Upstream Migrant American Eels: A 12-Year Time Series from an Eel Pass on the Lower Shenandoah River

Eel ladders have been used successfully to pass American Eels upstream at dams, but few monitoring studies have examined long-term datasets of upstream passage counts. We examined a 12-year time series of passage counts (2003–2014) of yellow-phase American Eels for an eel ladder at Millville Dam, lower Shenandoah River (Potomac River drainage). Over 23,000 yellow-phase American Eels were counted at the ladder during the time series. Passage counts were positively associated with river discharge and negatively associated with lunar illumination. Based on length measurements of 13,780 individuals, the average size of eels using the ladder was 302 mm TL with a range of 183 to 731 mm, although only 45 of 13,780 individuals exceeded 500 mm TL. Age estimates for 233 individuals (214–550 mm TL) ranged from 3 to 11 years. 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.