Acoustic telemetry reveals high site fidelity of yellow‐phase American eels <i>Anguilla rostrata</i> in a large river

Movement patterns and habitat use of yellow-phase American eels Anguilla rostrata in the Wolastoq/Saint John River were examined using acoustic telemetry. Tracking 72 tagged yellow-phase eels from late summer 2021 to autumn 2023 revealed overall restricted longitudinal movements (mean linear range ± standard deviation [SD] 6.4 ± 10.9 km) and high site fidelity. However, two eels exhibited small-scale (20-25 km) seasonal movements within freshwater habitats and one eel undertook large-scale (~83 km) seasonal movements between estuarine habitats in winter and freshwater habitats in summer. Tracking 24 yellow-phase eels equipped with depth sensors indicated that depth use correlated with diel period, season and fish length, with eels occupying shallower habitats at night, presumably for foraging, and deeper habitats during the day, likely for shelter. Sixteen tagged eels presumably matured to the silver phase and initiated out-migration in late summer. These eels moved rapidly through freshwater (mean ± SD 11.9 ± 8.8 km/day) but slowed considerably on reaching estuarine habitats (1.3 ± 1.3 km/day). These findings highlight the diversity of movement strategies in yellow-phase eels and provide new insights into the transition to the migratory silver phase.