Downstream passage of silver eels at a small hydroelectric facility

A 100-mm diameter bypass was installed in the dam face at a small hydropower station on the Mokau River, New Zealand, and its use by downstream migrating silver eels, Anguilla spp., was monitored by trapping at the outlet in autumn 2002 and 2003. In addition, a passive integrated transponder system was used to monitor passage over the spillway. Migrant eels were able to find the bypass, with 544 and 744 eels recorded using it in 2002 and 2003 respectively. Although the bypass was the sole means of safe passage at low flow, migrant eels passed down the spillway in preference to the bypass when the dam was overtopped during floods. A combination of spilling and small diameter bypasses would provide safe downstream passage at hydroelectric facilities for silver eels, so long as entrainment and impingement at the intake screens can be prevented.