Silver eel migration and mortality associated with five hydroelectric dams on the Shenandoah River

Turbines at hydroelectric dams can cause mortality for silver American eels (Anguilla rostrata) during outmigration in Atlantic Coast tributaries.  Five hydroelectric dams occur on the Shenandoah River, a tributary to the Potomac River in the Chesapeake Bay watershed.  We radio-tagged 145 large eels in the upper Shenandoah River watershed during the fall of 2007 through 2009 to evaluate environmental cues to downstream migration and turbine related mortality.  Downstream migrations occurred in every month of the year except for July, with peaks in late fall and spring.  Eel outmigration was highly correlated with increases in discharge and did not occur when water temperatures were less than 3^oC or greater than 23^oC.  Most silver eels migrated downstream between 17:00 and 05:00.  Cumulative survival for an outmigrating cohort was as low as 37%.  Night-time turbine shutdowns from September 15 to December 15 reduced the number of migrating eels passing through the turbines; however, the time of year for the shutdown period did not fully encompass the downstream migration season.