Three-dimensional movement of silver-phase American eels in the forebay of a small hydroelectric facility

Declines in the population of the American eel, Anguilla rostrata, along the northwestern Atlantic have stimulated resource managers to consider the impact of hydroelectric facilities on silver-phase eels as they migrate downstream to the sea. During the fall of 2002, we investigated the movement of migrant eels passing downstream of a small hydroelectric facility on the Connecticut River (Massachusetts). We used three-dimensional acoustic telemetry to monitor fine-scale movement of telemetered silver eels in the forebay (the first 100 m of area directly upstream of the dam). Eel movements were tracked approximately every three seconds, and individual swimming pathways were reconstructed to compare the three-dimensional results with biotelemetry methods previously used at this site; conventional telemetry systems included radio, PIT, and acoustic telemetry. We found that three-dimensional acoustic telemetry provided the necessary fine-scale resolution to characterize dominant movement patterns and locations of passage. Eels were detected at all depths throughout the forebay; however, they spent the greatest proportion of their time near the bottom, with occasional vertical movements to the surface. Eels exhibited a range of movements interpreted to be downstream searching behavior, including altered vertical and horizontal positions at or near the trash racks and various looping movements directly upstream of the trash racks and throughout the entire forebay. A substantial number of these eels (28%) were detected re-entering the acoustic array on multiple dates before passing the station. The majority (89%) were detected passing downstream of the dam through the turbines.