Abundance and Size Structure of Shortnose Sturgeon in the Altamaha River, Georgia

Abstract

Endangered Shortnose Sturgeon Acipenser brevirostrum inhabit large tidal rivers along the Atlantic coastline of North America, ranging from the St. John River, Canada, southward to the St. Johns River, Florida. Currently, long‐term assessments of the abundance and age structure of southern populations are completely lacking. To address this information gap, we assessed recent changes in Shortnose Sturgeon abundance and age structure by sampling with anchored entanglement gear in the Altamaha River, Georgia, during summer in 2004–2010. To estimate abundance, we used the Huggins closed‐capture model in Program MARK. We assessed size structure by interpreting the first, second, and third quartiles of the FL of individuals captured during each year. In total, we captured 1,737 Shortnose Sturgeon (72 recaptures). Total estimated abundance was variable, ranging from 1,206 individuals (95% lognormal confidence interval [CI] = 566–2,759) in 2009 to 5,551 individuals (95% CI = 2,804–11,304) in 2006. Much of the annual variation in total abundance was attributable to wide variations in juvenile abundance, which ranged from a low of 62 individuals (95% CI = 24–181) in 2009 to a high of 3,467 individuals (95% CI = 1,744–7,095) in 2006. Annual shifts in size structure were indicative of rapid population turnover, probably resulting from a combination of mortality and permanent emigration. Although the Altamaha River Shortnose Sturgeon population shares several characteristics with northern populations (e.g., variable juvenile abundance and stable adult abundance), our results suggest that southern populations are more susceptible to decline because of their accelerated life cycle and inherently lower abundances.