What otolith microchemistry and stable isotope analysis reveal and conceal about anguillid eel movements across salinity boundaries

Otolith microchemistry studies indicate that growth-phase (yellow stage) anguillid eels commonly shift at irregular intervals between fresh and saline waters, but this technique has not detected regular seasonal migrations across salinity zones. We tested the ability of otolith microchemistry and stable isotope analysis to detect migrations of American eels ( Anguilla rostrata ) between salinity boundaries in two small stream–estuary systems in Canada’s Bay of Fundy. Although the two methods showed concordant classifications of recent residence history, most eels caught in fresh water in spring (68.8–89.7 %) and fall (78.8–83.3 %) showed microchemical and isotopic signatures that reflected occupancy of saline waters. These eels were classified as migrants which had summered in saline waters and then migrated to freshwater wintering grounds where they retained their saline signatures. In summer, most eels (85.0–100.0 %) captured in fresh and saline water had recent microchemical and isotopic signatures matching the habitat of capture. Our results suggest that lifetime otolith microchemistry profiles are unable to detect eel wintering migrations, a failure that is likely due to winter depression of otolith accretion. Elucidation of seasonal eel movements requires cross-seasonal and cross-site sampling for the microchemistry and stable isotope methods, or tagging studies. Seasonal saline–fresh eel migrations may be more common than previously appreciated, underlining the need for conservation of both habitats, and connectivity between the two.