Pallid sturgeon larvae: The drift dispersion hypothesis

Abstract

The fate of drifting pallid sturgeon larvae [Scaphirhynchus albus, Forbes and Richardson, 1905] from spawning in the Missouri River below Fort Peck Dam, Montana, USA remains in question. Previous estimates of drift duration used flow modeling and experimental releases of free embryo pallid sturgeon that were recaptured in nets at incremental distances downstream. The recapture studies hypothesized that the released larvae drift along the river bottom in an intact plume that flushes into the downstream reservoir where they die. Results were interpreted that nearly all drifting embryos would reach Sakakawea Reservoir (~375 river km downstream) in just 6 days, suggesting that there is insufficient drift distance for successful reproduction of pallid sturgeon. Those experiments recaptured less than 1% of the released larvae, thus the dispersal fate of the remaining ~99% of drifting larvae remains unknown. We hypothesized that the river is extremely dispersive due to complex channel bathymetry and flow, which spreads the plume of drifting particles bank to bank within a few kilometers downstream of the release location, and that the same would be true for drifting free embryos. This is referred to as the Drift Dispersion Hypothesis. This hypothesis was tested by conducting high-resolution 3D flow and depth mapping, spanning the width of the Missouri River for 338 km in coordination with an embryo release and recapture study conducted in June 2016. Fort Peck Dam discharge was stabilized for the experiment. Hydraulic base maps were used to calculate particle drift paths and estimate drift speeds in (i) the bottom boundary layer (0.5 m) and (ii) the whole water column, as a surrogate for drifting larvae. Drift path patterns indicated wide lateral dispersion across the river, bank to bank. Drift simulations indicated that less than 10% of the fastest drifting embryos could reach Sakakawea Reservoir in just 6.56 days, but only if they remained mid-column and in the channel thalweg the entire distance. However, the fastest drifting particles that remained in the bottom boundary layer would not reach Sakakawea Reservoir for 31 days. Concluded was that most drifting embryos must be swept from the thalweg into low velocity habitats and stall, extending their river residence time. Stall locations were recorded along the entire study reach. The results indicate that drift duration is not the limiting factor for pallid sturgeon recruitment when spawning occurs near the embryo release site downstream of Fort Peck Dam.