Entrainment of Rainbow Smelt Through Oahe Dam: The Flood of 2011

Entrainment of fishes through impoundments is commonplace in many reservoirs.   In Lake Oahe, South Dakota, rainbow smelt Osmerus mordax are the dominant prey fish and are commonly entrained through Oahe Dam.  Previous research has documented increased smelt entrainment during times of high water discharges.  In 2011, the Missouri River system endured a flood of a magnitude never before seen.  During this time, concerns with the level of smelt entrainment were high and we sought to determine rainbow smelt entrainment rates during this event. To complete our objectives we used two different gears: stationary ichthyoplankton trawls downstream from each intake structure (i.e., the powerhouse and stilling basin) and repeated hydroacoustic transects on Lake Oahe.  Due to safety concerns during high water releases, downstream netting only took place after the peak discharges had passed.  Additionally, consumption of smelt by avian and aquatic predators immediately downstream from discharge locations likely created underestimation of net based entrainment rates. Estimated entrainment of rainbow smelt from 27-July through 30-September using nets where a loss of 62.7 million (41.2% of 2011 population estimate).  Repeated hydroacoustic transects resulted in a relative degree of decline and likely over estimated entrainment rates due to the loss of fish from natural mortality during the study period.  Estimated loss of rainbow smelt from 27-July through 30-September using standard hydroacoustic transects was 134.6 million (88.6% 2011 population estimate).  Despite large disparities in total entrainment estimates by both gears, a similar pattern was observed where entrainment was high through the beginning of August but leveled off by late August and into September.