Reproductive biology of middle Mississippi River shovelnose sturgeon: insights from seasonal and age variation in plasma sex steroid and calcium concentrations

Abstract

Shovelnose sturgeon Scaphirhynchus platorynchus are endemic to the Mississippi River drainage and are commercially harvested for roe in several states. The status of shovelnose sturgeon populations throughout much of its range is unknown or declining; Mississippi River stocks are experiencing recruitment overfishing and may be at risk of collapse. Restoration of shovelnose sturgeon populations will require additional information on their reproductive biology, including age at maturity and reproductive status and cycles. The objectives of this study were to determine the effects of reproductive stage, fish age, and season on plasma 17-β estradiol (E2) and calcium (Ca2+) concentrations in female shovelnose sturgeon and plasma testosterone (T) concentrations in male shovelnose sturgeon from the middle Mississippi River (MMR). The relationship was also assessed between plasma vitellogenin (VTG) and Ca2+concentrations for female shovelnose sturgeon. Plasma E2 and Ca2+concentrations in females and T concentrations in males differed among reproductive stages, consistent with results of prior research on shovelnose sturgeon and other sturgeon species. VTG and Ca2+concentrations were strongly correlated in female shovelnose sturgeon, indicating that Ca2+can be used as a surrogate for VTG assays for identification of vitellogenic females, as has been shown for other sturgeon species. Age at maturity was estimated at 10 years for males and 9 years for females based on T and E2 profiles, consistent with other recent age at maturity estimates in the MMR and Wabash River determined by gonadal examination. Peaks in plasma sex steroid and Ca2+concentrations during April and October possibly reflected the spring spawning season and provide additional evidence in support of recent documentation of autumn spawning by shovelnose sturgeon in the MMR. Additional research is needed to elucidate autumn spawning by shovelnose sturgeon in the MMR.