An 11-Year Series of Passage Counts of Upstream Migrant American Eels at the Millville Dam Eel Ladder, Lower Shenandoah River, USA

Upstream dam passage of American Eels is a current management concern for Atlantic coastal rivers of North America. Eel ladders have been used successfully to pass American Eels upstream at hydroelectric dams, but few studies have examined long-term datasets of upstream passage counts. We examined an 11-year time series of passage counts (2003–2013) of yellow-phase American Eels at an eel ladder at a hydroelectric dam on the lower Shenandoah River (Potomac River drainage), USA. Time series of daily American Eel counts were modeled with generalized estimating equations (GEE) by fitting time-varying environmental covariates of lunar phase, river discharge, and water temperature, including one- and two-day lags of each covariate. Over 22,000 yellow-phase American Eels were counted at the ladder during the 11-year time series. Passage counts were positively associated with river discharge and negatively associated with lunar illumination. Also, large spikes in river discharge, such as those associated with hurricane-induced rainfall during fall, were associated with mass upstream migration events. Our data support long-term success of upstream eel passage of an eel ladder on the lower Shenandoah River, and also emphasize the management utility of an eel ladder for monitoring upstream eel migration.