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, alongthe northwestern Atlantic have stimulated resource managers to consider the impactof hydroelectric facilities on silver-phase eels as they migrate downstream to thesea. During the fall of 2002, we investigated the movement of migrant eels passingdownstream of a small hydroelectric facility on the Connecticut River (Massachusetts).We used three-dimensional acoustic telemetry to monitor fine-scale movementof telemetered silver eels in the forebay (the first 100 m of area directly upstream ofthe dam). Eel movements were tracked approximately every three seconds, and individualswimming pathways were reconstructed to compare the three-dimensional resultswith biotelemetry methods previously used at this site; conventional telemetrysystems included radio, PIT, and acoustic telemetry. We found that three-dimensionalacoustic telemetry provided the necessary fine-scale resolution to characterize dominantmovement patterns and locations of passage. Eels were detected at all depthsthroughout the forebay; however, they spent the greatest proportion of their timenear the bottom, with occasional vertical movements to the surface. Eels exhibiteda range of movements interpreted to be downstream searching behavior, includingaltered vertical and horizontal positions at or near the trash racks and various loopingmovements directly upstream of the trash racks and throughout the entire forebay. Asubstantial number of these eels (28%) were detected re-entering the acoustic arrayon multiple dates before passing the station. The majority (89%) were detected passingdownstream of the dam through the turbines