Relative species abundance and microhabitat preferences of larval Scaphirhynchus sturgeon in the middle Mississippi River

The pallid sturgeon (Scaphirhynchus albus) and shovelnose sturgeon (S. platorynchus) are benthic freshwater fishes that are sympatric throughout the range of the pallid sturgeon. Pallid sturgeon was listed as endangered under the Endangered Species Act in 1990, while shovelnose is common throughout the range. Previous abundance studies estimate the ratio of adult pallid to shovelnose sturgeon to be on the order of 1:82 in the middle Mississippi River, respectively. Despite adult abundances, reproduction and/or recruitment of pallid sturgeon larvae is undocumented in the middle Mississippi River. The current study aims to confirm the presence of pallid sturgeon reproduction and estimate the relative species abundances of larval pallid, shovelnose and hybrid sturgeon in the middle Mississippi River. Since larval pallid, shovelnose and hybrid sturgeon are virtually identical morphologically, the use of DNA markers was required for species designations; sixteen previously developed microsatellite loci were used in this study. Of the 583 larval Scaphirhynchus sturgeon collected from the middle Mississippi River, 581 were shovelnose, one was a hybrid and one was a pallid. This study was the first to genetically confirm the presence of pallid sturgeon reproduction in the middle Mississippi River. Differences in species ratios between adult and larval Scaphirhynchus sturgeon could be explained by three potential hypotheses; life history characteristics accentuate species ratios between adult and larval stages, pallid are experiencing low reproduction and/or low recruitment, or pallid larvae reside in different microhabitat locations compared to shovelnose