Age interpretation in eulachon (Thaleichthys pacificus) as suggested by otolith microchemical signatures

Eulachon (Thaleichthys pacificus) are forage fish that play an important role in the ecosystem as prey for many species. Given their continuing population decline, determining accurate ages is necessary for age-structured stock assessments. This study employed microchemistry analysis to identify group differences and help interpret growth zone patterns on otolith surfaces. Specimens were collected off the coast of Oregon, in the coastal areas of Southeast Alaska, and in the southeastern Bering Sea. Laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) was used to measure elemental ratios in the otoliths along a continuous track from the core to the proximoventral margin. Ba:Ca, Sr:Ca, Zn:Ca, and Mg:Ca signatures suggested that eulachon specimens from three geographic regions are different based on their elemental profiles. For the Oregon specimens, fluctuations in Ba:Ca and Zn:Ca signatures appeared consistent with otolith growth zones and most likely were the result of seasonal coastal upwelling events. Variations in Ba:Ca and Zn:Ca were useful as annual markers for eulachon otoliths from the Bering Sea. For the Southeast Alaska specimens, analysis of the Ba:Ca and Zn:Ca oscillations was not straightforward. Further work is needed to understand the link between otolith microchemistry, fish physiology, and regional environmental factors.