Spatial Distribution of Drifting Pallid Sturgeon Larvae in the Missouri River Inferred from Two Net Designs and Multiple Sampling Locations

Abstract

Nearly 430,000 larval pallid sturgeon Scaphirhynchus albus (5–13 d posthatch) were released in the Missouri River and allowed to free-drift for 5.7 km and then were sampled at a river bend by boats positioned on the inside-bend, inside-bend channel border, mid-channel, and outside-bend locations of the channel. Boats were rigged with conical nets and rectangle nets designed to sample for drifting pallid sturgeon larvae along the bottom and at the surface of the river to address three objectives: (1) examine the vertical drift behavior of larvae; (2) compare numbers, concentrations, and lengths of larvae between conical and rectangle nets; and (3) examine the lateral distribution of free-drifting larvae. The vertical distribution of larvae was not uniform as nearly all larvae (≥98%) were sampled in nets fished on the bottom, and larval concentrations (expressed as larvae/m³) were greater in bottom-fished nets. Large-opening rectangle nets (0.375 m²) fished on the bottom sampled greater numbers of larvae than small-opening conical nets (0.196 m²) fished on the bottom; however, larval concentrations were similar between the two net designs. Numbers and concentrations of larvae differed among lateral sampling locations and increased from the inside-bend location to the outside-bend location. Lengths of sampled larvae were similar between net designs and vertical locations in the water column, but larvae sampled at the mid-channel location were slightly smaller than larvae sampled at other lateral locations. These results serve as a guide for sampling larval pallid sturgeon in large rivers. Specifically, sampling adjacent to the bottom in the high-velocity thalweg of the channel maximizes the likelihood of collecting larvae and quantifying numbers of drifting larvae of this federally endangered species.