Evaluating Population Recovery Characteristics and Potential Recovery Actions for a Long‐Lived Protected Species: A Case History of Gulf Sturgeon in the Apalachicola River

Abstract

In the U.S. Gulf of Mexico, Gulf Sturgeon Acipenser oxyrinchus desotoi supported an intense and short‐lived commercial fishery in the early twentieth century. Thereafter, it persisted at very low levels until the fishery was closed by individual U.S. states in the Gulf of Mexico region in the mid‐1980s. Despite the closure of the fishery, the stock has not recovered and there have been threats to population recovery including the potential effects of the Deepwater Horizon oil spill, storm events, and harmful algal blooms. We developed an age‐structured population model for Gulf Sturgeon to examine their population recovery characteristics. We paired this model with simple population reference points to assess the factors that influence the population recovery rate and strategies that resource managers could adopt to promote the recovery of the species. We used the Gulf Sturgeon population in the Apalachicola River as a case history and the date of 2023 that has been identified in Gulf Sturgeon Recovery Plan (GSRP) as the point at which to evaluate recovery, and under current management we predict that (1) age‐4+ Gulf Sturgeon (fish 4 years of age and older) will be approaching 50% of current estimated carrying capacity; (2) the dynamic spawning potential ratio is likely >0.3, suggesting a low chance of recruitment overfishing; (3) the population age structure is likely slowly recovering; (4) the recovery of the Gulf Sturgeon population is sensitive to increases in total mortality; and (5) the estimated values for exploitation rate (UMSY) and biomass at maximum sustainable yield (BMSY) are about 0.058 and 1,859 kg (or <200 age‐4+ fish per year [age‐4 and older fish]), respectively. Our results demonstrate the relative efficacy and influence of various recovery efforts and threats, respectively, and demonstrate that “recovery” is much different when it is based on historic versus currently available habitat. These model results provide reference points for comparing field assessments as part of the planned restoration efforts and upcoming population status reviews for Gulf Sturgeon that are funded as part of the Natural Resource Damage Assessment following the Deepwater Horizon oil spill.

Effects of Hurricane Michael on Gulf Sturgeon in the Apalachicola River system, Florida

“…Although they do not constitute a regular pressure faced by Gulf Sturgeon, stochastic mass mortality events can affect abundance recovery timelines. Gulf Sturgeon populations are inherently slow growing because of their late age of maturity and intermittent spawning periodicity, and they are generally sensitive to increases in adult mortality (Rudd et al 2014; Flowers et al 2020). Pine et al (2001) estimated a growth rate of only about 5% per year for the Suwannee River population during its recovery phase.…”

Experimental passage of adult male Gulf sturgeon around Jim Woodruff Lock and Dam on the Apalachicola River, Florida

“…After months of fasting throughout the summer, adult Gulf sturgeon must outmigrate to marine waters to restore their energy by foraging on the rich benthic resources present in these areas (Sulak et al., 2016). Population modeling has indicated that Gulf sturgeon population growth is highly sensitive to increases in adult mortality rates (Flowers et al., 2020; Rudd et al., 2014), thus the costs and benefits of continuing Gulf sturgeon passage in this system – and involving both males and females – must be carefully considered (George et al., 2009), perhaps by way of theoretical modeling before further experimentation (Jager, 2006).…”

“…Currently there are 7 reproducing populations of Gulf sturgeon, although a few more existed historically. Dams in the Mobile and Ochlockonee river systems are thought to have led to the extirpation of resident populations (Sulak et al., 2016), whereas other systems like the Apalachicola River still support a remnant population decades after the species was granted federal protections under the Endangered Species Act in 1991 (Flowers et al., 2020).…”