Size, age composition, and upstream migration of American eels at the Millville Dam eel ladder, Shenandoah River, West Virginia

Abundances of American eel (Anguilla rostrata) are declining along the east coast of the United States and Canada, possibly due to habitat loss and barriers to migration. In Atlantic coast watersheds, dams detain upstream migration of juveniles, and little is known about age class composition, age-length relationships, or environmental cues to upstream migration of yellow-phase eels. An eel ladder was installed on the Millville hydroelectric dam, lower Shenandoah River, WV, to facilitate and monitor the upstream movement of yellow phase eels. Daily length measurements (TL cm) and weights (g) were taken on eels using the ladder during three sampling periods; spring/summer 2004 (May 14-July 23), fall 2004 (Sept 10-30), and spring/summer 2005 (June 1-July 31). Additionally, otolith-based ages were estimated from a subsample of eels. To examine environmental variables associated with upstream migration, candidate models were fit to daily count data and included combinations of four environmental covariates (barometric pressure, local precipitation, lunar illumination, and river flow) and a year effect. A total of 4,847 eels used the ladder during the three sampling periods. Eel sizes were similar among sampling periods (range 19-75 cm TL), and age estimates from 74 eels (21.4-55 cm TL) ranged from 3 to 10 years. Estimates of mean length at age of eels from Shenandoah River were low relative to published estimates from southern and northern latitudes. Peaks in eel counts coincided primarily with low levels of lunar illumination or with rise in river flow, and the data supported an additive model of lunar illumination and river flow. The data did not support single-variable or additive models with covariates of barometric pressure or local precipitation, or models with a year effect. Management strategies for American eels will benefit from short and long-term studies of eel ladders, including additional focus on eel counts, size and age composition, and upstream migration