Otolith chemistry and American shad migration patterns.
Walther, Benjamin*,1, Thorrold, Simon1, 1 Woods Hole Oceanographic Institution, Woods Hole, MA
ABSTRACT- Otolith geochemical analysis has emerged as a reliable method to determine natal origins of fish that spawn in chemically distinct habitats. This "natural tag" approach is important in uncovering degrees of population connectivity as well as the stock identities of marine migrants. We selected American shad (Alosa sapidissima) as our model system to apply otolith chemistry techniques to questions in migration and life history theory. In order to assemble a database of groundtruthed chemical signatures indicative of spawning habitat, juvenile shad were collected in rivers in 2004. River water samples were collected from a subset of these rivers as well as from rivers for which no juveniles were available. Our collections spanned the current spawning range of shad from Florida to Quebec and represented the major extant spawning populations. We analyzed a suite of elemental and isotopic ratios (Sr/Ca, Mg/Ca, Mn/Ca, Ba/Ca, Pb/Ca, 18O, and 87Sr) using laser ablation ICP-MS and isotope ratio mass spectrometry in juvenile shad in both otolith and water samples. The relationship between otolith and water chemistry allowed us to predict otolith signatures from rivers where only water samples were available. Multivariate statistics including discriminant function analysis allowed us to assign natal origin to unknown fish using this and older databases of juvenile signatures. We were thus able to determine natal origins of returning spawners and marine migrants and compare our results to traditional mark-recapture studies on shad.
Key words: otolith, geochemistry, migration, Alosa sapidissima
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