 70%) and including (> 90%) oxygen isotopes. Reclassification accuracies improved for both models by including genetic results in a hierarchical assignment model. Though all natural tags were effective for stock discrimination, the accuracy varied depending on the markers included; while inclusion of oxygen isotopes resulted in the highest reclassification rates, accurate application requires intensive sampling because of high interannual variability. Genetic markers reduce the effects of interannual variation because they are generally stable over generations.
Variations in otolith chemistry across an otolith (i.e. the fish's life history) can provide information about movements among habitats and especially along salinity gradients. Juvenile alewife within the Hudson River, NY (a large watershed) moved among multiple freshwater habitats, and trends varied widely among individuals while in the Peconic River, NY (a small, coastal watershed) three distinct movement patterns were observed. Retrospective analysis of Hudson River adult otoliths showed that multiple nursery habitats contribute to the spawning stock. Throughout the coast, retrospective analysis of adult otoliths showed that juveniles used fresh waters, estuaries, or a combination of both as nursery habitats and migratory behavior varied among populations, and were correlated to the latitude of the watershed, the watershed area, the amount of accessible river kilometers, and the percentage of the watershed in urban use.