Reducing Uncertainty to Improve Gulf Sturgeon Stock Assessment and Inform Recovery Criteria

Gulf sturgeon Acipenser oxyrinchus desotoi populations in Florida were extensively harvested by commercial fisheries in the late nineteenth and early twentieth century possibly leading to large population declines and ultimately closure of the fishery in Florida in 1984.  Research on this species in Florida has occurred since the early 1970s with most efforts focused on basic population ecology including movement, growth, food habits, and population size estimation from tagging programs.  We developed two types of population models using information from commercial landings and data from these previously conducted studies to develop an assessment of trends in Gulf sturgeon population abundance in the Apalachicola and Suwannee rivers.  We used a stock reduction analysis (SRA) to reconstruct historical population biomass prior to the onset of commercial fishing.  Our results suggest that population biomass for this species in Florida was severely reduced by about 90% during a short, but intense, period of commercial harvest.  After large-scale fishing was abandoned, a small (exploitation less than 5% annually), but sustainable, fishery persisted until the fishery was closed in 1984.  We also used a series of age-structured mark-recapture models (ASMR) to estimate abundance, recruitment, and mortality of Gulf sturgeon in the Apalachicola and Suwannee rivers, the two rivers that likely supported the largest fisheries.  A key result of our research is a demonstration of large uncertainty in population trajectories since the late 1990s, concurrent with changes in the monitoring programs, depending on model structure is used.  Assessing Gulf sturgeon population status is difficult due to a combination of life history characteristics, behavior, and variations in sampling programs that all contribute to uncertainty in current population estimates.  This highlights key uncertainties in available data for assessing status and trends in Gulf sturgeon populations.  This uncertainty could lead to divergent management policies related to this listed species.