Entrainment of Rainbow Smelt through Oahe Dam during the 2011 Missouri River Flood

Entrainment of fishes through impoundments is common, and Rainbow Smelt Osmerus mordax, which is the dominant forage fish species in the Missouri River reservoir, Lake Oahe, is frequently entrained through Oahe Dam at low rates. In 2011, the Missouri River endured a flood of a magnitude never before experienced in recorded history. Concerns regarding Rainbow Smelt entrainment were numerous; thus we sought to determine Rainbow Smelt entrainment rates. To evaluate entrainment, we used stationary trawls downstream from each Oahe Dam outlet structure and repeated hydroacoustic sampling above Oahe Dam on Lake Oahe. Trawl sampling estimated that 231 million adult and 433 million age-0 Rainbow Smelt were entrained during the summer of 2011. Hydroacoustic sampling estimated a loss of 213 million adult and 520 million age-0 Rainbow Smelt from Lake Oahe. Thus, both gears provided an estimate of Rainbow Smelt loss that was surprisingly similar. Additionally, Rainbow Smelt entrainment rates were substantially higher from 1500 to 2100 hours. This corresponds to the time of day when Rainbow Smelt exhibit a diel migration into the water column likely making them more susceptible to entrainment as they pass by Oahe Dam intake structures. By reducing discharge during this critical time period, entrainment of Rainbow Smelt could be dramatically reduced.