Bradbury, I.R.; DiBacco, C.; Thorrold, S.R.; Snelgrove, P.V.R.; Campana, S.E. (2011). Resolving natal tags using otolith geochemistry in an estuarine fish, rainbow smelt Osmerus mordax. Marine Ecology-progress Series. 433 195-U576.
Dispersal and connectivity are central to stability and persistence in natural populations. The use of otolith composition as geo-referenced tags may provide unparalleled resolution of spatial movements in marine and anadromous fish, although these geochemical signatures remain largely undescribed and the factors influencing otolith composition poorly understood. We examined spatial variation in the otolith geochemistry of juvenile rainbow smelt Osmerus mordax using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and isotope ratio monitoring mass spectrometry to evaluate its potential as a natural tag to resolve fine-scale geographic patterns and dispersal of estuarine early life history stages. Otolith element ratios (Mg:Ca, Mn:Ca, Sr:Ca, Ba:Ca) and isotope ratios (delta(13)C, delta(18)O, (87:86)Sr) varied significantly among locations(n = 9) and provided 83% cross-validated accuracy using a quadratic discriminant function analysis. Assignments based only on the 3 isotope variables resulted in the highest rates of correct assignment (87%), largely driven by a significant increase(similar to 11%) in correct assignments at nearby locations(< 20 km distant). Five of the 7 elements examined were significantly correlated with each other, consistent with a common response to estuarine differences. We also observed a significant effect of habitat on assignment success, in that sites with partially restricted marine access (i.e. sand bars) assigned correctly at significantly higher rates. This study demonstrates that geochemical otolith signatures are intimately linked to estuarine structure, which in turn directly influences assignment power. It also demonstrates that assignment over fine spatial scales (< 20 km) can be maximized by the examination of isotope signatures and sampling of specific habitat types.