Estimated population size of pallid sturgeon Scaphirhynchus albus in a novel reach of the lower Missouri River

The recovery criterion for pallid sturgeon Scaphirhynchus albus consists of attaining a self-sustaining genetically diverse population for two generations. The suppressed pallid sturgeon population is theorized as a potential factor limiting recovery; therefore, the Pallid Sturgeon Conservation and Augmentation Program (PSCAP) was implemented to proliferate the overall population. The pallid sturgeon population has been quantified in the lower basin of the Missouri River (Gavins Point Dam [rkm 1,305.2] to the confluence of the Missouri and Mississippi rivers [rkm 0.0]), but previous population estimates have only covered less than ten percent of the lower basin. Therefore, the objectives of this study were to quantify the annual pallid sturgeon population (2015-2017) in a novel, unquantified 30.1 rkm reach of the lower Missouri River basin and compare these results to previously published population estimates in the lower Missouri River basin. The study area included a six-bend reach in the upper channelized Missouri River, approximately 226.3 rkm below the last main-stem dam. Population estimates (N-sp) for the "super-population" of pallid sturgeon within the 30.1 rkm study area varied from 593 (95% CI 471-716) in 2015 to 471 (373-569) in 2016 and 608 (482-734) in 2017. The population estimates (fish/rkm +/- SE) ranged from 15.6 +/- 1.0 to 20.2 +/- 1.3 fish/rkm. This study aligned with a previously published estimate in the same proximal reach but was two or three times higher compared to an estimate reported from approximately 500 rkm downstream. Understanding the temporal and spatial variations of the pallid sturgeon population is critical as recovery efforts continue, especially to seed input parameters into population prediction models that provide management guidance.