Upstream Migration Dynamics of Juvenile American Eels (Anguilla rostrata) Towards a Hydropower Dam in a Large River

Abstract

Young American eels (Anguilla rostrata) undergo a remarkable migration from oceanic spawning grounds to continental growth habitats. Impaired river connectivity can prevent facultatively catadromous eels from completing their upstream migration. The effects of barriers on the upstream migration dynamics of this endangered species remain poorly understood. This study evaluated the timing of juvenile eel migration and the spatial distribution and size/age structure of migrating eels within the tidal and hydropower-regulated Wolastoq | Saint John River in Canada. Upstream eel migration in this river is restricted by the Mactaquac Dam and Generating Station, located 150 km upstream of the sea. Approximately 36,000 eels were caught with fyke nets at various locations between the river mouth and the Mactaquac Dam, measured for length, and a subsample (n = 401) was aged via sagittal otolith readings. While glass eels (not fully pigmented juvenile eels) were observed up to 90 km upstream, approximately 20 km beyond the point of saltwater intrusion, eels arriving at the Mactaquac Dam were exclusively fully pigmented, with 80% being continental age 1+ or 2+. The mean length of juvenile eels increased with distance upstream. These results indicate that upstream eel migration in the Wolastoq | Saint John River can take multiple years and involves phases of juvenile residency. Our study provides important insights into juvenile eel migration in large rivers and implications for effective management of passage success at barriers such as hydropower dams.