A Population Model to Assess Influences on the Viability of the Shortnose Sturgeon Population in the Ogeechee River, Georgia

Abstract

AbstractSouthern populations of the federally endangered Shortnose Sturgeon Acipenser brevirostrum are considered to be at greater risk of extirpation than northern populations. Our study focused on the Ogeechee River, Georgia, a small, undeveloped, coastal river that supports a population with fewer than 300 Shortnose Sturgeon. We designed a population viability analysis (PVA) model to represent and quantify the demographic influences of three factors (poor water quality, intrusion of saline water via rice canals, and incidental harvest) on the viability of this population. As an isolated population, only 75% of simulated populations persisted beyond a 20‐year time horizon with all factors simulated. However, immigration from the Altamaha River may help to support the population. We quantified population persistence with and without simulating each factor and found that (1) incidental harvest had no effect on simulated persistence, (2) poor water quality decreased simulated persistence by 29%, primarily due to low oxygen conditions in summer, and (3) roughly one‐third of this effect was attributed to rice canals (premature exposure of juveniles to high salinities). Simulated recruitment to age 1 was limited by a habitat squeeze between density‐dependent starvation upstream near the spawning grounds and premature exposure to salinity downstream. These results highlight a need for research on availability of summer refuge and freshwater rearing habitat. As these results derived from a PVA model, which required many assumptions, they should be considered preliminary. Further field research is needed to confirm those results where it is possible to test intermediate predictions. We conclude by suggesting that efforts to maintain or increase the number of viable populations of Shortnose Sturgeon in southern U.S. rivers will probably require an understanding of (1) source‐sink dynamics between populations in rivers with access to adequate freshwater rearing habitat and those without, and (2) the effects of climate change.