Downstream Passage and Movements of Silver-Phase American Eels at Three Hydroelectric Projects on the Shetucket River, Connecticut

Traditional downstream bypass structures installed at hydroelectric projects have not been well studied for effectiveness of passage of adult downstream migrant eels. We quantified downstream movement and passage of telemetered migrant, silver-phase American eels at three consecutive hydroelectric projects on the Shetucket River, Connecticut, over two migratory seasons. Downstream migration frequently occurred during significant flow events, often well after time of release. Eels usually progressed downstream rapidly in reaches between dams, but could be delayed at dams, depending on flow and operation characteristics. Passage routes selected by eels were also variable; increased numbers of eels passed via spillways during periods of significant spill flows, but the majority of fish passed through turbines. Although all three projects operate as run-of-river, hydropeaking operation of a fourth project further upstream may have influenced migration rate, timing, and activity, route selection, and use of bypasses. Passage via bypasses depended on project intake configuration, bypass entrance siting, and approach velocities.  An angled bar rack with 2.5 cm clear spacing and low approach velocity generally passed a higher proportion of eels via the bypass than more traditional sluice bypasses at trashracks oriented perpendicular to the flow.