Rivers and estuaries along the Atlantic coast support both military installations and populations of the federally endangered shortnose sturgeon (Acipenser brevirostrum). The Ogeechee River, as an individual population segment, contains an order of magnitude fewer sturgeon than the neighboring Altamaha River and two orders of magnitude fewer fish than the largest known population in the Hudson River. Population estimates for this population have varied between 75 and 400 individuals since 1990. Fort Stewart, Georgia is the largest military installation in the southeastern U.S., nearly all of which drains to the Canoochee River, a tributary of the Ogeechee River.
Population viability analysis (PVA) can be used to partition influences on population recovery, and to distinguish those under the control of the military. This project used PVA to evaluate threats to the recovery of shortnose sturgeon (Acipenser brevirostrum) in the Ogeechee river, which is adjacent to Fort Stewart. Potential threats were evaluated including groundwater withdrawal, poor water quality in summer, saltwater intrusion via rice canals, mercury effects, harvest as by-catch, and sedimentation of spawning habitat.
This project assessed and ranked potential threats to population recovery using PVA. A workshop ruled out groundwater withdrawal and sedimentation of spawning habitat caused by activities on Fort Stewart. Models were developed to assess two potential threats to sturgeon growth and reproduction: high summer water temperatures and mercury uptake. To assess effects of summer water quality, temperature, salinity, and dissolved oxygen in the Canoochee and Ogeechee Rivers were monitored and were used to model and simulate spatial and temporal variation in water quality. In addition, a side-scan sonar survey of sediments was conducted to evaluate the availability of sturgeon spawning habitat. A sophisticated riverine PVA model was developed to simulate sturgeon growth, survival, and reproduction in response to water quality, saltwater intrusion via rice canals, and harvest. Simulation experiments that removed potential threats evaluated their effects on short-term extinction risk. To examine Fort Stewart’s influence on water quality, a novel survey of headwater watersheds was designed with the goal of minimizing correlation among predictors and including watersheds with extreme land-use characteristics. Relationships were then developed between water quality and watershed attributes, including military training activities.
During the course of the project, two threats were eliminated and there was a greater focus on the remaining four. Six putative spawning locations were identified with coarse substrate in deep pools. PVA simulations found poor water quality, saltwater intrusion, and by-catch mortality to have significant individual and interactive influences on persistence. Of these, only water quality is potentially influenced by Fort Stewart’s activities. It was found that military training was associated with higher suspended sediment and organic carbon in Fort Stewart’s headwater streams. Downstream, dissolved oxygen in the Ogeechee River was frequently below levels considered suitable for sturgeons.
The project’s results suggest that future research to improve management for this species in southeastern rivers should focus on understanding the apparent sensitivity of shortnose sturgeon to availability of upstream river habitat for spawning and rearing of early lifestages. The PVA modeling suggested a general theory for understanding coastal rivers and the anadromous populations that they support as a habitat “squeeze” for juveniles between density-dependent factors upstream and high salinities downstream. The length of suitable river habitat decreases seasonally as water temperatures rise. One can envision suitable river habitat as a series of accordions, shrinking and expanding seasonally. Locating refuge during periods of poor water quality in summer remains an important direction of research. River habitat is effectively “shortened” by man-made influences, including man-made canals and climate changes that result in sea-level rise. Therefore, research to understand future climate change effects and the potential for closing rice canals as an adaptive measure is recommended.