Passage of Downstream Migrant American Eels through an Airlift Deep Bypass System

Because eels typically spend the majority of their time near the bottom in forebay environments, deep bypass entrance structures with bottom entrances hold some promise for increased effectiveness as protection measures for this species.  A new design of deep-entrance bypass system that uses airlift technology (the Conte Airlift Bypass) to induce flow in a bypass pipe was hydraulically and biologically tested in a simulated forebay environment under controlled laboratory conditions. Migratory silver-phase American eels (Anguilla rostrata) tested at night under dark conditions readily located, entered, and passed the bypass; avoidance and rejection rates were lower at higher entrance velocities. Eels appeared to swim against the flow while being transported downstream through the pipe; median transit times for each test velocity ranged from 5.8 to 13.7 sec, with transit time decreasing with increasing entrance velocity. There was no strong avoidance of the vertical section of the pipe which contained injected air. No mortality or injury of bypassed eels was observed, and individual eels repeatedly passed through the bypass at rates of up to 30 passes per hour, suggesting that individuals do not avoid repeated entrainment through the bypass.