Larval Surveys Indicate Low Levels of Endangered Pallid Sturgeon Reproduction in the Middle Mississippi River

Abstract

AbstractThe pallid sturgeon Scaphirhynchus albus is an endangered riverine species that is less abundant than the sympatric shovelnose sturgeon S. platorynchus with which it hybridizes. The two species are morphologically similar, and due to morphological variation within species, allometry, and the occurrence of morphological intermediates, morphological identification of specimens can be problematic. When the pallid sturgeon was listed under the U.S. Endangered Species Act in 1990, perceived threats included habitat loss due to habitat alteration (dams and channelization), hybridization, and commercial harvest. Previous studies estimated the ratio of adult pallid sturgeon to adult shovelnose sturgeon to be approximately 1:77 in the middle Mississippi River (MMR; the reach between the Missouri River confluence and the Ohio River confluence). One study using morphological identification estimated that 8–26% of Scaphirhynchus larvae collected in the MMR during May and June were pallid sturgeon. Given that larval pallid sturgeon, shovelnose sturgeon, and their hybrids have overlapping morphologies, DNA markers may provide more accurate species identification. The present study aimed to estimate relative larval abundances of pallid sturgeon, shovelnose sturgeon, and hybrids in the MMR. We genotyped 583 Scaphirhynchus larvae collected from the MMR at 16 DNA microsatellite loci and determined that 581 larvae were shovelnose sturgeon, one was a hybrid, and one was a pallid sturgeon. This study is the first to provide genetically verified larval abundance estimates of Scaphirhynchus species in the MMR. The present data indicate that the rate of pallid sturgeon reproduction relative to shovelnose sturgeon reproduction is an order of magnitude lower than expectations based on adult ratios and that hybrid sturgeon larvae are as common as pure pallid sturgeon.