Morphological Identification Overestimates the Number of Pallid Sturgeon in the Lower Mississippi River due to Extensive Introgressive Hybridization

Pallid Sturgeon Scaphirhynchus albus and Shovelnose Sturgeon S. platorynchus are congeners occupying overlapping habitats within the Missouri and Mississippi River basins. The Pallid Sturgeon is listed as federally endangered under the Endangered Species Act in the United States, whereas the Shovelnose Sturgeon is not. Observational data across the range of both species include specimens that appear to be phenotypically and genotypically intermediate between Pallid Sturgeon and Shovelnose Sturgeon, though these occurrences are most frequent in the Mississippi River. Based on both morphological and genetic data, the variability and overlap within Scaphirhynchus spp. phenotypes has resulted in debate about the accuracy of existing identification methods of Pallid Sturgeon within the Mississippi River portion of its range (i.e., distinguishing Pallid, Shovelnose, and hybrid sturgeons). As a result, increasing questions have surfaced relative to species identification, implications of purported hybridization, the accuracy of local demographic estimations, and how those effect management actions. This project is the first comprehensive analysis to compare genotypic, phenotypic, and field identification data in order to evaluate the accuracy of existing field identification of the two congener Scaphirhynchus spp. and their purported hybrids in the lower Mississippi River. Our results indicate current field identification methods are not reliable for identifying Pallid Sturgeon in the lower Mississippi River, likely due to introgressive hybridization between Pallid and Shovelnose sturgeons and the resulting continuum of morphotypes and genotypes in hybridized individuals. We conclude that reliance on current morphological-based field identification methods to quantify Pallid Sturgeon may lead to inaccurate assessments that could negatively impact the conservation and management of this endangered species, including the use of hybrid broodstock for conservation stocking and erroneous estimates of stock and recruitment.