Potential Influence of Harvest on Shovelnose Sturgeon Populations in the Missouri River System

Abstract

The collapse of the European and Asian caviar industry has raised concern about the overexploitation of shovelnose sturgeon Scaphirhynchus platorynchus in the Missouri River. Unfortunately, little is known about the potential effects of harvest on the population dynamics of this species. Therefore, this study was conducted to describe the population characteristics (e.g., growth, longevity, and mortality) and to determine the influence of exploitation and harvest regulations (minimum length limits) on the yield, size structure, and egg production of shovelnose sturgeon from three sites in the Missouri River using a Beverton–Holt equilibrium model. Despite differences in the population characteristics (e.g., growth and longevity) of shovelnose sturgeon among sites, all populations responded similarly to harvest at the conditional natural mortality rates (death rate in the absence of harvest) used in our simulations (i.e., 5% and 20%). Our simulations of yield indicated that growth overfishing (i.e., shovelnose sturgeon being harvested before reaching their full growth potential) occurred with and without length limits at low conditional natural mortality rates in all populations. At a higher conditional natural mortality rate, only a 508-mm (fork length) minimum length limit prevented growth overfishing. Size structure (relative stock density of preferred-length fish (RSD-P); ≥510 mm) was highly sensitive to exploitation and was reduced up to 87% in simulations without a harvest restriction or with a 406-mm length limit. A 508-mm length limit prevented RSD-P from declining more than 18% in all simulations. As with size structure, maximum lifetime egg production was reduced up to 74% at low exploitation rates (≤20%), indicating the potential for recruitment overfishing. A 508-mm length limit prevented the proportion of maximum lifetime egg production from reaching 20% in all populations, except at the uppermost site (upstream of Fort Peck Lake, Montana). Although shovelnose sturgeon are thought to be more resilient to exploitation than the other sturgeon species, these results suggest that shovelnose sturgeon are sensitive to low levels of exploitation. We believe that a proactive approach to their management is warranted due to their current status and possible threats in the future.