A Mark‐Recapture Estimate of the Number of American Eels Arriving at the Lowermost Barrier on the Wolastoq|Saint John River, Canada

ABSTRACT The upstream migration of juvenile American eels ( Anguilla rostrata ) is frequently obstructed by dams and other in‐stream barriers, leading to habitat loss and fragmentation, factors that are believed to contribute to the species' population decline. While fishways can help restore access to upstream habitats, their effective design and implementation require reliable estimates of the number of eels arriving at migration barriers. The Mactaquac Dam and Generating Station (672 MW) is the lowermost barrier on the Wolastoq|Saint John River in Canada and currently does not provide an eel‐specific fish passage. To inform the design and required capacity of a potential future eel passage at this site, this study provides mark‐recapture‐based estimates of the number of American eels arriving in the tailrace during three consecutive upstream migration seasons (2021–2023). During the peak migration period from late June to early August, a total of 30,144 juvenile eels were captured, tagged with visible implant elastomer tags, and released in the spillway bays of the dam over multiple sampling occasions spanning 30 to 42 days. Data were analyzed using POPAN Jolly‐Seber models and resulted in population estimates ranging from approximately 150,000 individuals in 2021 and 2022 to approximately 950,000 in 2023. Although confidence intervals were relatively wide, these results highlight the importance of an upstream passage system for eels at this dam, provide guidance for future research and management, and demonstrate the potential for long‐term monitoring at a future eel ladder to support local stock assessment.