Reproductive physiology of shovelnose sturgeon from the middle Mississippi River in relation to seasonal variation in plasma sex steroids, vitellogenin, calcium, and oocyte diameters

The shovelnose sturgeon Scaphirhynchus platorynchus, smallest of the North American sturgeons at < 2.5 kg, has inhabited the Mississippi River drainage basin for almost 100 million years. Shovelnose sturgeon have been heavily harvested from the middle Mississippi River (MMR) due to the importation ban of beluga caviar into the U.S. from the Caspian and Black Seas and the Volga River. Many states in the Mississippi River and Missouri River drainages still allow commercial harvest of shovelnose sturgeon, which may increase the risk of population collapses within these areas. Therefore, there is a need for more information on the reproductive biology of shovelnose sturgeon in the middle Mississippi River including age at maturity, reproductive status and cycles, fecundity, and methods (i.e., hormone assays) for development of best management strategies. The objectives of this study were to determine reproductive parameters from analyses of plasma sex steroids, vitellogenin, calcium, and oocytes. A total of 395 shovelnose sturgeon were harvested from the MMR between river kilometers (RKM) 201-198 near Modoc, Illinois and RKM 127-124 near Grand Tower, Illinois from October 2005 to February 2007. Each analyzed reproductive biochemical had its own specific concentration pattern during gametogenesis in shovelnose sturgeon. The techniques used for determining sex steroid concentrations were radioimmunoassay, enzyme linked immunosorbant assay, and colormetric assay. Age at maturation was determined at 10 years for males and 9 years for females based on testosterone (T) and 17beta-estradiol (E2) concentration profiles. Evidence provided by peak mean plasma T, E2, and calcium concentrations during April and October and optimal spawning temperatures indicated that a fall spawning by shovelnose sturgeon may be occurring in the MMR. Gravid (FIV) females had a mean oocyte diameter of 2.58 mm, similar to much larger species of sturgeon. Female shovelnose sturgeon oocyte diameters were not significantly different between young and older fish, but the diameters of black oocytes positively increased with age. In this study, the mean fecundity estimate of 20,277 +/- 7,570 was within the range of other shovelnose sturgeon populations in Missouri, Illinois, and Indiana. Mean monthly polarization indices ranged from 0.25-0.35, well above the standard < 0.10 PI value for ovulation. Monthly mean polarization indices seemed to contradict the notion of fall spawning by shovelnose sturgeon in the MMR because October and November samples had the highest PI values. Additional research is needed to elucidate fall spawning by shovelnose sturgeon in the MMR.